Chapter V: Muscles of the Leg and Foot (12)

The cerebral arteries are very variable in arrangement, and the
foregoing account is a brief statement of the more usual disposition
of the larger vessels. The internal carotid artery often arises with
the occipital by a common trunk of variable length.

THE EXTERNAL CAROTID ARTERY (Figs. 436, 437)

This artery (A. carotis externa) by its size and direction constitutes the continuation of the common carotid. It passes forward on the lateral wall of the pharynx at the lower border of the guttural pouch, covered by the submaxillary gland and the stylo-maxillaris, digastricus, and stylo-hyoideus muscles. It then emerges between the stylo-hyoideus and the great cornu of the hyoid bone, passes upward on the latter parallel with the posterior border of the lower jaw, and terminates about two inches (ca. 5 cm.) below the temporo-maxillary articulation by dividing into superficial temporal and internal maxillary branches. It is crossed deeply near its origin by the superior laryngeal and pharyngeal branches of the vagus nerve. Just before its emergence its superficial face is crossed by the hypoglossal nerve, and the glosso-pharyngeal nerve passes over its inner surface at the ventral border of the great cornu. The chief collateral branches are the masseteric, external maxillary, and posterior auricular. It also furnishes variable branches to the submaxillary and parotid glands, the guttural pouch, and the pharyngeal lymph glands, as well as twigs to some adjacent muscles.

1. The =inferior masseteric= or maxillo-muscular artery (A. masseterica inferior) is given off from the external carotid at its emergence from beneath the stylo-hyoideus. It passes downward and slightly forward under cover of the parotid gland and over the tendon of insertion of the sterno-cephalicus to the posterior border of the mandible and appears on the masseter muscle, into which it plunges after a short course on its surface. It gives branches also to the internal pterygoid and stylo-maxillaris muscles and the parotid gland.

2. The =external maxillary=, =facial=, or =submaxillary artery= (A. maxillaris externa)[159] arises from the external carotid on the inner surface of the posterior belly of the digastricus (Fig. 436). It runs downward and forward on the lateral wall of the pharynx across the deep face of the stylo-hyoideus toward the great cornu of the hyoid bone, accompanied by the glosso-pharyngeal nerve in front and the hypoglossal nerve behind. After giving off the lingual artery at the posterior border of the great cornu, it inclines more ventrally on the inner surface of the internal pterygoid muscle, crosses over the hyo-glossus muscle, the hypoglossal nerve, the submaxillary duct, and the intermediate tendon of the digastricus, and turns forward in the submaxillary space. Here it lies on the lower part of the internal pterygoid muscle, and is related internally to the submaxillary lymph glands, above to the anterior belly of the digastricus, and below to the homonymous vein. At the anterior border of the masseter it turns around the lower border of the jaw and runs upward on the face in front of that muscle.[160] At the turn the artery is in front, the vein in the middle, and the parotid duct posterior. The artery and vein pass upward along the anterior border of the masseter, under cover of the facial panniculus and the zygomaticus, and are crossed superficially by branches of the facial nerve and deeply by the parotid duct. The artery terminates over the levator labii superioris proprius by dividing into the dorsal nasal and the angular artery of the eye. The chief branches of the external maxillary are as follows:

(1) The =pharyngeal artery= (A. palatina ascendens) arises usually behind the stylo-pharyngeus, passes between that muscle and the great cornu, and runs forward on the lateral wall of the pharynx under the elastic pharyngeal fascia. It is distributed to the soft palate, pharynx, and tonsil.

FIG. 435.—SUPERFICIAL DISSECTION OF HEAD OF HORSE. MOST OF THE
PANNICULUS IS REMOVED.

_a_, Dilatator naris superior; _b_, levator labii superioris proprius;
_c_, levator nasolabialis; _d_, dilatator naris lateralis; _e_,
buccinator; _f_, zygomaticus; _g_, depressor labii inferioris; _h_,
stump of retractor anguli oris; _i_, masseter; _k_, _k_, scutularis;
_l_, scutulo-auriculares superficiales; _m_, zygomatico-auricularis;
_n_, parotido-auricularis; _o_, stylo-maxillaris; _p_,
sterno-cephalicus; _p′_, tendon of _p_; _q_, omo-hyoideus; _r_,
splenius; _s_, tendon of splenius and trachelo-mastoideus; _t_,
corrugator supercilii; _u_, orbicularis oris; _v_, parotid gland;
_w_, zygomatic arch; _x_, scutiform cartilage; _y_, upper commissure
of nostril; _1_, facial nerve; _2_, superior buccal nerve; _3_,
inferior buccal nerve; _4_, transverse facial nerve; _5_, cervical
branch of facial nerve; _6_, posterior auricular branch of second
cervical nerve; _7_, cutaneous cervical branch of same; _8_,
infratrochlear nerve; _9_, frontal nerve; _10_, lacrimal nerve;
_10′_, end of auriculo-palpebral nerve; _11_, inferior masseteric
artery and vein; _12_, transverse facial artery and vein; _13_,
facial artery; _14_, inferior labial artery; _15_, superior labial
artery; _16_, lateral nasal artery; _17_, dorsal nasal artery; _18_,
angular artery of the eye; _19_, posterior auricular artery; _20_,
_21_, jugular vein; _22_, external maxillary vein; _23_, great
auricular vein; _24_, parotid duct; _24′_, origin of same. (After
Ellenberger-Baum, Top. Anat. d. Pferdes.)
]

(2) The =lingual artery= (A. lingualis) is a large branch which diverges from the parent trunk at an acute angle, runs along the ventral border of the great cornu of the hyoid bone, and dips under the hyo-glossus muscle. It then passes across the kerato-hyoideus, turns inward under the intercornual joint of the hyoid bone, and runs forward in the tongue between the hyo-glossus and genio-glossus. This part (A. profunda linguæ) is flexuous and is accompanied by branches of the hypoglossal and lingual nerves. It is the chief artery of the tongue, and anastomoses with the opposite artery and the sublingual.

(3) The =sublingual artery= (A. sublingualis) is a smaller vessel which arises at the anterior extremity of the submaxillary gland (Fig. 436). It passes forward on the anterior belly of the digastricus between the ramus of the mandible and the mylo-hyoideus, perforates the latter, runs along the lower border of the sublingual gland, and ramifies in the mucous membrane of the anterior part of the floor of the mouth. It detaches branches to the muscles and skin in the submaxillary space, the submaxillary lymph glands, and the sublingual gland. It also gives off the small =submental artery=, which runs forward superficially toward the lower lip, supplying twigs to the skin and the mylo-hyoideus.

FIG. 436.—PAROTID, MASSETERIC, AND LINGUAL REGIONS OF HORSE: DEEP
DISSECTION, THIRD LAYER.

