Chapter V: Muscles of the Leg and Foot (14)

7. The =external maxillary= or =facial vein= (V. maxillaris externa) arises by radicles which correspond in general to the branches of the artery of like name. It passes down over the cheek along the anterior border of the masseter muscle behind the artery, crossing over the parotid duct, which lies behind the vein lower down. Thus on the ramus and as they turn around its lower border the artery is in front, the vein in the middle, and the duct posterior. In the submaxillary space the vein is ventral to the artery for some distance, then parts company with the artery, runs straight backward along the lower border of the parotid gland, and opens into the jugular vein at the posterior angle of the gland.[171] The chief differences in the tributaries of the vein as compared with the branches of the corresponding artery are as follows:

The =labial veins= (Vv. labiales) form a plexus in the submucous tissue of the cheek from which two veins emerge. The upper one passes back and joins the buccinator vein. The lower one (V. labialis communis) joins the external maxillary vein.

Three veins connect with the external maxillary at the anterior border of the masseter.

The upper one is the =transverse facial=, which unites close to the end of the facial crest.

A little lower is the large valveless =vena reflexa= or =alveolar vein=. This passes back under the upper part of the masseter on the maxilla, turns around the tuber maxillare, perforates the periorbita, and joins the ophthalmic vein. It is relatively small at each end, but presents one or two large fusiform dilatations. It receives the following tributaries: (_a_) The =palatine vein= (V. palatina major), which separates from the palatine artery at the anterior palatine foramen and passes in the groove between the tuber maxillare and the palate bone. The palatine veins form a very rich plexus of valveless vessels in the submucosa of the hard palate, which consists of several layers anteriorly. (_b_) The =sphenopalatine vein= (V. sphenopalatina) forms a rich plexus of valveless vessels on the turbinal bones and the septum nasi. It is usually joined by the =infraorbital vein= (V. infraorbitalis) to form a short common trunk.

FIG. 462.—DISSECTION OF SUBMAXILLARY SPACE AND ADJACENT PART OF NECK
OF HORSE.

_a_, Ramus of mandible; _b_, sterno-cephalicus muscle; _c_, _c′_,
omo-hyoidei and sterno-hyoidei (portion removed on right side); _d_,
hyoid bone; _e_, anterior belly of digastricus; _f_, _g_,
mylo-hyoideus; _h_, submaxillary lymph glands (portion removed on
right side); _i_, parotid gland; _k_, submaxillary gland; _l_, chin;
_m_, stylo-maxillaris; _1_, parotid duct; _2_, facial vein; _3_,
facial artery; _4_, external maxillary vein; _5_, sublingual vein;
_6_, sublingual artery; _7_, ventral branch of first cervical nerve;
_8_, mylo-hyoid nerve. (After Ellenberger, in Leisering’s Atlas.)
]

The venous plexuses are remarkably developed in certain parts of the
nasal mucosa. On the septum a little below its middle and on the
turbinals the veins are in several layers. The olfactory region does
not share in this arrangement and the veins here are small and join
the ethmoidal vein.

(_c_) The =ophthalmic vein= (V. ophthalmica) is a short trunk, connected in front with the vena reflexa and behind with the cavernous sinus through the foramen lacerum orbitale. It receives veins which correspond to the arterial branches.

The =buccinator vein= (V. buccinatoria) extends backward from the external maxillary along the lower border of the depressor labii inferioris and buccinator under cover of the masseter, passes between the ramus of the mandible and the tuber maxillare and is continued as the internal maxillary vein. It has a large fusiform dilatation and is valveless. It receives a large common labial vein from the labial plexus, and is usually connected with the masseteric vein.

FIG. 463.—DEEPER VESSELS AND NERVES OF HEAD OF HORSE.

The parotid gland, most of the masseter muscle, and a portion of the
ramus of the mandible are removed. _a_, Remnant of masseter muscle;
_b_, internal pterygoid muscle; _c_, stylo-mandibularis; _d_, _d′_,
levator labii superioris proprius (portion removed); _e_,
periorbita; _1_, masseteric artery; _1′_, parotid branch; _2_, trunk
for anterior (_3_) and posterior (_4_) auricular arteries; _5_,
_5′_, superficial temporal artery; _5″_, transverse facial artery;
_6_, inferior alveolar (or dental) artery; _6′_, mental continuation
of _6_; _7_, buccinator artery; _8_, infraorbital artery; _9_, _11_,
jugular vein; _10_, external maxillary vein; _12_, inferior cerebral
vein; _13_, facial vein; _14_, angular vein of eye; _15_, dorsal
nasal vein; _16_, lateral nasal vein; _17_, superior labial vein;
_18_, _19_, inferior labial veins; _20_, labial plexus; _21_, _22_,
emergent veins of plexus; _23_, vena reflexa; _24_, trunk of
sphenopalatine and infraorbital veins; _25_, palatine vein; _26_,
great auricular vein; _27_, _28_, superficial temporal vein; _29_,
transverse facial vein; _30_, buccinator vein; _31_, dorsal lingual
vein; _32_, inferior alveolar or dental vein; _33_, pterygoid vein;
_34_, deep temporal vein; _35_, superior cerebral vein; _36_,
external nasal nerve; _37_, anterior nasal nerve; _38_, superior
labial nerve; _39_, masseteric nerve; _39′_, end branches of _39_;
_40_, buccinator nerve; _41_, pterygoid nerve; _41a_, inferior
alveolar or dental nerve; _41b_, mylo-hyoid nerve; _42_, posterior
auricular nerve; _43_, auricular branch of vagus; _44_, internal
auricular nerve; _45_, auriculo-palpebral nerve (cut); _46_,
digastric nerve. (After Leisering’s Atlas.)
]

The =lingual vein= (V. lingualis) is not a satellite of the artery. It is formed at the side of the lingual process of the hyoid bone by the confluence of several veins which come from the substance of the tongue. One or two considerable vessels run partly in the substance of the hyo-glossus, and another in the genio-hyoideus. The vein is at first covered by the mylo-hyoideus, perforates that muscle, runs back along the omo-hyoideus in relation to the submaxillary lymph glands, and joins the external maxillary near the posterior border of the jaw. Near its termination it receives the =sublingual vein=, or the latter may open into the external maxillary directly.