_a_, Mylo-hyoideus, anterior part, reflected; _b_, genio-hyoideus;
_c_, genio-glossus; _d_, sublingual gland; _e_, ramus of mandible,
the greater part of which is removed; _e′_, stump of masseter; _f_,
maxillary tuberosity; _g_, great cornu of hyoid bone; _h_, wing of
atlas; _i_, intermediate tendon of digastricus; _i′_, anterior
belly, _i″_, posterior belly of digastricus; _k_, posterior part of
mylo-hyoideus; _l_, hyo-glossus; _m_, pterygoideus internus (cut);
_n_, stylo-hyoideus; _o_, stylo-maxillaris (cut); _p_,
crico-pharyngeus; _q_, obliquus capitis ant.; _r_, tendon of
trachelo-mastoideus; _s_, rectus cap. ant. major; _t_,
mastoido-humeralis (cut); _u_, sterno-cephalicus (cut); _v_,
sterno-thyroideus (cut); _w_, sterno-hyoideus (cut); _x_,
omo-hyoideus; _y_, obliquus cap. post.; _z_, splenius (cut); _1–4_,
upper cheek teeth; _4′_, last cheek tooth; _5_, stump of facial
nerve; _6_, stump of buccinator nerve; _7_, lingual nerve; _7′_,
superficial branch, _7″_, deep branch of lingual nerve; _8_, stumps
of inferior alveolar artery, vein and nerve; _9_, mylo-hyoid nerve
(cut); _10_, glosso-pharyngeal nerve; _11_, hypoglossal nerve; _12_,
superior laryngeal nerve; _13_, ventral branch of first cervical
nerve; _14_, vagus and sympathetic; _15_, dorsal branch of spinal
accessory nerve; _16_, ventral branch of same; _17_, inferior
cerebral vein; _18_, submaxillary duct; _19_, common carotid artery;
_20_, parotid branch; _21_, thyro-laryngeal artery; _22_, pharyngeal
artery; _23_, laryngeal artery; _24_, internal carotid artery; _25_,
occipital artery; _26_, external carotid artery; _27–31_, external
maxillary artery; _28_, posterior palatine artery; _29_, lingual
artery; _30_, sublingual artery; _32_, internal maxillary artery;
_33_, internal maxillary vein (origin); _34_, remnant of parotid
gland; _35_, thyroid gland; _36_, jugular vein; _37_, pharyngeal
lymph glands. (After Ellenberger-Baum, Top. Anat. d. Pferdes.)
]

In some cases the sublingual artery arises from the lingual and the
submental from the external maxillary. Sometimes the sublingual
remains on the external face of the mylo-hyoideus—thus resembling the
submental of man—and the sublingual gland is supplied by a special
branch of the lingual.

Not uncommonly a considerable branch, given off in the submaxillary
space, turns round the lower border of the jaw and enters the middle
of the lower part of the masseter muscle. In some cases this artery is
of large size and its pulsation can be felt. It is accompanied by a
vein.

(4) The =inferior labial artery= (A. labialis inferior) arises from the facial a little before it reaches the depressor labii inferioris (Fig. 435). It passes forward, dips under the depressor muscle, and continues to the lower lip. It supplies branches to the muscles and skin in this region, to the inferior buccal glands, the mucous membrane of the cheek, and the lower lip, anastomosing with the mental artery and the corresponding vessels of the opposite side. It detaches a branch (A. anguli oris) to the angle of the mouth, which anastomoses with the superior labial.

(5) The =superior labial artery= (A. labialis superior) arises from the facial in front of the facial crest (Fig. 435). It passes forward under the dilatator naris lateralis and levator nasolabialis to the upper lip, gives twigs to the upper part of the cheek and the lateral nasal region, and ramifies in the upper lip, anastomosing with the opposite artery and the palato-labial.

(6) The =lateral nasal artery= (A. lateralis nasi) arises usually a little above the preceding one, and runs forward parallel with it and under the levator nasolabialis to the nostril (Fig. 435). It supplies branches to the lateral nasal region and the nostril.

The vessel is often double. It may arise from the superior labial at
the point of bifurcation of the facial or with the dorsal nasal from
the infraorbital artery (as in the ox). In some cases it gives off a
dorsal nasal branch.

(7) The =dorsal nasal artery= (A. dorsalis nasi) arises on the levator labii superioris proprius and passes forward under the levator nasolabialis to the dorsum nasi (Fig. 435).

(8) The =angular artery of the eye= (A. angularis oculi) runs toward the internal canthus of the eye, where it anastomoses with the orbital branch (Ramus malaris) of the infraorbital artery (Fig. 435).

In addition to the preceding, unnamed branches are supplied to the submaxillary salivary gland and the submaxillary lymph glands.

3. The =posterior auricular artery= (A. auricularis posterior) arises at an acute angle from the external carotid just above the origin of the masseteric. It passes upward under cover of the parotid gland, to which it gives branches, and divides into several branches which supply the skin and muscles of the external ear (Fig. 435). The =posterior branch= passes to the posterior part of the base of the ear, where it divides into two branches; of these, one (Ramus intermedius) passes up the convex surface of the external ear to the apex, while the other (Ramus medialis) winds around to the anterior (inner) border, and forms an arch with the intermediate branch. The =external branch= (Ramus lateralis) passes up the posterior (external) border of the ear and forms an arch with the intermediate branch. The =deep branch= (A. auricularis profunda) enters the interval between the external auditory canal and mastoid process, and passes through an opening into the interior of the external ear and ramifies in the skin which lines it. It gives off the =stylo-mastoid artery=, which passes through the stylo-mastoid foramen into the tympanum, forms an arch around the membrana tympani, and supplies the middle ear and its muscles.

THE SUPERFICIAL TEMPORAL ARTERY

This artery (A. temporalis superficialis) is much the smaller of the two terminal branches of the external carotid. It passes upward behind the posterior border of the ramus of the mandible under cover of the parotid gland, and divides below the level of the condyle into the anterior auricular and transverse facial arteries. It is crossed superficially by the facial nerve.

The =anterior auricular artery= (A. auricularis anterior) passes upward behind the temporo-mandibular articulation under cover of the parotid gland, crosses the zygomatic arch, and reaches the temporalis muscle. It is crossed deeply at its origin by the superficial temporal nerve and is accompanied by a satellite vein and the auriculo-palpebral branch of the facial nerve. It is distributed to the skin and the temporal and anterior auricular muscles, and sends a branch through the conchal cartilage to the skin which lines it. Collateral twigs are detached to the parotid gland, and an anterior branch anastomoses with the supraorbital artery. A branch sometimes passes into the parieto-temporal canal and anastomoses with the posterior meningeal (mastoid) artery.