8. The =thyroid vein= (V. thyreoidea) is a large vessel which joins the jugular near the external maxillary vein. It receives anterior thyroid, laryngeal, and pharyngeal radicles, and sometimes a posterior thyroid vein.

9. =Muscular=, =tracheal=, and =œsophageal= veins.

10. The =cephalic vein= (Vena cephalica) enters the jugular near its termination. It will be described with the veins of the thoracic limb.

11. The =inferior cervical vein= (V. cervicalis ascendens) accompanies the artery. It may open into the brachial vein.

THE SINUSES OF THE DURA MATER

These (Sinus duræ matris) are blood-spaces between the meningeal and periosteal layers of the dura mater and are lined with endothelium. In many places the lumen is crossed by fibrous strands. They receive the veins of the brain, communicate with the meningeal and diploic veins, and with veins outside of the cranium; their connections with the latter are by means of small =emissary veins= (Emissaria). They convey the blood directly or indirectly to the jugular veins. Some are paired, others unpaired. They may be divided into dorsal and basilar systems. The =dorsal system= comprises the following:

The =superior longitudinal= or =sagittal sinus= (S. sagittalis superior) is situated in the upper border of the falx cerebri along the internal sagittal crest. It begins at the crista galli and ends at the tentorium osseum by dividing into two transverse sinuses. It receives the superior cerebral veins. Along each side are small pouches (Lacunæ laterales) into which the veins open. The lumen of the sinus is traversed by fibrous bands and is partially divided by a longitudinal septum.

FIG. 464.—MEDIAN SECTION OF HEAD OF HORSE, UPPER PART WITH SEPTUM NASI
REMOVED.

_a_, Lateral mass of ethmoid bone; _b_, superior turbinal; _c_,
inferior turbinal; _d_, _d′_, turbinal folds; _e_, frontal sinus;
_f_, falx cerebri; _g_, tentorium cerebelli; _h_, medial surface of
hemisphere; _i_, cerebellum; _k_, occipital bone; _k′_, occipital
condyle; _k″_, paramastoid process; _l_, external auditory meatus;
_m_, temporal condyle; _n_, parieto-temporal canal; _1_, branches of
ethmoidal artery; _2_, _2′_, branches of sphenopalatine artery; _3_,
_3′_, branches of sphenopalatine vein; _4_, branches of ethmoidal
nerve; _5_, _5′_, branches of sphenopalatine nerve; _6_, artery of
corpus callosum; _7_, superior sagittal sinus; _8_, straight sinus;
_9_, vena magna cerebri; _10_, inferior sagittal sinus; _11_, _11′_,
transverse sinuses; _12_, superior petrosal sinus; _13_, superior
occipital sinus; _14_, superior cerebral vein; _15_, corpus
callosum; _16_, fornix. (After Ellenberger, in Leisering’s Atlas.)
]

The =transverse sinuses= (S. transversi), right and left, pass outward in the transverse grooves of the parietal bones, enter the parieto-temporal canals, and are continued by the superior cerebral veins to the superficial temporal veins. The two sinuses are connected by the sinus communicans.

The =inferior longitudinal= or =sagittal sinus= (S. sagittalis inferior) runs backward on the upper surface of the corpus callosum along the concave edge of the falx cerebri and joins the great cerebral vein (of Galen) to form the straight sinus.

The =straight sinus= (S. rectus) passes upward and backward between the cerebral hemispheres and in the tentorium cerebelli and joins the superior sagittal sinus. The point of meeting is the confluence of the sinuses (Confluens sinuum).

The =occipital sinuses= (S. occipitales) lie on either side of the vermis cerebelli. They empty anteriorly into the sinus communicans and communicate behind with the spinal veins.

The =superior petrosal sinuses= (S. petrosi superiores) pass in the tentorium cerebelli to end in the transverse sinuses.

The =basilar system= consists of the following:

The =cavernous sinuses= (S. cavernosi) lie in the inner grooves of the root of the temporal wings of the sphenoid bone at either side of the sella turcica. The two are connected by a wide traverse branch (Sinus intercavernosus) behind and below the posterior part of the pituitary body. Each is continuous in front with the ophthalmic vein and below with the inferior petrosal sinus. The third, sixth, and the ophthalmic and maxillary divisions of the fifth nerve lie along the outer wall of the sinus. The internal carotid artery traverses the sinus and is connected with its fellow by a transverse branch which lies in the corresponding part of the sinus. An oval opening in the floor communicates with the inferior petrosal sinus and transmits the internal carotid artery.

The sinus is not subdivided by strands of fibrous tissue as in man,
but a few delicate bands attach the artery to its wall.

The =inferior petrosal sinuses= (S. petrosi inferiores)[172] lie along the borders of the basilar part of the occipital bone, inclosed in the thick dura which closes the foramen lacerum. The anterior part extends about half an inch (ca. 12 mm.) under the temporal wing of the sphenoid. Here communications exist with veins in the pterygo-palatine fossa. The posterior end is bulbous and lies in the condyloid fossa; it communicates with the condyloid vein issuing from the hypoglossal foramen and is drained by the inferior cerebral vein; it also receives an emissary vein from the parieto-temporal canal. The roof of the sinus is perforated by an oval opening at the carotid notch which opens into the cavernous sinus and transmits the internal carotid artery; the latter forms the first bend of its =S=-shaped curve in the petrosal and the second in the cavernous sinus.

The =basilar plexus= (Plexus basilaris) is a venous plexus situated on the upper surface of the basilar part of the occipital bone. It is connected through the hypoglossal foramen with the inferior petrosal sinus and communicates behind with the spinal veins.

THE VEINS OF THE CRANIUM

The =veins of the brain= (Venæ cerebri) do not in general accompany the cerebral arteries. They have very thin walls, no muscular coat, and no valves. They are arranged in two sets, superficial or cortical and deep or central. The superficial veins are more numerous and larger than the arteries. They lie on the surface of the brain in the pia mater and the subarachnoid space.