The =transverse facial artery= (A. transversa faciei) is larger than the preceding. It turns around the neck below the condyle of the mandible and emerges from beneath the parotid gland (Fig. 435). It then passes forward a short distance on the masseter about half an inch below the zygomatic arch and plunges into the muscle, in which it commonly divides into two chief branches. It is accompanied by a vein and a branch of the superficial temporal nerve. It supplies the masseter and the skin of this region, and anastomoses with the external maxillary and posterior deep temporal arteries.

FIG. 437.—DEEP DISSECTION OF HEAD OF HORSE.

The left ramus of the mandible and structures connected with it have
been removed. _a_, _a_, Stumps of stylo-glossus; _b_, genio-glossus;
_c_, genio-hyoideus; _d_, omo-hyoideus; _e_, kerato-hyoideus; _f_,
thyro-hyoideus; _g_, thyro-pharyngeus; _h_, crico-thyroideus; _i_,
sterno-thyroideus; _k_, thyroid gland; _m_, crico-pharyngeus; _n_,
palatinus and palato-pharyngeus; _o_, pterygoideus externus; _p_,
tensor palati; _q_, levator palati; _r_, temporalis; _s_, rectus
cap. ant. major; _t_, obliquus cap. ant.; _u_, guttural pouch; _v_,
great cornu of hyoid bone, posterior extremity of which is removed
and indicated by dotted line; _w_, position of small cornu, dotted
line; _x_, thyroid cornu; _y_, tongue; _z_, anterior pillar of soft
palate; _1_, superficial temporal nerve; _2_, chorda tympani; _3_,
stump of inferior alveolar nerve; _4_, _4_, lingual nerve,
intermediate part removed; _5_, deep temporal nerve; _6_, masseteric
nerve; _7_, buccinator nerve; _8_, great palatine nerve; _9_,
infraorbital nerve; _10_, sphenopalatine and posterior nasal nerves;
_11_, spinal accessory nerve; _12_, vagus; _13_, pharyngeal branch
of vagus; _14_, superior laryngeal nerve; _15_, vago-sympathetic
trunk; _16_, sympathetic, with superior cervical ganglion a little
further back; _17_, glosso-pharyngeal nerve; _18_, pharyngeal and
_19_, lingual branches of glosso-pharyngeal; _20_, hypoglossal
nerve; _21_, left recurrent nerve; _22_, common carotid artery;
_23_, parotid branch; _24_, thyro-laryngeal artery; _24′_, laryngeal
artery; _25_, occipital artery; _26_, internal carotid artery; _27_,
external carotid artery; _28_, _30_, external maxillary artery;
_29_, pharyngeal artery; _31_, lingual artery; _32_, internal
maxillary artery; _33_, stump of inferior alveolar artery; _34_,
middle meningeal artery; _35_, deep temporal artery; _36_,
buccinator artery; _37_, palatine artery; _38_, end of internal
maxillary artery; _39_, right external maxillary artery; _40_,
satellite vein of _39_; _41_, right parotid duct; _42_, submaxillary
lymph glands; _43_, pharyngeal lymph glands; _44_, trachea; _45_,
wing of atlas; _46_, dotted line indicating outline of submaxillary
gland; _47_, lacrimal gland. (After Ellenberger-Baum, Top. Anat. d.
Pferdes.)
]

The pulse may usually be felt in this artery as it passes below the
articulation of the jaw.

THE INTERNAL MAXILLARY ARTERY (Figs. 437, 438, 439)

This artery (A. maxillaris interna) is much the larger of the two terminal branches of the external carotid. It begins at the inner side of the posterior border of the mandible about two inches (ca. 5 cm.) below the articulation of the jaw and ends in the anterior part of the pterygo-palatine fossa. On account of its complex course and the large number of branches given off it is convenient to divide it into three parts.

The =first part= is much the longest, forms an =S=-shaped curve, and is in great part in contact with the guttural pouch. It passes upward and forward on the inner surface of the mandible a distance of about an inch (ca. 2 to 3 cm.) and is related here to the vein, which is ventral. It then turns inward below the external pterygoid muscle and the inferior maxillary nerve, passes between that muscle and the tensor palati, and runs forward to enter the alar (pterygoid) foramen. This part gives off the following branches:

1. The =inferior alveolar= or =dental artery= (A. alveolaris mandibulæ s. inferior) passes downward and forward with the homonymous vein and nerve, being at first between the external and internal pterygoid muscles, then between the latter and the ramus of the mandible. It enters the mandibular foramen, passes downward and forward in the mandibular canal, and terminates at the mental foramen by dividing into mental and incisor branches. The =mental branch= (A. mentalis) emerges through the mental foramen and anastomoses in the lower lip with the opposite artery and the inferior labial. The =incisor branch= continues forward in the bone, supplies twigs to the canine and incisor teeth, and anastomoses with its fellow of the opposite side. Collateral branches are detached to the pterygoid and mylo-hyoid muscles, and within the bone to the teeth, alveolar periosteum, the gums, and the spongy substance of the mandible.

2. The =pterygoid arteries=, two or three in number, are distributed to the pterygoid and tensor and levator palati muscles.

3. The =tympanic artery= is a very small vessel which passes along the Eustachian tube to the petro-tympanic fissure (of Glaser) and enters the middle ear.

FIG. 438.—RIGHT EYE OF HORSE.

_a_, Remnants of periorbita; _b_, levator palpebræ superioris; _c_,
obliquus oculi inferior; _d_, rectus oculi inferior; _e_, rectus
oculi externus; _f_, rectus oculi superior; _g_, sclera; _g′_,
cornea; _h_, lacrimal gland; _i_, frontal nerve; _k_, frontal
artery; _l_, branch of lacrimal nerve to gland; _m_, lacrimal
artery; _n_, zygomatic nerve; _o_, branch of ophthalmic artery; _p_,
branch of oculomotor nerve to obliquus oculi inferior; _q_,
maxillary nerve; _r_, infraorbital nerve; _s_, posterior nasal
nerve; _t_, great palatine nerve; _u_, small palatine nerve; _v_,
internal maxillary artery; _w_, buccinator artery (cut); _x_,
infraorbital artery; _x′_, malar artery; _y_, sphenopalatine artery;
_z_, great palatine artery; _z′_, small palatine (or staphyline)
artery; _1_, posterior deep temporal artery; _2_, _3_, stumps of
orbital margin; _4_, facial crest; _5_, temporal fossa; _6_, foramen
lacerum orbitale; _7_, anterior end of alar canal; _8_, posterior
opening of same. (After Ellenberger, in Leisering’s Atlas.)
]

4. The =middle meningeal artery= (A. meningea media) arises beneath the buccinator nerve where the internal maxillary turns forward. It passes backward across the temporal wing of the sphenoid to the antero-external part (foramen spinosum) of the foramen lacerum. Entering the cranium, it divides into branches which course in the grooves on the temporal and parietal bones and supply the dura mater. It anastomoses with the posterior meningeal.