The =superior cerebral veins= drain the upper and outer part of the cerebral cortex. They receive veins from the medial surfaces of the hemispheres and open into the superior sagittal sinus. Their terminal parts are bulbous and their openings are directed obliquely forward, _i. e._, contrary to the direction of the bloodstream in the sinus.

The =inferior cerebral veins= lie on the inferior and external aspect of the hemispheres. They open into the inferior system of sinuses. The =middle cerebral vein= runs in the lateral fissure (of Sylvius) and opens into the cavernous sinus.

The =deep cerebral veins= issue from the central or ganglionic parts of the brain at the transverse fissure. They converge to form the =great cerebral vein= (of Galen) (V. magna cerebri). This passes upward and backward behind the splenium of the corpus callosum and is continued as the straight sinus to join the sagittal sinus.

The =superior cerebellar veins= ramify on the upper surface of the cerebellum. They open into the superior system of sinuses and the great cerebral vein. The =inferior cerebellar veins= are larger and go chiefly to the basilar plexus. The veins of the medulla and pons end in the inferior system of sinuses.

The =meningeal veins= (Venæ meningeæ) arise in capillary plexuses in the superficial and deep faces of the dura mater. Some end in the sinuses of the dura, others accompany the meningeal arteries.

The =diploic veins= (Venæ diploicæ) are anastomosing channels in the spongy substance of the cranial bones. Their walls are thin, consisting in many places only of the endothelium, and they have no valves. Some open inward into venous sinuses, others into extracranial veins.

SPINAL VEINS

Two =longitudinal spinal veins= or =sinuses= (Sinus columnæ vertebralis) extend along the floor of the vertebral canal, one on either side of the superior common ligament. They are continuous in front with the basilar plexus. They lie in the grooves on the bodies of the vertebræ and are connected by a series of transverse anastomoses which pass between the central part of the bodies of the vertebræ and the superior common ligament or in channels in the bone. They receive veins from the spinal cord, the dura mater, and the bodies of the vertebræ (Venæ basis vertebræ). Through the intervertebral foramina efferent vessels connect with the vertebral, intercostal, lumbar, and lateral sacral veins.

VEINS OF THE THORACIC LIMB

The =brachial vein= (V. brachialis) is a satellite of the extrathoracic part of the brachial artery. It arises at the inner side of the distal end of the shaft of the humerus and passes upward in the arm behind the artery under cover of a layer of fascia and the posterior superficial pectoral muscle. At the shoulder it is ventral to the artery, crosses the anterior border of the first rib, and concurs with its fellow and the two jugulars in the formation of the anterior vena cava. The roots of the vein are somewhat variable, but most often four radial veins in addition to a large oblique branch from the cephalic unite in its formation. Its tributaries correspond in general to the branches of the artery, but a few differences are worthy of notice. The =thoracico-dorsal vein= joins the brachial directly or opens into the deep brachial. The =external thoracic= or =“spur” vein= (V. thoracica externa) is a large vessel which arises in the ventral wall of the abdomen, passes forward (embedded more or less in the panniculus) along the outer border of the posterior deep pectoral muscle, and joins the brachial vein near the first rib. It often communicates with the subscapular vein.

FIG. 465.—SPINAL VESSELS OF HORSE.

The vertebral canal has been opened by sawing off the arches. The
nerve-roots are cut on one side and the spinal cord turned over to
right. _1_, Ventral or middle spinal artery; _2_, reinforcing
branches from vertebral, intercostal, or lumbar arteries (according
to region); _3_, longitudinal vertebral sinuses; _a_, ventral
surface of spinal cord; _b_, dura mater (cut); _c_, nerve-roots;
_d_, superior common ligament. (After Ellenberger, in Leisering’s
Atlas.)
]

The =cephalic vein= (V. cephalica) arises at the inner side of the carpus as the continuation of the internal superficial metacarpal vein. It runs upward on the deep fascia of the forearm at first in the furrow between the flexor carpi internus and the radius. Toward the middle of the forearm it inclines gradually forward on the inner surface of the radius, accompanied by a cutaneous branch of the median nerve, and arrives at the insertion of the biceps. Here it detaches a large branch (Ramus communicans) which passes upward and backward over the inner insertion of the biceps, the posterior radial artery, and the median nerve, and joins the brachial vein. The vein to this point is often termed the internal subcutaneous vein of the forearm (V. cephalica antebrachii). It is continued (as the V. cephalica humeri) in the furrow between the mastoido-humeralis and the anterior superficial pectoral with a branch of the inferior cervical artery, crosses the deep face of the cervical panniculus, and opens into the terminal part of the jugular or the brachial vein. It receives an =accessory cephalic vein= (V. cephalica accessoria), which arises from the carpal network, runs upward along the inner border of the extensor carpi, turns along the lower border of the brachialis, passes under the superficial tendon of the biceps and joins the cephalic.

FIG. 466.—DISSECTION OF PECTORAL REGION AND ANTERIOR PART OF ABDOMINAL
WALL OF HORSE.

_1_, Jugular vein; _2_, loose connective tissue of axillary space;
_3_, ascending branch of inferior cervical artery; _4_, descending
branch of same; _5_, cephalic vein; _6_, median nerve; _7_, ulnar
nerve; _8_, brachial artery; _9_, brachial vein; _10_, external
thoracic or “spur” vein; _11_, anterior abdominal artery and vein;
_12_, branches of sixth cervical nerve; _13_, cutaneous branch of
axillary nerve; _14_, cutaneous branch of musculo-cutaneous nerve;
_15_, cervical panniculus; _16_, sterno-cephalicus; _17_, scalenus;
_18_, mastoido-humeralis; _19_, anterior superficial pectoral; _20_,
posterior superficial pectoral; _21_, anterior deep pectoral; _22_,
posterior deep pectoral; _23_, abdominal panniculus; _24_, obliquus
abdominis externus; _25_, rectus abdominis; _26_, coraco-brachialis;
_27_, tensor fasciæ antibrachii; _a_, prescapular lymph glands; _b_,
prepectoral lymph glands; _c_, axillary lymph glands; _d_, cubital
lymph glands; _c.x._, xiphoid cartilage. (After Schmaltz, Atlas d.
Anat. d. Pferdes.)
]

The deep veins of the forearm are variable. Commonly two =posterior radial veins= (Venæ medianoradiales) accompany the artery of like name, one in front and one behind. A third vein arises by radicles emerging from the proximal part of the deep flexor; it joins the posterior satellite of the posterior radial artery or forms one of the roots of the brachial vein. The =common interosseous vein= joins the posterior satellite. The =anterior radial vein= (V. collateralis radialis) is a satellite of the artery. The =ulnar vein= is usually double at its proximal part, and communicates with the deep brachial vein.