The size of this artery is variable, and is in inverse ratio to that
of the posterior meningeal.

5. The =posterior deep temporal artery= (A. temporalis profunda aboralis) arises from the internal maxillary just before the latter enters the canal of the pterygoid process. It passes upward and backward in the temporal fossa on the deep face of the temporalis muscle, in which it ramifies. It sends a branch outward to the masseter, and anastomoses with the superficial temporal and middle meningeal arteries.

In some cases the tympanic and middle meningeal arise from this
artery.

The =second part= lies in the alar canal and is about an inch (ca. 2 to 3 cm.) in length. It gives off two branches—the anterior deep temporal and the ophthalmic.

1. The =anterior deep temporal artery= (A. temporalis profunda oralis) emerges from the canal through the temporal or small alar foramen, and passes upward in the anterior part of the temporal fossa on the deep face of the temporalis muscle, in which it is chiefly distributed. It gives twigs to the orbital fat and the skin of the frontal region.

FIG. 439.—LEFT EYE OF HORSE, DEEPER DISSECTION.

The outer plate of bone has been removed behind the pterygoid crest to
expose the vessels and nerves. _a_, _a_, Remnants of periorbita;
_b_, stump of rectus oculi superior; _c_, obliquus oculi inferior;
_d_, rectus oculi inferior; _e_, rectus oculi lateralis; _e′_,
retractor oculi; _f_, rectus oculi internus; _g_, _g_, obliquus
oculi superior; _h_, eyeball; _i_, trochlear nerve; _k_, ophthalmic
nerve; _k′_, nasal nerve; _k″_, infratrochlear nerve; _k‴_,
ethmoidal nerve; _l_, optic nerve; _m_, frontal nerve; _n_, lacrimal
nerve; _o_, zygomatic nerve; _p_, nerve to obliquus inferior (from
oculomotor); _q_, maxillary nerve; _r_, infraorbital nerve; _s_,
sphenopalatine nerve; _t_, great palatine nerve; _u_, small palatine
nerve; _v_, internal maxillary artery; _w_, buccinator artery (cut
off); _x_, infraorbital artery; _x′_, malar artery; _y_,
sphenopalatine artery; _z_, great palatine artery; _z′_, small
palatine (or staphyline) artery; _1_, _2_, stumps of zygomatic arch;
_3_, stump of supraorbital process; _4_, facial crest; _5_, temporal
fossa; _6_, ophthalmic artery; _7_, muscular branch of _6_; _8_,
lacrimal artery (cut); _9_, frontal artery; _10_, anterior deep
temporal artery; _11_, ethmoidal artery. (After Ellenberger-Baum,
Top. Anat. d. Pferdes.)
]

2. The =ophthalmic artery= (A. ophthalmica externa) emerges from the anterior opening of the alar canal, and enters the apex of the periorbita or ocular sheath. Within this it forms a semicircular bend under the rectus oculi superior and is continued by the ethmoidal artery. Its branches are as follows:

(_a_) The =supraorbital= or =frontal artery= (A. supraorbitalis)[161] is a small vessel which often arises from the anterior deep temporal or the internal maxillary. It passes along the inner wall of the orbit in company with the nerve of the same name to the supraorbital foramen, through which it emerges. It is distributed to the orbicularis oculi, the corrugator supercilii, and the skin of the supraorbital region.

(_b_) The =lacrimal artery= (A. lacrimalis) runs upward and forward within the periorbita along the outer edge of the levator palpebræ superioris to the lacrimal gland, in which it is chiefly distributed. It also sends twigs to the upper eyelid.

(_c_) =Muscular branches= (Rami musculares) supply the orbital muscles, the periorbita, the third eyelid, and the conjunctiva.

(_d_) =Ciliary branches= (Aa. ciliares), two sets of very slender arteries, arise from the ophthalmic direct and from the muscular branches. The anterior ciliary arteries (Aa. ciliares anteriores) pierce the sclera in front of the equator and ramify chiefly in the ciliary body and the iris. The posterior ciliary arteries (Aa. ciliares posteriores) pierce the posterior part of the sclera; most of them ramify in the choroid coat as the short ciliary arteries, but two of larger size, the long ciliary arteries, run forward, one on each side, between the sclera and choroid to the periphery of the iris. Here they divide into branches which anastomose and form a circle (Circulus iridis major). From this secondary branches are detached which form a second circle around the pupil (Circulus iridis minor).

FIG. 440.—SAGITTAL SECTION OF HEAD OF HORSE, CUT A LITTLE TO THE RIGHT
OF THE MEDIAN PLANE.

_1_, Skin; _2_, nasal bone; _3_, frontal bone; _4_, parietal bone;
_4′_, tentorium osseum; _5_, occipital bone; _6_, sphenoid bone;
_7_, hard palate; _8_, premaxilla; _9_, mandible; _10_, hyoid bone;
_11_, septum between frontal sinuses; _12_, alar cartilage; _13_,
dilatator naris transversus; _14_, septum nasi with venous plexuses;
_15_, palato-labial artery; _16_, _16′_, upper and lower septal
branches of _15_; _17_, septal branch of sphenopalatine artery and
satellite vein; _18_, septal branch of ethmoidal artery; _19_,
sphenoidal sinus; _20_, guttural pouch; _21_, pharynx; _22_,
pharyngeal orifice of Eustachian tube; _23_, posterior naris; _24_,
soft palate; _25_, palatinus muscle; _26_, pharyngeal muscles; _27_,
œsophagus; _28_, dotted line indicating position of posterior pillar
of soft palate; _29_, lamina; _29′_, arch of cricoid cartilage;
_30_, arytenoideus transversus muscle; _31_, epiglottis; _32_, body
of thyroid cartilage; _33_, vocal process and cord; _34_, arytenoid
cartilage; _35_, lateral ventricle of larynx; _36_, trachea; _37_,
ventral straight muscles of head; _38_, longus colli; _39_, atlas;
_40_, axis; _41_, lig. nuchæ; _42_, dorsal spinal muscles; _43_,
muscles of external ear; _44_, omo- and sterno-hyoideus; _45_,
tongue; _46_, mylo-hyoideus; _47_, genio-hyoideus; _48_,
genio-glossus; _49_, longitudinalis inferior; _50_, longitud.
superior; _51_, hyo-epiglotticus; _52_, chin and mentalis muscle;
_53_, venous plexus of hard palate; _54_, corpus callosum; _55_,
septum pellucidum; _56_, fornix; _57_, thalamus; _58_, pineal body;
_59_, corpora quadrigemina; _60_, cerebral peduncle; _61_, corpus
mammillare; _62_, pituitary body; _63_, chiasma opticum; _64_,
intercarotid artery; _65_, medulla oblongata; _66_, interventricular
foramen; _67_, infundibulum; _68_, third ventricle; _69_, cerebral
aqueduct; _70_, anterior medullary velum; _71_, fourth ventricle;
_72_, posterior medullary velum; _73_, basilar artery; _74_, spinal
cord; _75_, ethmoidal nerve; _76_, septal branch of posterior nasal
nerve; _76′_, branch of same to organ of Jacobson; _77_, olfactory
nerve to organ of Jacobson; _78_, nasal branches of palatine artery.
(After Ellenberger, in Leisering’s Atlas.)
]