There are three chief =metacarpal veins=. The =internal metacarpal vein= (V. metacarpea volaris superficialis medialis) arises from the volar venous arch above the fetlock. It is the largest vein of the region and lies in front of the large metacarpal artery. It separates from the artery at the proximal end of the metacarpus, passes upward on the inner part of the posterior surface of the carpus, under cover of the superficial layer of the posterior annular ligament, and is continued as the cephalic vein. It communicates at its proximal part with the origin of the posterior radial veins by a short but relatively large branch. The =external metacarpal vein= (V. metacarpea volaris superficialis lateralis) arises from the venous arch above the fetlock and passes upward behind the external border of the suspensory ligament in front of the external branch of the median nerve and accompanied by a small artery. At the proximal end of the metacarpus it is connected with the deep metacarpal vein by two transverse anastomoses which pass across the suspensory ligament. It then passes upward as a satellite of the external volar metacarpal artery and concurs in the origin of the ulnar and posterior radial veins. The =deep metacarpal vein= (V. metacarpea volaris profunda medialis) arises from the venous arch, passes forward between the two branches of the suspensory ligament, and ascends on the posterior surface of the large metacarpal bone. At the proximal end of the latter it communicates with the other metacarpal veins, ascends with the small metacarpal artery, and concurs in forming the radial veins.

The =venous arch= (Arcus venosus volaris), from which the metacarpal veins arise, is situated above the sesamoids of the fetlock between the suspensory ligament and the deep flexor tendon. It is formed by the junction of the two digital veins.

The =digital veins=, internal and external (V. digitalis medialis, lateralis), drain the venous plexuses of the foot. They arise at the upper edge of the lateral cartilages and ascend in front of the corresponding arteries.

It is convenient to recognize the following venous plexuses of the foot, which, however, communicate very freely:

1. The =coronary plexus= encircles the upper part of the foot. It is attached to the terminal part of the extensor tendon, the lateral cartilages, and the bulbs of the plantar cushion.

2. The =dorsal= (or laminal) =plexus= covers the dorsal or wall surface of the third phalanx in the deep layer of the matrix of the wall of the hoof. It forms the circumflex vein of the third phalanx or vein of the distal border of the third phalanx, which corresponds to the artery of like name.

3. The =volar plexus= is in the deep layer of the matrix of the sole of the hoof and on the deep surface of the lateral cartilages. It communicates around the inferior border of the third phalanx with the dorsal plexus and through the lateral cartilages with the coronary plexus.

The deep vein of the third phalanx accompanies the terminal part of the digital artery. It drains the =intraosseous plexus=.

THE POSTERIOR VENA CAVA (Figs. 270, 450)

The =posterior vena cava= (V. cava caudalis) returns almost all of the blood from the abdomen, pelvis, and pelvic limbs. It is formed by the confluence of the right and left common iliac veins at the fifth lumbar vertebra, above the terminal part of the aorta and chiefly to the right of the median plane.

The mode of origin is variable. In some cases there is a common trunk
formed by the union of the two internal iliac veins so that the
arrangement resembles the termination of the aorta. In other cases the
internal iliac vein does not exist.

It passes forward on the ventral face of the psoas minor to the right of the abdominal aorta. At the last thoracic vertebra it separates from the aorta and runs forward between the right crus of the diaphragm and the pancreas till it reaches the liver. Here it inclines downward along the inner border of the right lobe and the parietal surface of the liver, largely embedded in the gland substance, and passes through the foramen venæ cavæ of the diaphragm. It then runs forward and somewhat downward between the mediastinal lobe and the main mass of the right lung at the upper margin of a special fold of the right pleura, accompanied by the right phrenic nerve, and opens into the posterior part of the right atrium. It receives the following tributaries:

1. The =lumbar veins= (Vv. lumbales) correspond to the arteries. Five pairs usually empty into the vena cava. Sometimes the corresponding veins of opposite sides unite to form a common trunk. The first communicate with the vena azygos.

2. The =spermatic veins= (Vv. spermaticæ internæ) (Fig. 450) accompany the arteries of like name. In the spermatic cord they form the pampiniform plexus about the artery and nerves. Their termination is variable. The right one commonly joins the vena cava near the renal vein, often by a common trunk with the left one. Frequently the left vein joins the left renal, and sometimes the right one ends similarly.

2_a_. The =utero-ovarian veins= are much larger than the preceding and are satellites of the arteries in the broad ligaments. The ovarian branch is plexiform near the ovary. The uterine branches form a rich plexus in the wall of the uterus. The trunk is very short.

3. The =renal veins= (Vv. renales), satellites of the arteries, are of large caliber and thin-walled. The right vein passes inward and backward on the ventral face of the kidney between the artery in front and the ureter behind. It joins the vena cava above the right adrenal. The left vein is somewhat longer. It passes inward at first like the right one, then bends around the posterior end of the adrenal, crosses the origin of the renal artery and opens into the vena cava a little further back than the right one. They receive veins from the adrenals, but some adrenal veins open directly into the vena cava.

4. The =hepatic veins= (Vv. hepaticæ) return the blood from the liver, and open into the vena cava as it lies in the groove in the liver. Three or four large vessels open into the vena cava just before it leaves the liver, and numerous small ones discharge into its embedded part.

5. The =phrenic veins= (Vv. phrenicæ), two or three in number, return the blood from the diaphragm. They are very large in comparison with the arteries, and join the vena cava as it lies in the caval opening.

In some cases there is a small middle sacral vein (V. sacralis media),
a satellite of the artery. It opens into the angle of junction of the
common iliac veins or into one of the latter.

THE PORTAL VEIN

The =portal vein= (V. portæ) is a large trunk which returns the blood carried to the viscera by the gastric, splenic, and mesenteric arteries. Its peripheral tributaries correspond closely with the branches of the arteries, but the terminal trunks do not. The vein is formed behind the pancreas and below the posterior vena cava by the confluence of the =anterior= and =posterior mesenteric= and =splenic veins=. It passes forward, traverses the posterior part of the pancreas very obliquely, inclines ventrally and a little to the right, and reaches the portal fissure of the liver. Here it divides into three branches which enter the liver and ramify in the substance of the gland like an artery, terminating in the lobular capillaries. From the lobules the blood passes into the hepatic veins and through these to the posterior vena cava. Thus the blood which is distributed to the stomach, nearly the entire intestinal tract, the pancreas, and the spleen, passes through two sets of capillaries prior to its return to the heart, viz., the capillaries of these viscera and of the liver.

1. The =anterior mesenteric vein= (V. mesenterica cranialis) is the largest of the portal radicles. It is situated to the right of the artery of like name, and its tributaries correspond in general to the branches of the artery. Usually a single colic vein corresponds to the two arteries of the right portions of the colon.

2. The =posterior mesenteric vein= (V. mesenterica caudalis) is the smallest of the radicles of the portal. It accompanies the artery in the colic mesentery and its rectal branches anastomose with those of the internal pudic vein.

3. The =splenic vein= (V. lienalis) is the very large satellite of the splenic artery. It is formed by the union of two radicles at the base of the spleen. On leaving the hilus of the spleen it passes inward between the anterior pole of the left kidney and the saccus cæcus of the stomach and above the left end of the pancreas, receives commonly the =posterior gastric vein= (V. gastrica caudalis), and unites with the anterior mesenteric at the posterior border of the pancreas.

The collateral tributaries of the portal veins are as follows:

(1) =Pancreatic veins= (Rami pancreatici).

(2) The =gastro-duodenal vein= (V. gastroduodenalis) corresponds mainly to the extrahepatic branches of the hepatic artery.

(3) The =anterior gastric vein= (V. gastrica cranialis) joins the portal at the portal fissure.

THE COMMON ILIAC VEINS (Fig. 456)

These (V. iliacæ communes) are two very large but short trunks which result from the union of the internal and external iliac veins of each side at the sacro-iliac articulation. The left one is the longer and crosses obliquely over the terminal part of the aorta. The chief tributaries of each are as follows:

1. The last =lumbar vein=.

2. The =circumflex iliac veins= (Vv. circumflexæ ilium profundæ) are the two satellites of each corresponding artery, on either side of which they are placed. They may open directly into the posterior vena cava or into the external iliac vein.

3. The =ilio-lumbar vein= (V. iliolumbalis) may open into the common iliac, the external iliac, or the internal iliac vein.

THE INTERNAL ILIAC VEINS

The =internal iliac= or =hypogastric veins= (Vv. hypogastricæ), right and left, are usually formed by the confluence of lateral sacral, gluteal, and internal pudic veins. The obturator vein may open into them also. They are short trunks and are smaller than the external iliac veins. The tributaries correspond in general to the branches of the three arteries of like names. The internal pudic veins receive affluents from the venous plexuses of the prepuce and penis in the male; of the mammary gland, vulva, vagina, and vestibular bulb in the female.

THE VEINS OF THE PELVIC LIMB

The =external iliac vein= (V. iliaca externa) (Figs. 450, 451) lies behind the corresponding artery at the brim of the pelvis. It is the upward continuation of the femoral vein, and unites at the sacro-iliac joint with the internal iliac to form the common iliac vein. Its tributaries are as follows:

1. The =obturator vein= (V. obturatoria) is a satellite of the artery and usually opens into the external iliac at the insertion of the psoas minor. Its radicles anastomose with those of the internal and external pudic veins (Figs. 451, 455).

2. The =iliaco-femoral= or =external circumflex veins= (Vv. circumflexæ femoris laterales) are the two satellites of the homonymous artery. They open a little higher than the obturator.

The =femoral vein= (Figs. 450, 451) lies behind the artery in the upper part of the thigh, external to it lower down. Its chief tributaries are:

1. A very large but short trunk formed by the union of the deep femoral and the external pudic vein. The =deep femoral vein= (V. profunda femoris) corresponds otherwise to the artery. The =external pudic vein= (V. pudenda externa) arises chiefly from a rich plexus of large veins situated above and along the sides of the penis and prepuce in the male, the mammary glands in the female. It passes through a foramen in the anterior part of the tendon of origin of the gracilis and runs outward in the subpubic groove behind the pectineus to unite with the deep femoral vein. The right and left veins are connected by a large transverse anastomosis and each has a large connection with the obturator vein. Each receives the =posterior abdominal vein= (V. epigastrica caudalis) which accompanies the artery of like name. The =subcutaneous abdominal vein= (V. abdominalis subcutanea) arises in the skin and panniculus of the ventral abdominal wall, anastomoses with the internal and external thoracic and deep abdominal veins, and joins the external pudic or posterior abdominal vein.

A small vein accompanies the external pudic artery in the inguinal canal.

2. The =anterior femoral vein= (V. femoris cranialis) accompanies the artery.

3. The =saphenous vein= (V. saphena) (Figs. 451, 458, 460) arises at the inner side of the flexion surface of the tarsus as the upward continuation of the internal metatarsal vein. Its course is distinctly visible. It ascends on the subcutaneous surface of the tibia and the popliteus muscle, inclosed between layers of the deep fascia, inclines a little backward to the proximal part of the leg, then deviates slightly forward, runs upward on the gracilis, passes between that muscle and the sartorius, and joins the femoral or the external pudic vein.[173] On the upper part of the capsule of the hock joint it forms an arch with the anterior tibial vein. The vein has numerous valves. The satellite artery is relatively small and lies in front of the vein as far as the junction with the recurrent tibial vein, which it accompanies on the leg. It receives the =recurrent tibial vein= (V. recurrens tibialis)[174] at the proximal fourth of the leg. This vessel arises at the inner surface of the tarsus and forms an arch with the posterior tibial vein at the level of the tuber calcis. It ascends in the furrow in front of the gastrocnemius tendon, inclines forward at the proximal third of the leg, and joins the saphenous vein at an acute angle. It has numerous valves. A smaller vein from the anterior face of the metatarsus joins the saphenous at the hock.