(_e_) The =central artery of the retina= (Arteria centralis retinæ) is a small vessel which arises from the ophthalmic or from a posterior ciliary artery. It pierces the optic nerve a short distance behind the sclera and runs in its center to the lamina cribrosa, where it breaks up in thirty to forty fine branches. These appear in the fundus of the eye at the margin of the optic papilla and radiate in the posterior part of the retina.

(_f_) The =ethmoidal artery= (A. ethmoidalis) is the continuation of the ophthalmic. It enters the cranial cavity through the ethmoidal foramen, passes inward on the cribriform plate, and divides into meningeal and nasal branches. The former ramify in the anterior part of the dura mater and anastomose with branches of the artery of the corpus callosum. The nasal branch passes through the cribriform plate, gives branches to the mucous membrane of the lateral mass of the ethmoid and the adjacent part of the septum nasi, and runs forward on the superior turbinal.

The =third part= passes forward in the pterygo-palatine fossa, accompanied by branches of the maxillary nerve. On reaching the posterior palatine foramen it is continued by the palatine artery. Its branches are as follows:

(1) The =buccinator artery= (A. buccinatoria) arises from the lower aspect of the internal maxillary shortly after its emergence (Fig. 437). It turns around the maxillary tuberosity, accompanied by the buccinator nerve, and under the masseter muscle, enters the cheek, and runs forward in it. It supplies branches to the cheek, the superior buccal glands, and the masseter and pterygoid muscles. Near its origin it gives off a branch to the orbital fat behind the periorbita.

(2) The =infraorbital= or =superior dental artery= (A. infraorbitalis) arises from the upper aspect of the internal maxillary a little in front of the preceding vessel. It passes upward and forward to the maxillary foramen, runs in the infraorbital canal in company with the nerve of the same name, and is continued forward within the jaw to the incisor teeth. It gives branches to the teeth and gums, and detaches a branch through the infraorbital foramen which anastomoses with the lateral nasal and superior labial. About midway between its origin and the maxillary foramen it gives off the =malar= or =orbital branch= (Ramus malaris), which passes along the floor of the orbit to end in the lower lid and anastomose with the angularis oculi. It gives twigs to the inferior oblique muscle and the lacrimal sac.

The infraorbital artery is usually small at its emergence upon the
face, but in some cases it is rather large and may partially replace
the superior labial and lateral nasal arteries.

(3) The =staphyline= or =small palatine artery= (A. palatina minor) is a small vessel which passes forward in the groove at the inner side of the maxillary tuberosity to the soft palate. In the groove it is accompanied by the nerve of the same name and the palatine vein.

(4) The =sphenopalatine artery= (A. sphenopalatina) arises in the extreme anterior part of the pterygo-palatine fossa and passes into the nasal cavity, where it divides into internal and external branches. The internal branch is distributed to the mucous membrane of the septum nasi; the external one goes to the inferior turbinal, the inferior meatus, the posterior nares, and the maxillary and frontal sinuses. It may arise from the infraorbital.

(5) The =palatine= or =palato-labial artery= (A. palatina major) is the direct continuation of the internal maxillary. It passes through the palatine canal to the roof of the mouth, accompanied by the palatine nerve, and runs forward in the palatine groove, where it is joined by the vein. A little behind the plane of the corner incisor teeth it curves inward over a bar of cartilage to the foramen incisivum, where it unites with its fellow of the opposite side. The single artery thus formed passes up through the foramen and divides under the transverse dilator of the nostril into two branches. These ramify in the upper lip and anastomose with the lateral nasal and superior labial arteries. In its course in the roof of the mouth the palatine artery gives off branches to the hard and soft palate and the gums, and others which pass through the accessory palatine foramina to be distributed in the mucous membrane of the lower part of the nasal cavity. Commonly two branches, right and left, are detached from the convexity of the arch formed by the union of the two arteries; these run forward in the anterior part of the hard palate.

ARTERIES OF THE THORACIC LIMB

THE BRACHIAL ARTERY

The =brachial artery=, after turning around the lower border of the scalenus at the first rib, passes backward and a little downward across the origin of the coraco-brachialis and the insertion of the subscapularis muscle at the inner side of the shoulder joint. At the posterior border of the subscapularis it gives off the subscapular artery, and turns downward on the inner surface of the arm. In its course in the arm it inclines a little forward, crosses the humerus very obliquely, and divides at the antero-internal aspect of the distal third of the shaft of the bone into the anterior and posterior radial arteries.[162]

It is related internally to the serratus magnus and the posterior deep pectoral muscles, and the cubital lymph glands. Externally it lies on the subscapularis, the brachialis, the insertion of the teres major and latissimus dorsi, and the inner head of the triceps. The coraco-brachialis muscle lies along the front of the artery, which, however, overlaps the muscle distally. The vein crosses the inner face of the artery and runs down behind it, accompanied part way by the ulnar and radial nerves. The musculo-cutaneous and median nerves form a loop around the artery close to its origin, and the latter nerve then passes down in front of the artery. The chief branches are as follows:

1. The =suprascapular artery= (A. thoracico-acromialis) is a small and somewhat flexuous vessel, which arises near the anterior border of the subscapularis, runs upward, and dips in between that muscle and the supraspinatus. It gives branches to these muscles, the anterior deep pectoral, and the mastoido-humeralis. A branch passes in front of the coraco-brachialis to the shoulder joint and the proximal end of the humerus.

This vessel may be double, or be represented by one or more branches
of the inferior cervical artery.

2. The =subscapular artery= (A. subscapularis) is a very large vessel which arises at the posterior border of the subscapularis muscle. It passes upward in the interstice between that muscle and the teres major on the inner surface of the long head of the triceps, turns around the posterior border of the scapula below the posterior angle, and ends in the infraspinatus and deltoid. Besides collaterals (Rami musculares) to the subscapularis, teres major, triceps, and tensor fasciæ antibrachii it gives off the following named branches:

(_a_) The =thoracico-dorsal= (A. thoracicodorsalis) is a long artery which is usually given off about an inch from the origin of the subscapular, crosses the inner face of the teres major, and runs upward and backward on the latissimus dorsi. It gives branches to these muscles, the abdominal panniculus and the axillary lymph glands.

(_b_) The =posterior circumflex artery= (A. circumflexa humeri posterior) arises a little above the preceding vessel and passes outward behind the shoulder joint between the long and external heads of the triceps with the axillary (or circumflex) nerve. It gives branches to these muscles, the joint capsule, and the muscles and skin of the outer side of the shoulder, anastomosing with the anterior circumflex artery.

(_c_) The =circumflex artery of the scapula= (A. circumflexa scapulæ) arises about two or three inches (ca. 5 to 7.5 cm.) above the shoulder joint, passes forward to the posterior border of the scapula, and divides into two branches. The outer one runs forward on the external surface of the scapula below the spine and gives branches to the supraspinatus, infraspinatus, and teres minor. The inner branch passes forward in a similar fashion on the costal surface of the scapula and supplies branches to the subscapularis.

FIG. 441.—DISSECTION OF SHOULDER AND ARM OF HORSE, INNER SURFACE.

_A_, Brachial artery; _B_, subscapular artery; _C_, thoracico-dorsal
artery; _D_, deep brachial artery; _E_, ulnar artery; _F_, anterior
radial artery; _G_, posterior radial (or median) artery; _H_,
brachial vein; _I_, external thoracic (or “spur”) vein; _J_,
brachial vein; _K_, _K_, _K_, posterior radial veins; _L_, cephalic
vein; _L′_, communicating vein; _M_, brachialis muscle; _N_,
internal epicondyle of humerus; _O_, prescapular lymph glands; _P_,
position of axillary lymph glands; _Q_, position of cubital lymph
glands; _1_, suprascapular nerve; _2_, subscapular nerves; _3_,
thoraco-dorsal nerve; _4_, musculo-cutaneous nerve; _4′_, cutaneous
branch of musculo-cutaneous nerve; _5_, median nerve; _6_, axillary
nerve; _7_, ulnar nerve; _8_, radial nerve; _9_, thoracic nerves.
(After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 442.—DISSECTION OF RIGHT FOREARM AND CARPUS OF HORSE, INTERNAL
SURFACE.

_C.b._, Coraco-brachialis muscle; _E.m._, internal epicondyle of
humerus; _R_, subcutaneous surface of radius. (After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]

3. The =anterior circumflex= (or prehumeral) =artery= (A. circumflexa humeri anterior) arises usually at the upper border of the teres major. It passes forward and outward between the two parts of the coraco-brachialis or between the latter and the humerus. It gives branches to the coraco-brachialis and deep pectoral and ends in the upper part of the biceps and in the mastoido-humeralis. It anastomoses with the posterior circumflex.

Not rarely it arises below the middle of the humerus and passes upward
between the biceps and coraco-brachialis. In these cases a small
branch for the coraco-brachialis is usually given off at the usual
point of origin of the anterior circumflex.

4. The =deep brachial artery= (A. profunda brachii) is a large but short trunk which arises usually about the middle of the humerus. It passes backward and divides into several branches which supply the triceps, tensor fasciæ antibrachii, anconeus, and brachialis. A branch runs in the musculo-spiral groove with the radial nerve to the front of the elbow joint and anastomoses with the anterior radial. A slender branch passes down along the external border of the extensor carpi and supplies cutaneous twigs. Anastomoses occur with the ulnar and recurrent interosseous arteries.

The point of origin is inconstant and it is not uncommon to find two
arteries instead. Often a large branch for the posterior deep pectoral
muscle is detached close to the origin or arises from the brachial
directly.

5. =Muscular branches= (Rami musculares) are distributed to the teres major, deep pectoral, coraco-brachialis, and biceps. The largest and least variable of these supplies the lower part of the biceps.

6. The =ulnar artery= (A. collateralis ulnaris superior) arises a little below the nutrient foramen of the humerus and passes downward and backward along the lower edge of the internal head of the triceps under cover of the brachial vein and the tensor fascia antibrachii. It gives branches to these muscles, the posterior superficial pectoral, the cubital lymph glands, panniculus, and skin. At the internal epicondyle of the humerus it is joined by the ulnar nerve and turns downward under the ulnar head of the flexor carpi medius. It continues with the vein and nerve under the deep fascia of the forearm between the ulnar and humeral heads of the perforans, and in the distal half of the region between the external and middle flexors of the carpus. It unites just above the carpus (under cover of the flexor carpi externus) with a branch of the radial artery, with which it forms the supracarpal arch. It detaches small collaterals to the muscles along which it passes and terminal twigs to the outer surface of the carpus.

7. The =nutrient artery of the humerus= (A. nutritia humeri) is a short vessel which enters the nutrient foramen of the humerus. It often arises from the ulnar.

8. The =anterior radial artery= (A. collateralis radialis inferior) passes downward and a little outward on the anterior face of the humerus under cover of the biceps and brachialis to the front of the elbow joint, where it is joined by the radial nerve. It then descends on the anterior surface of the radius under cover of the anterior extensor of the digit to the carpus, where it concurs in the formation of the rete carpi dorsale, anastomosing with the posterior radial and interosseous arteries. It supplies branches to the elbow joint, the biceps, brachialis, and the extensors of the carpus and digit. A cutaneous branch emerges between the distal end of the biceps and the brachialis.

THE POSTERIOR RADIAL ARTERY

The =posterior radial= or =median artery= (A. mediana) is the direct continuation of the brachial. It passes downward and slightly backward, at first on the inner surface of the humerus, and then over the capsule and internal lateral ligament of the elbow joint, under cover of the posterior superficial pectoral muscle.[163] Below the elbow it dips under the flexor carpi internus and passes down the inner part of the posterior surface of the radius. In the distal part of the forearm it inclines backward and is separated from the radius by the reinforcing band (Caput tendineum) of the superficial flexor of the digit and is continued by the large metacarpal artery.

It is accompanied by the median nerve, which lies in front of the artery at its origin, then crosses over it obliquely at the elbow joint and becomes posterior. In the arm the brachial vein lies behind and partly upon the artery; lower down there are usually two satellite veins, anterior and posterior. The chief collateral branches are as follows:

FIG. 443.—DISSECTION OF LEFT FOREARM OF HORSE, EXTERNAL SURFACE.
(After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

1. =Articular branches= for the elbow joint.

2. =Muscular branches= to the flexors of the carpus and digit. The largest of these arise at the proximal third of the forearm.

FIG. 444.—DEEP DISSECTION OF RIGHT CARPUS OF HORSE, POSTERIOR VIEW.

_1_, Radial carpal bone; _2_, accessory carpal bone; _3_, second
carpal bone. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

FIG. 445.—CROSS-SECTION OF RIGHT CARPUS OF HORSE.