4. =Muscular branches= which correspond to the arteries.

5. The =posterior femoral= or =femoro-popliteal= vein (V. femoris caudalis) is a satellite of the artery. It receives the =recurrent tarsal= or =external saphenous vein= (V. tarsea recurrens), which arises at the outer side of the hock, ascends on the deep fascia of the external surface of the leg in front of the tendo Achillis, passes between the biceps femoris and semitendinosus, and joins the posterior femoral vein. It is connected with the recurrent tibial vein by a large anastomotic branch which crosses in front of the tuber calcis. Usually a branch from it ascends along the great sciatic nerve and anastomoses with the obturator vein.

The =popliteal vein= (V. poplitea) lies along the inner side of the artery (Fig. 459). It is formed by the confluence of anterior and posterior tibial veins.

Two =anterior tibial veins= (Vv. tibiales anteriores) usually accompany the artery of like name; the outer vein is much the larger. In other cases there is a single large vein in the proximal part of the leg, two lower down. They arise from a number of anastomosing radicles on the front of the capsule of the hock joint, chiefly as the continuation of the perforating tarsal vein. The origin of the chief vein is connected with the saphenous by a large anastomotic branch.

The =posterior tibial vein= (V. tibialis posterior) is commonly double (Fig. 458). It arises at the level of the tuber calcis, where it has a communication with the recurrent tibial vein. It is a satellite of the artery.

The =internal= or =great metatarsal vein= (V. metatarsea dorsalis medialis) (Figs. 458, 460) arises from the venous arch above the sesamoids at the fetlock, but is practically the upward continuation of the internal digital vein. It ascends along the inner border of the deep flexor tendon, then in the groove on the inner aspect of the proximal part of the large metatarsal bone to the capsule of the hock joint, and is continued by the saphenous vein.

The =external metatarsal vein= (V. metatarsea plantaris lateralis) arises from the venous arch above the fetlock, but appears to be the upward continuation of the external digital vein. It ascends along the outer border of the deep flexor tendon in front of the plantar nerve, and is connected with the deep metatarsal vein at the proximal part of the metatarsus by a transverse branch. It then passes upward along the deep flexor tendon in relation to the plantar nerves and the inner tarsal artery and is continued by the recurrent tibial vein.

The =deep metatarsal vein= (V. metatarsea plantaris medialis) arises from the plantar venous arch, passes forward between the branches of the suspensory ligament, and ascends on the posterior face of the large metatarsal bone. At the proximal end of the metatarsus it is connected with the external metatarsal vein by a transverse branch. It then passes (as the perforating tarsal vein) through the vascular canal of the tarsus and forms the chief radicle of the anterior tibial vein.

The =plantar venous arch= (Arcus venosus plantaris) and the digital veins are arranged like those of the thoracic limb.

The Lymphatic System

The =lymphatic system= (Systema lymphaticum) is subsidiary to the venous part of the circulatory system, from which it arises in the embryo. It consists of the lymph vessels and glands.

The =lymph vessels= (Vasa lymphatica) contain a colorless fluid, the lymph, which contains numerous lymphocytes.[175] They resemble the veins in structure but have thinner walls and are provided with more numerous valves. The vessels are sacculated opposite the segments of the valves and have a characteristic beaded appearance when distended. The collecting lymph vessels do not usually form rich plexuses, as veins often do, their branching is more limited and less tree-like than that of the blood-vessels, and their caliber therefore increases less from the periphery toward their termination. All of the lymph is ultimately carried into the venous system by two trunks, the =thoracic duct= and the =right lymphatic duct=. Almost all of the lymph passes through at least one group of lymph glands before entering the blood-vascular system.

The =lymph glands= or =nodes= (Lymphoglandulæ) are intercalated in the course of the lymph vessels. They vary widely in size, some being microscopic, others several inches in length. In form they may be globular, ovoid and flattened, elongated, or irregular. In certain situations they are aggregated into groups, and a knowledge of the position of these and the territory drained into them is important. It is convenient, when possible, to indicate their position with regard to arteries on the course of which they are placed. In color they are usually gray or yellowish-brown in the dead subject, pink or reddish-brown during life, but this varies according to their position and functional state. The bronchial glands are often blackened by infiltration with carbon. The mesenteric glands are creamy or white while the chyle is passing through, but pink at other times. Vessels which carry lymph to a gland are called =afferent=; the =efferent= vessels which convey it away are larger and fewer. Each gland has a depression, the =hilus=, at which the blood-vessels enter and the efferent lymph vessels emerge.

=Lymph nodules= or =follicles= (Noduli lymphatici) are minute masses of lymphoid tissue which occur in certain mucous membranes. They may be solitary, as in the solitary glands of the intestine, or aggregated into masses or patches, as in the tonsils and the so-called Peyer’s patches.

The =lymph nodule= or =follicle= is the unit of structure of the lymph
gland. It consists essentially of an artery surrounded by a
=reticulum= of connective tissue, the meshes of which contain numerous
=lymphocytes=. Surrounding this is a rich plexus of lymph vessels,
forming the so-called =sinus=, inclosed in some cases by a fibrous
capsule. The gland consists of a mass of follicles, inclosed in a
fibrous =capsule=, from which =trabeculæ= pass in and unite the
follicles. Beneath the capsule is the =peripheral sinus=, which
consists of a very rich plexus of lymph vessels; to this the afferent
vessels pass at various points of the surface. In the =cortical
substance= the cells are in rounded masses, the cortical nodules,
while in the =medullary substance= they lie around the arteries,
forming the so-called medullary cords. The =medulla= is redder than
the cortex, since it is more vascular; it contains the =central lymph
sinuses=, which have a similar structure to the peripheral sinus.