The soft structures are cut at the level of the radio-carpal
articulation, exposing the proximal row of carpal bones. _Cr_,
Radial carpal bone; _Ci_, intermediate carpal bone; _Cu_, ulnar
carpal bone; _Ca_, accessory carpal bone. (After Schmaltz, Atlas d.
Anat. d. Pferdes.)
]

3. The =common interosseous artery= (A. interossea communis) is a vessel of considerable size which arises at the level of the interosseous space, through which it passes. Before entering the space it gives off a small branch, the volar interosseous (A. interossea volaris), which descends to the radial head of the perforans. In the space it supplies the nutrient arteries of the radius and ulna. Emerging from the space it gives off branches to the flexor carpi externus, a small =recurrent branch= (A. interossea recurrens) which passes upward on the outer surface of the ulna and anastomoses with the deep brachial and ulnar arteries, and is continued as the =dorsal interosseous artery=. This vessel (A. interossea dorsalis) passes down between the anterior and lateral extensors of the digit and concurs with the anterior radial in forming a network on the anterior surface of the carpus, the =rete carpi dorsale=. From the latter arise two small vessels, the =internal= and =external dorsal metacarpal arteries= (A. metacarpea dorsalis medialis, lateralis), which run distally in the grooves between the large and small metacarpal bones and anastomose with the volar metacarpal arteries.

4. The =artery of the rete carpi volare= (A. retis carpi volaris) is a small vessel which arises at the distal third of the forearm and passes downward to the posterior surface of the carpus, where it concurs with branches of the volar metacarpal arteries in forming the rete carpi volare.

5. The =external volar metacarpal artery= (A. metacarpea volaris lateralis) is a small vessel which arises just above the carpus under cover of the flexor carpi medius and anastomoses with the ulnar artery, forming the supracarpal arch. From the latter a branch descends with the outer branch of the median nerve, inclines outward toward the posterior border of the accessory carpal bone, and arrives at the head of the external metacarpal bone. Here it is connected with the internal volar metacarpal artery, usually by two transverse branches, thus forming the =deep volar= or =subcarpal arch= (Arcus volaris profundus). One of these branches lies between the subcarpal check ligament and the suspensory ligament; the other (not always present) lies beneath the latter on the large metacarpal bone. A small branch descends to the fetlock with the external metacarpal nerve. Below the arch the artery pursues a flexuous course downward on the posterior face of the large metacarpal bone alongside of the external small metacarpal and under cover of the suspensory ligament. At the distal third of the metacarpus it commonly unites with the corresponding vessel of the inner side to form a short trunk which passes backward through the angle of divergence of the branches of the suspensory ligament and joins the external digital or the common digital artery.

FIG. 446.—CROSS-SECTION OF MIDDLE OF RIGHT METACARPUS OF HORSE.

_a_, Common digital (or large metacarpal) artery; _b_, internal
metacarpal vein; _c_, internal metacarpal nerve; _d_, external
metacarpal nerve; _e_, branch of external volar metacarpal artery;
_f_, external metacarpal vein; _g_, _g_, _g_, deep volar or
interosseous metacarpal arteries; _h_, tendon of anterior extensor;
_i_, tendon of lateral extensor; _k_, anastomotic branch connecting
metacarpal nerves; _l_, deep flexor tendon; _m_, check ligament;
_n_, superficial flexor tendon; _o_, suspensory ligament; _p_, large
metacarpal bone; _q_, _q_, small metacarpal bones; _r_, skin (After
Ellenberger, in Leisering’s Atlas.)
]

6. The =internal volar= or =small metacarpal artery= (A. metacarpea volaris medialis) is given off from the posterior radial at an acute angle, usually a little above the external one or by a common trunk with it. It passes down the inner side of the carpus behind the tendon of the flexor carpi internus and embedded in the posterior annular ligament. Arriving at the proximal end of the inner metacarpal bone it becomes more deeply placed and is connected with the external volar artery by one or two transverse branches as stated above. It then pursues a flexuous course downward alongside of the inner small metacarpal bone, like the corresponding external artery, with which it commonly unites as described above. It is larger than the external artery and supplies the nutrient artery to the large metacarpal bone.

The foregoing account describes the most common arrangement of the
dorsal and volar metacarpal arteries. Variations in their origin and
connections are common, but have no great surgical importance.
Collateral branches are omitted for the same reason. In some cases the
internal volar metacarpal is connected with the common digital or
large metacarpal artery a little below the carpus by a branch passing
obliquely across the inner border of the deep flexor tendon; this
forms a superficial volar arch.

THE COMMON DIGITAL ARTERY

FIG. 447.—DISSECTION OF RIGHT CARPUS, METACARPUS, AND DIGIT OF HORSE,
INNER VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]

The =common digital= or =large metacarpal artery=[164] is the direct continuation of the posterior radial. It descends in the carpal canal along the inner side of the flexor tendons in company with the inner branch of the median nerve and a satellite vein. Continuing down the limb it preserves this relation to the tendons to the distal third of the metacarpus, where it inclines toward the middle line of the limb behind the suspensory ligament, and divides into the internal and external digital arteries. In the metacarpus the artery is related to the vein in front and the artery behind, and is covered by the fascia and skin. It furnishes collateral branches to the suspensory ligament, the flexor tendons, and the skin.

THE DIGITAL ARTERIES

The =internal= and =external digital arteries= (A. digitalis medialis, lateralis) are formed by the bifurcation of the common digital at the distal third of the metacarpus. They diverge, pass down over the outer surface of the corresponding sesamoid at the fetlock, and descend parallel with the borders of the deep flexor tendon to the volar grooves and foramina of the third phalanx. Entering the latter the two arteries unite in the semilunar canal and form the =terminal arch= (Arcus terminalis), from which numerous branches pass through the bone to the wall surface and ramify in the matrix of the wall and sole of the hoof. A number of branches emerge through the foramina at the lower border, where they anastomose with each other in arciform fashion. These branches were named by Spooner the inferior communicating arteries, and the anastomotic arch formed by them is termed the circumflex artery of the third phalanx (Chauveau) or the artery of the lower border of the third phalanx (Leisering).

Each artery is accompanied by a vein and by the digital nerves. Above the fetlock the artery is most deeply placed and is covered by the vein; the nerve is behind the vein. At the fetlock the artery has become superficial and is related to the vein in front and the posterior branch of the nerve behind. The anterior branch of the nerve crosses over the artery obliquely to the lateral aspect of the first phalanx. The vessels and nerves are crossed obliquely by a small tendinous band, the ligament of the ergot, which begins in the fibrous basis of the ergot at the back of the fetlock and extends downward and forward to end in the fascia at the side of the pastern joint.

In addition to branches to the joints, tendons and synovial sheath, ergot, and skin, the digital arteries give off:

1. The =artery of the first phalanx= or perpendicular artery (A. phalangis primæ), a short trunk which arises at a right angle about the middle of the first phalanx, and divides into dorsal and volar branches (Rami dorsales et volares). The dorsal (anterior) branch passes between the first phalanx and the extensor tendon and ramifies on the front of the digit, anastomosing with its fellow. The volar (posterior) branch dips in between the flexor tendons and the inferior sesamoidean ligaments and anastomoses with the opposite artery.

2. The =artery of the plantar cushion= (A. toricæ phalangis tertiæ) arises at the proximal border of the lateral cartilage and passes backward and downward to ramify in the plantar cushion and the matrix of the heels and frog.

3. The =dorsal artery of the second phalanx= (Ramus dorsalis phalangis secundæ) arises a little above the navicular bone, and passes forward under cover of the lateral cartilage and the extensor tendon to the front of the second phalanx, where it anastomoses with the opposite vessel. It gives branches to the skin, the tendon, the coffin joint, and the coronary matrix of the hoof.

4. The =volar artery of the second phalanx= (Ramus volaris phalangis secundæ) is smaller than the preceding, opposite to which it arises. It passes above the proximal border of the third sesamoid and unites with the opposite artery.

The arteries of the second phalanx form what is termed by Chauveau the
=coronary circle=. The dorsal (anterior) part of the circle gives off
commonly an artery (A. coronalis phalangis tertiæ) near either side of
the extensor tendon, which divides into two branches. The central
branch unites with that of the opposite side, while the other joins a
branch of the artery of the plantar cushion. In this way is formed the
=circumflex artery of the coronary cushion=, an anastomotic arch which
lies on the extensor tendon at the coronet. In some cases descending
branches of the arteries of the first phalanx concur in the formation
of the arch.

5. The =dorsal artery of the third phalanx= or preplantar artery (A. dorsalis phalangis tertiæ) arises at the deep face of the wing of the third phalanx, passes outward through the notch or foramen there, and runs forward in the groove on the wall surface. It gives off ascending and descending branches, which ramify in the matrix of the wall of the hoof, anastomosing above with the circumflex artery of the coronary cushion and below with the circumflex artery of the third phalanx. Before passing through the wing it detaches a retrograde branch to the plantar cushion, and after emerging one which ramifies on the outer aspect of the lateral cartilage.

The terminal part of the digital artery after giving off the preceding
vessel is sometimes termed the plantar.

BRANCHES OF THE THORACIC AORTA

In addition to the coronary arteries and the common brachiocephalic trunk (which have been described), the thoracic part of the aorta gives off branches to the thoracic walls and viscera and to the spinal cord and its membranes. The =visceral branches= (Rami viscerales) are the bronchial and œsophageal, which arise by a broncho-œsophageal trunk. The =parietal branches= (Rami parietales) are the intercostal and phrenic arteries.

1. The =broncho-œsophageal= (Truncus broncho-œsophageus) is a short, usually bulbous, trunk which arises at the sixth thoracic vertebra from the aorta or in common with the first aortic intercostal arteries. It passes (under cover of the vena azygos) down the right face of the aorta toward the bifurcation of the trachea and divides into bronchial and œsophageal branches.

(_a_) The =bronchial artery= (A. bronchialis) crosses the left face of the œsophagus to the bifurcation of the trachea, where it divides into right and left branches. Each enters the hilus of the corresponding lung above the bronchus, which it accompanies in its ramification. It supplies the lung tissue and also detaches twigs to the bronchial lymph glands and the mediastinum.

(_b_) The =œsophageal artery= (A. œsophagea) (Fig. 429) is a small vessel which passes backward above the œsophagus in the posterior mediastinum and anastomoses with the œsophageal branch of the gastric artery. It detaches twigs to the œsophagus and the mediastinal lymph glands and pleura, and also gives off two branches which pass between the layers of the ligaments of the lungs and ramify in the subpleural tissue. Very commonly there is another artery which runs backward ventral to the œsophagus.

In some cases there is no broncho-œsophageal trunk, the bronchial and
œsophageal arising separately. In other cases the second aortic
intercostal arises in common with them also.

2. The =intercostal arteries= (Aa. intercostales) (Figs. 428, 429) number eighteen pairs. The first arises from the deep cervical artery, the next three from the subcostal branch of the dorsal artery, and the remainder from the aorta. The aortic intercostals arise from the dorsal face of the aorta in pairs close together; the fifth and sixth usually spring from a common stem. Each passes across the body of a vertebra to the corresponding intercostal space, detaches twigs to the vertebræ and the pleura, and divides into dorsal and ventral branches. The =dorsal branch= (Ramus dorsalis) gives off a spinal branch (Ramus spinalis) which passes through the intervertebral foramen, gives twigs to the membranes of the spinal cord, perforates the dura, and reinforces the ventral spinal artery. A muscular branch passes to the muscles and skin of the back. The =ventral branch= (Ramus ventralis) is much the larger. It passes downward, at first almost in the middle of the intercostal space between the intercostal muscles, then gains the posterior border of the rib and is subpleural. Each is accompanied by a vein and nerve, the artery being in the middle and the vein in front. At the lower part of the space it unites with a ventral intercostal branch of the internal thoracic or the asternal artery. It supplies the intercostal muscles, the ribs and the pleura, and gives off perforating branches which pass out to the serratus magnus, the abdominal muscles, and the skin.

3. The =phrenic arteries= (Aa. phrenicæ) are two or three small vessels which arise at the hiatus aorticus from the ventral aspect of the aorta, often by a common trunk. They supply the crura of the diaphragm. In some cases they arise in common with an intercostal artery.

BRANCHES OF THE ABDOMINAL AORTA

The collateral branches of the abdominal part of the aorta are distributed chiefly to the walls and contents of the abdominal cavity, but some branches are supplied to the spinal cord and its membranes, and others extend into the pelvis and to the scrotum. The =visceral branches= are the cœliac, anterior mesenteric, renal, posterior mesenteric, and the spermatic or utero-ovarian. The =parietal branches= are the lumbar arteries.

FIG. 448.—PLAN OF BRANCHES OF CŒLIAC ARTERY OF HORSE.

_1_, Cœliac artery; _2_, gastric artery; _3_, hepatic artery; _4_,
splenic artery; _5_, posterior gastric artery; _6_, anterior gastric
artery; _7_, œsophageal branch; _8_, gastro-duodenal artery; _9_,
pancreatico-duodenal artery; _10_, right gastro-epiploic artery;
_11_, pyloric artery; _12_, left gastro-epiploic artery; _13_, short
gastric branches of splenic.
]