The =hæmolymph glands= differ from the lymph glands in color and structure. They are of a deep red color, which is due to the high vascularity of the cortical substance. The peripheral sinuses especially are greatly developed and contain numerous red blood-cells. There is no clear division into cortical and medullary substance, and the trabeculæ contain smooth muscle-cells. Some have afferent and efferent lymph vessels and others do not. They resemble the spleen in some respects, but their significance is not yet clear. They are numerous in the ox and sheep, much fewer in the dog, and apparently are absent in the horse. They occur along the course of the aorta, in the perineal fat, at the portal fissure, and with the gastric and mesenteric lymph glands. In the ox they are also found under the trapezius muscle, under the skin of the upper part of the flank, and in other places less constantly.

The =tissue= or =lymph-spaces= are interstices of varying size between cells or in the meshes of connective tissue. They contain a fluid derived from the blood-plasma, which is usually called =lymph=. They are drained by the veins and lymph vessels. The large serous sacs are often included in this category.

The exact relationship between the lymphatic capillaries and the
tissue spaces is still a matter of controversy. It is held by some
that the lymph vessels are in direct communication with the tissue
spaces, while others maintain that the lymphatics are complete closed
tubes. Communication between the spaces and vessels is in general very
free. Mall has shown that granules injected into the hepatic artery
are returned by the lymphatics as well as by the veins, and
intramuscular injections will enter the lymph vessels of the tendon in
spite of the absence of lymphatics in muscle.

LYMPHATIC SYSTEM OF THE HORSE

THE THORACIC DUCT (Figs. 428, 429)

The =thoracic duct= (Ductus thoracicus) is the chief collecting trunk of the lymphatic system. It begins as an elongated irregular dilatation, the =cisterna= or =receptaculum chyli=, which is situated between the right side of the aorta and the right crus of the diaphragm at the first and second lumbar vertebræ. The duct enters the thorax through the hiatus aorticus and runs forward on the right of the median plane between the vena azygos and the aorta, covered by the pleura. At the sixth or seventh thoracic vertebra it inclines somewhat ventrally, crosses obliquely over the left face of the œsophagus, and passes forward on the left side of the trachea to the inlet of the thorax. The extrathoracic terminal part passes downward and forward a variable distance (3 to 4 cm.) on the deep face of the left scalenus muscle, bends inward and backward under the bicarotid trunk, and opens into the upper part of the origin of the anterior vena cava just behind the angle of junction of the jugular veins. The terminal bend is ampullate and sometimes divides into two very short branches which open close together.

Since the duct develops from a plexus of ducts in the embryo,
considerable variation from the more usual course occurs. There is
often a left duct which arises at the cisterna or at a variable point
from the right duct, runs across the left intercostal arteries
parallel to the latter, and unites with it over the base of the heart
or further forward. The two are connected by cross-branches. In some
cases the left duct is the larger, and there may indeed be none on the
right side. Other variations are common.

The chief tributaries of the thoracic duct are as follows:

1. The two =lumbar trunks= (Trunci lumbales) are formed by the confluence of the efferent ducts of the lumbar glands, and commonly unite with each other and with the posterior intestinal trunk before opening into the cistern.

2. The =intestinal trunks= (Trunci intestinales), two or three in number, receive the efferents of the lymph glands of the intestine, stomach, liver, and spleen.[176]

In its course through the thorax the thoracic duct receives efferents from the intercostal, mediastinal, and bronchial glands. At the thoracic inlet it is joined by ducts from the prepectoral and right axillary glands, and by the left tracheal duct. The duct is provided with several pairs of valves; the best developed are at its termination.

THE RIGHT LYMPHATIC DUCT

This vessel (Ductus lymphaticus dexter)—when present—collects the lymph from the right side of the head, neck, and thorax, and from the right thoracic limb. It is most frequently absent, being represented by a number of short ducts which terminate in the thoracic duct, the right jugulo-brachial junction, or the origin of the anterior vena cava. When present in its typical form, it results from the confluence of efferent ducts from the right axillary and prepectoral lymph glands with the right tracheal duct. It lies on the deep face of the scalenus muscle above the terminal part of the right jugular vein. It is more or less ampullate and usually opens into the anterior vena cava to the right of the thoracic duct. It may be connected with the latter by considerable anastomoses and may join it.

The duct is very variable in form and in regard to its afferents.
Often it is a very short, irregular, and bulbous trunk; in some cases
it is about an inch and a half (ca. 3 to 4 cm.) in length and receives
the tracheal duct at its terminal bend. The lymphatico-venous
connections here need further study.

THE LYMPH GLANDS AND VESSELS OF THE HEAD AND NECK

1. The =submaxillary lymph glands= (Lg. submaxillares) (Figs. 437, 462) are arranged in two elongated groups in the submaxillary space along each side of the omo-hyoid muscles. The two groups are in apposition in front of the insertion of these muscles and diverge posteriorly in the form of a =V=, extending backward about four or five inches (ca. 10 to 12 cm.). They are covered by the skin and a thin layer of fascia and panniculus, and are therefore palpable. Anteriorly they are firmly attached to the mylo-hyoidei, but otherwise they are rather movable in the normal state. Each group is related externally to the external maxillary artery and the anterior belly of the digastricus, below to the external maxillary vein, and above to the lingual and sublingual veins.

They receive =afferent vessels= from the lips, nostrils, nasal region, cheeks, the anterior part of the tongue, the jaws, the floor of the mouth, and the greater part of the hard palate and nasal cavity. The =efferent vessels= pass to the anterior cervical and pharyngeal glands.

The superficial lymph vessels of the face converge to twelve to
fifteen trunks which turn around the lower border of the jaw with the
facial vessels. Those of the lips form plexuses at the commissures.
The nasal mucous membrane is richly supplied with lymph vessels which
accompany the veins; posteriorly they communicate with the subdural
and subarachnoid spaces and send efferents to the pharyngeal and
anterior cervical glands.

2. The =pharyngeal lymph glands= (Figs. 436, 437, 569) may consist of two groups. One lies on the lateral surface of the pharynx along the course of the external carotid artery. These glands are related externally to the stylo-maxillaris and digastricus and often to the submaxillary gland also, above to the guttural pouch. Other glands (Lg. retropharyngeales) are commonly found on the guttural pouch along the course of the internal carotid artery. They lie below the artery and are covered by the aponeurosis of the mastoido-humeralis and the cervical end of the submaxillary gland.

They receive afferent vessels from the cranium, the posterior part of the tongue, the soft palate, pharynx, guttural pouch, larynx, posterior part of the nasal cavity, and efferents from the submaxillary glands.

3. The =anterior cervical lymph glands= (Lg. cervicales craniales) are situated chiefly along the course of the common carotid artery in the vicinity of the thyroid gland, under cover of the cervical angle of the parotid gland. Some occur between the thyroid and the submaxillary salivary gland, others above and partly upon the thyroid. They are related deeply to the posterior part of the larynx, the trachea, the thyroid gland, and the œsophagus; below to the external maxillary vein and the outer border of the omo-hyoideus.[177]

These glands are variable. Often there are none in front of the
thyroid and the group may extend back a considerable distance along
the course of the carotid artery.

Their afferents are deep lymph vessels from the head, the pharynx, larynx, guttural pouch, and thyroid gland, and efferents from the submaxillary and pharyngeal glands. Their efferent vessels go to the middle and posterior cervical glands.

4. The =middle cervical lymph glands= (Lg. cervicales mediæ) form an inconstant group situated a little in front of the middle of the neck on the trachea below the carotid artery. The group is usually small and in some cases is absent, being replaced by a number of glands occurring at intervals along the course of the carotid artery. In other subjects the group consists of several glands of considerable size. They are intercalated in the course of the tracheal lymph ducts.

5. The =posterior cervical= or =prepectoral lymph glands= (Lg. cervicales caudales) form a large group below the trachea at the entrance to the thorax (Fig. 466). They occupy the interstices between the vessels and muscles and extend forward a variable distance on the ventral aspect of the trachea. They are covered by the panniculus and sterno-cephalicus. Their afferent vessels come from the head, neck, thorax, and thoracic limb. They receive efferent ducts of the anterior and middle cervical, prescapular, and axillary glands. Their efferents go to the thoracic duct on the left, to the right lymphatic duct on the right, or open directly into the vena cava.

6. The =prescapular= or =superficial cervical lymph glands= (Lg. cervicales superficiales) lie on the anterior border of the anterior deep pectoral muscle, in relation to the omo-hyoideus internally and the mastoido-humeralis externally (Figs. 431, 441, 466). They are on the course of the ascending branch of the inferior cervical artery. They receive afferents from the neck, breast, shoulder, and arm. Their efferents pass to the prepectoral glands.

The =tracheal ducts=, right and left (Ductus trachealis dexter, sinister), are collecting trunks for the lymph of the head and neck. They lie on the trachea in relation to the carotid arteries. The right one goes to the prepectoral glands or to the right lymphatic duct, the left one to the terminal part of the thoracic duct.

THE LYMPH GLANDS AND VESSELS OF THE THORAX

1. The =intercostal lymph glands= (Lg. intercostales) are small and are situated at the sides of the bodies of the thoracic vertebræ, in series corresponding to the intercostal spaces. They receive afferents from the vertebral canal, the spinal muscles, the diaphragm, intercostal muscles, and pleura. The efferent vessels go to the thoracic duct.

In the young subject these glands are more numerous than in the adult,
and there are also glands along the dorsal face of the thoracic aorta
which seem to disappear later.

2. The =anterior mediastinal lymph glands= (Lg. mediastinales craniales) (Fig. 428) are numerous. Some are situated on the course of the brachial arteries and their branches; on the right side they are related deeply to the trachea, on the left to the œsophagus also. They are variable in size and disposition and are continuous in front with the prepectoral glands. Other glands lie along the ventral face of the trachea on the anterior vena cava and the right atrium of the heart; these are continuous behind with the bronchial glands. A few glands usually occur along the dorsal surface of the trachea, and there is often one at the angle of divergence of the brachiocephalic trunk. The afferent vessels come chiefly from the pleura, the pericardium, the heart, the thymus or its remains, the trachea, and the œsophagus. Their efferent vessels pass to the prepectoral glands and the thoracic duct.

The glands along the ventral face of the trachea are frequently
enlarged and usually pigmented in dissecting-room subjects. The left
recurrent nerve lies above them on the ventral face of the trachea.

3. The =bronchial lymph glands= (Lg. bronchiales) are grouped around the terminal part of the trachea and the bronchi. One group lies on the upper surface of the bifurcation of the trachea and is continued a short distance backward under the œsophagus (Fig. 428). Others lie below the trachea and bronchi and also occupy the angle between the aortic arch and the pulmonary artery, concealing the left recurrent nerve and often covering the vagus in this part of its course. Small glands occur along the chief bronchi in the substance of the lungs (Lg. pulmonales). They receive the deep and most of the superficial lymph vessels of the lungs, and the efferents from the posterior mediastinal glands. Their efferent vessels go to the thoracic duct and the anterior mediastinal glands. The deep lymph vessels of the lung arise in plexuses which surround the terminal bronchi and accompany the bronchi to the root of the lung. The superficial vessels form a rich network under the pleura; most of them pass to the bronchial glands.

The bronchial glands are commonly pigmented except in young subjects
and are often enlarged and indurated.

4. The =posterior mediastinal lymph glands= (Lg. mediastinales caudales) are usually small and are scattered along the posterior mediastinum above the œsophagus. They receive afferent vessels from the œsophagus, mediastinum, diaphragm, and liver. The efferents go to the bronchial and anterior mediastinal lymph glands, partly to the thoracic duct directly.

Sometimes one or two small glands are situated in the acute angle between the posterior vena cava and the diaphragm.

THE LYMPH GLANDS AND VESSELS OF THE ABDOMEN AND PELVIS

The =lymph glands of the abdomen= consist of two main groups, parietal and visceral. The =parietal glands= lie in the subperitoneal or subcutaneous tissue; they receive the lymph vessels from the abdominal and pelvic walls, from parts of the viscera, and from the proximal lymph glands of the pelvic limbs. The =visceral glands= lie on the walls of the viscera or in the peritoneal folds which connect the organs with the wall or with adjacent viscera. They receive all or most of the lymph vessels from the organs with which they are connected.

The =parietal glands= comprise the following: