Chapter V: Muscles of the Leg and Foot (20)

6. =Bronchial branches= (Rr. bronchiales) are detached at the roots of the lungs and unite with sympathetic filaments in forming the =pulmonary plexuses=. From the latter numerous branches proceed in a plexiform manner along the bronchi and vessels into the substance of the lungs.

The vagus and glosso-pharyngeal nerves are so closely associated in
origin and central connections that they may be described together in
this respect.

The =sensory fibers= arise from the petrous and jugular ganglia, and
their central parts enter the lateral aspect of the medulla and divide
into anterior and posterior branches like the fibers of the dorsal
roots of the spinal nerves. Most of the fibers end in arborizations
about the cells of the vago-glosso-pharyngeal nucleus of termination,
which consists of two parts. Of these the =dorsal sensory nucleus=
(Nucleus alæ cinereæ) is situated in the posterior part of the floor
of the fourth ventricle and in the adjacent part of the closed portion
of the medulla near the median plane. The other part is termed the
=nucleus of the solitary tract=, and is so named because its cells are
grouped about the bundle (Tractus solitarius) formed by the posterior
divisions of the afferent nerve-fibers. It ends about the level of the
pyramidal decussation. The secondary central connections are similar
to those of the sensory part of the trigeminus.

The =motor fibers= (and those of the medullary part of the accessory)
arise from the =dorsal motor nucleus= and the =ventral motor nucleus=.
The cells of the former lie in groups along the ventro-medial side of
the dorsal sensory nucleus. The latter, also termed the nucleus
ambiguus, is situated more deeply in the lateral part of the formatio
reticularis.

THE ELEVENTH OR SPINAL ACCESSORY NERVE

The =spinal accessory nerve= (N. accessorius) is purely motor. It consists of two parts which differ in origin and function.

The =medullary part= arises from the lateral aspect of the medulla by several rootlets which are behind and in series with those of the vagus (Fig. 499). The =spinal part= arises from the cervical part of the spinal cord by a series of fasciculi which emerge between the dorsal and ventral roots. The bundles unite to form a trunk which is very small at its origin at the fifth segment of the cord, but increases in size when traced toward the brain, since it continually receives accessions of fibers. It passes through the foramen magnum and joins the medullary part. The trunk thus formed sends its medullary fibers to the tenth and ninth nerves and emerges through the foramen lacerum posterius. It then runs backward and downward with the vagus in a fold of the guttural pouch, separates from that nerve, crosses the deep face of the submaxillary gland and the occipital artery, and divides in the recessus atlantis into dorsal and ventral branches.

FIG. 518.—NECK OF HORSE, AFTER REMOVAL OF PART OF PANNICULUS AND
TRAPEZIUS.

_a_, _a′_, Mastoido-humeralis; _b_, anterior superficial pectoral
muscle; _c_, cervical panniculus; _d_, sterno-cephalicus; _e_,
omo-hyoideus; _f_, sterno-thyro-hyoideus; _g_, trachea; _h_,
_h′_, _h″_, tendons of splenius, mastoido-humeralis, and
trachelo-mastoideus; _i_, trapezius cervicalis; _k_,
supraspinatus; _l_, anterior deep pectoral muscle; _m_,
rhomboideus cervicalis; _n_, serratus cervicis; _o_, splenius,
upper and lower borders of which are indicated by dotted lines;
_p_, parotid gland; _q_, parotido-auricularis muscle; _r_, wing
of atlas; _s_, spine of scapula; _1_, external maxillary vein;
_2_, _3_, jugular vein; _4_, carotid artery; _5_, descending
branch of inferior cervical artery; _6_, cephalic vein; _7–11_,
ventral branches of second to seventh cervical nerves; _12_,
cutaneous branch of second cervical nerve; _13_, cervical branch
of facial nerve; _14_, terminal branches of dorsal divisions of
cervical nerves; _15_, dorsal branch of spinal accessory nerve.
(After Ellenberger-Baum, Top. Anat. d. Pferdes.)
]

It is connected by anastomotic branches with the vagus and hypoglossal
nerves and the superior cervical ganglion of the sympathetic, and
contributes a branch to the pharyngeal plexus.

The =dorsal branch= (R. dorsalis) (Figs. 433, 517, 518) receives a twig from the second and third cervical nerves and turns around the atlantal tendon of the splenius under cover of the mastoido-humeralis. It then passes obliquely through the latter muscle and continues backward on the splenius and the cervical part of the serratus, inclines upward across the anterior deep pectoral and the supraspinatus, and enters the deep face of the trapezius, in which it ramifies.

The =ventral branch= (R. ventralis) (Fig. 436) is smaller. It enters the sterno-cephalicus muscle behind the cervical angle of the parotid gland.

The fibers of the spinal part of the accessory arise from the
ventro-lateral cells of the ventral gray column of the cord as far
back as the fifth cervical segment. The fibers of the medullary part
come chiefly from the nucleus ambiguus in common with the motor fibers
of the vagus.

THE TWELFTH OR HYPOGLOSSAL NERVE

The =hypoglossal nerve= (N. hypoglossus) is purely motor and innervates the muscles of the tongue (Fig. 437). Its root-fibers arise from the ventral face of the medulla in linear series about 3 to 4 mm. lateral to the posterior half of the pyramid (Fig. 499). The filaments converge to three or four bundles which perforate the dura mater and unite to form the trunk. The latter emerges through the hypoglossal foramen (Fig. 515) and runs downward and backward between the guttural pouch and the capsule of the atlanto-occipital articulation for a distance of a little less than an inch (ca. 2 cm.). It then passes between the tenth and eleventh nerves, turns downward and forward, crosses the external face of the external carotid artery, and continues over the pharynx parallel with the great cornu of the hyoid bone and behind the external maxillary artery. It then crosses beneath the artery, runs forward on the external face of the hyo-glossus muscle, and divides into its terminal branches (Rami linguales). The smaller branch supplies the stylo-glossus, hyo-glossus, and lingualis. The larger branch ramifies on the genio-glossus and supplies the remaining muscles. Anastomoses occur with branches of the lingual nerve.

In the first part of its course the nerve communicates with the superior cervical ganglion and with the ventral branch of the first cervical nerve, and gives filaments to the pharyngeal branch of the vagus and the pharyngeal plexus.

The fibers of the nerve arise from the hypoglossal nucleus, an
elongated group of large multipolar cells situated chiefly under the
posterior part of the floor of the fourth ventricle close to the
median plane. The two nuclei are connected by commissural fibers. The
other central connections include: (_a_) communications by the medial
longitudinal fasciculus with the nuclei of termination of other
cranial nerves; (_b_) cortico-nuclear fibers which come from the
cortex by way of the internal capsule and the pyramids and go largely
to the nucleus of the opposite side; (_c_) fibers which join the
dorsal longitudinal bundle of Schütz, a tract which underlies the
floor of the fourth ventricle and is traceable forward below the
cerebral aqueduct.

THE SPINAL NERVES

The =spinal nerves= (Nervi spinales) are arranged in pairs, of which there are usually forty-two in the horse. They are designated according to their relations to the vertebral column as =cervical= (8), =thoracic= (18), =lumbar= (6), =sacral= (5), and =coccygeal= (5). Each nerve is connected with the spinal cord by two roots, dorsal and ventral (Fig. 497).

The =dorsal= (or superior) =root= (Radix dorsalis) is the larger of the two. Its fibers (Fila radicularia) spread out in fan shape and join the cord in a linear series along the dorso-lateral groove. Laterally the fibers converge to form a compact bundle, on which is a gray nodular enlargement, the =spinal ganglion= (Ganglion spinale). Beyond the ganglion the dorsal root joins the ventral root to constitute the nerve. The ganglia are external to the dura mater, and are situated in the intervertebral foramina, except in the case of the sacral and coccygeal nerves, the ganglia of which lie within the vertebral canal. Those of the coccygeal nerves are intradural.

The ganglia vary greatly in size; that of the first cervical nerve is
scarcely as large as a hemp seed, while that of the eighth cervical is
about 2 cm. long and 1 cm. wide. On the large roots connected with the
cervical and lumbar enlargement of the cord there are multiple ganglia
of varying sizes interposed in the course of the root-bundles. The
fibers of the dorsal roots arise from the cells of the spinal ganglia;
each cell gives off a fiber which enters the spinal cord and another
which passes into the nerve.

The =ventral= (or inferior) =root= (Radix ventralis) contains fewer fibers than the dorsal root, except in the case of the first cervical nerve. It arises from the ventral surface of the spinal cord (Fig. 495) by means of numerous small bundles of fibers which do not form a linear series but emerge from the cord over an area three to five millimeters in width (ventral root zone). The fibers are processes of the large cells of the ventral gray columns of the spinal cord. There is no ganglion on the ventral root.

In the cervical, thoracic, and anterior lumbar regions the bundles of
both roots pass through separate openings in linear series in the dura
mater before uniting into a root proper. Further back the bundles of
each root unite within the dura. In the anterior part of the cervical
region and in the thoracic part of the cord there are intervals of
varying length between adjacent roots, but in some places the fibers
of adjacent roots overlap and an exchange of fibers may be observed.
Many of the roots are directed almost straight outward or incline
slightly backward, but the posterior lumbar, sacral, and coccygeal
roots and nerves run backward to reach the foramina through which they
emerge. The distance thus to be traversed increases from before
backward, so that these nerves form a tapering sheaf around the conus
medullaris and filum terminale in the last lumbar vertebra and the
sacrum which is known as the cauda equina.

FIG. 519.—VERTEBRAL CANAL OPENED BY SAWING OFF THE ARCHES. (After
Schmaltz. Atlas d. Anat. a. Pferdes.)
]

The =size= of the spinal nerves varies greatly. The largest are connected with the cervical and lumbar enlargements.

In or immediately after its emergence from the intervertebral foramen each spinal nerve gives off a small =meningeal= or =recurrent branch= (Ramus meningeus). This is joined by a bundle of fibers from the ramus communicans and enters the vertebral canal, in which it is distributed. Each nerve then divides into two primary branches, =dorsal= and =ventral= (Ramus dorsalis, ventralis). The =dorsal branches= are smaller than the ventral, except in the cervical region. They are distributed chiefly to the muscles and skin of the dorsal part of the body. The =ventral branches= supply in general the muscles and skin of the ventral parts of the body, including the limbs. Each nerve or its ventral branch is connected with an adjacent ganglion of the sympathetic system by at least one small short branch known as a =ramus communicans=. Many nerves have two and some have three such rami. A nerve may be connected with two ganglia, and a ganglion may be connected with two nerves.

The dorsal root is sensory or afferent, _i. e._, it conveys impulses
to the central system. Its fibers are axones of the cells of the
spinal ganglion. The ventral root is motor or efferent, and conveys
impulses toward the periphery. Its fibers are axones of the large
cells in the ventral gray columns of the spinal cord. The common trunk
or nerve formed by the union of the two roots contains both kinds of
fibers, as do also their primary divisions. In addition to these
fibers, which are distributed to the skeletal muscles and the skin,
the spinal nerves contain fibers derived from the sympathetic system
through the rami communicantes; these go to the glands and unstriped
muscle and are designated secretory and vasomotor fibers.

THE CERVICAL NERVES

The =cervical nerves= (Nervi cervicales) (Figs. 431, 433, 518, 523) number eight pairs. Of these the first emerges through the intervertebral foramen of the atlas, the second through that of the axis, and the eighth between the last cervical and the first thoracic vertebræ.

The =dorsal branches= are distributed to the dorso-lateral muscles and skin of the neck. They divide usually into inner and outer branches. The internal branches (Rami mediales) run in general across the multifidus and the lamellar part of the ligamentum nuchæ to the skin of the upper border of the neck; they supply the deep lateral muscles and the skin. The external branches (Rami laterales) are chiefly muscular in their distribution. The dorsal branches of the third to the sixth nerves are connected by anastomotic branches to form the =dorsal cervical plexus=.

The =ventral branches= are smaller than the dorsal ones—an exception to the general rule. They increase in size from first to last. They supply in general the muscles and skin over the lateral and ventral aspect of the vertebræ, but the last three enter into the formation of the brachial plexus, and the three preceding the last give off the roots of the phrenic nerve. An irregular =ventral cervical plexus= is formed by anastomoses established between the ventral branches. The following special features may be noted:

The =first cervical nerve= emerges through the intervertebral (antero-internal) foramen of the atlas. Its =dorsal branch= (N. occipitalis) passes upward and outward between the obliqui capitis and the recti capitis dorsales and supplies branches to these muscles, the scutularis and posterior auricular muscles, and the skin of the poll. The =ventral branch= descends through the alar (antero-external) foramen of the atlas, crosses over the ventral straight muscles and the carotid artery under cover of the parotid gland, and divides into two branches. The anterior branch enters the omo-hyoideus muscle. The posterior branch passes downward and backward under cover of that muscle, unites with a branch of the ventral division of the second cervical nerve, and continues its course on the ventro-lateral surface of the trachea to enter the sterno-thyro-hyoideus behind the intermediate tendon. In the recessus atlantis the ventral branch is connected by one or more twigs with the superior cervical ganglion of the sympathetic, and a little lower with the hypoglossal nerve. It also sends branches to the ventral straight muscles of the head and the thyro-hyoideus. Below the atlas the ventral branch is crossed superficially by the spinal accessory nerve, the occipital artery, and the inferior cerebral vein.

The =second cervical nerve= is larger than the first. It emerges from the vertebral canal through the intervertebral foramen of the anterior part of the arch of the axis. Its =dorsal branch= ascends between the complexus and the ligamentum nuchæ and ramifies in the skin of the poll. The =ventral branch= gives off =muscular branches= to the rectus capitis anterior major, and =anastomotic branches= to the spinal accessory and the ventral divisions of the first and third cervical nerves; one of these crosses over the carotid artery and concurs in the formation of the nerve to the sterno-thyro-hyoideus mentioned above. The ventral branch then becomes superficial by passing between the two parts of the mastoido-humeralis, and divides into posterior auricular and cutaneous branches. The =posterior auricular nerve= passes upward and forward on the parotid gland parallel with the posterior border of the parotido-auricularis to ramify on the convex face of the external ear. The =cutaneous nerve= of the neck (N. cutaneus colli) crosses the mastoido-humeralis muscle and turns backward along the course of the jugular vein. On the lower part of the parotid gland it is connected by a twig with the cervical cutaneous branch of the facial nerve. It gives off twigs to the subcutaneous muscles and the skin of the parotid and laryngeal regions and a long branch which passes forward in the submaxillary space.

The =third cervical nerve= leaves the vertebral canal through the foramen between the second and third cervical vertebræ. Its =dorsal branch= emerges between two bundles of the intertransversalis muscle accompanied by a branch of the vertebral artery, turns upward on the multifidus, and divides into several branches which radiate on the deep face of the complexus. It gives branches to these muscles and to the skin, and a twig which joins the corresponding branch of the fourth nerve. The =ventral branch= emerges through the intertransversalis below the bundle above which the dorsal branch appears. It gives branches to the trachelo-mastoideus, rectus capitis anterior major, longus colli, splenius, and mastoido-humeralis. It also gives off a large cutaneous nerve which passes out between the two parts of the mastoido-humeralis and divides into several divergent branches.

The =fourth= and =fifth cervical nerves= are distributed in general like the third. Their =dorsal branches= are united by anastomotic twigs with each other and with those of the third and sixth nerves to form the dorsal cervical plexus. The =ventral branch= of the fifth nerve often contributes a small twig to the phrenic nerve.

The =sixth cervical nerve= has a smaller =dorsal branch= than the fifth. Its =ventral branch= is larger and goes in part to the brachial plexus; it supplies twigs to the intertransversales, the longus colli, the mastoido-humeralis, and the cervical parts of the serratus and rhomboideus, furnishes a root of the phrenic nerve, and gives off several considerable subcutaneous branches. One of the latter ramifies on the thick part of the cervical panniculus, to which it gives branches; another and larger branch (N. supraclavicularis) sends twigs to the skin over the shoulder joint, and descends to the skin over the superficial pectoral muscles (Fig. 466).

The =seventh= and =eighth cervical nerves= have small =dorsal branches=, which ascend between the longissimus and multifidus, giving twigs to these muscles, the spinalis and semispinalis, the rhomboideus, and the skin. Their =ventral branches= are very large and concur in the formation of the brachial plexus; that of the seventh nerve contributes the posterior root of the phrenic nerve.

PHRENIC NERVE

The =phrenic nerve= (N. phrenicus) (Figs. 428, 429, 433), the motor nerve to the diaphragm, is formed by the union of two or three roots which cross the superficial face of the scalenus muscle obliquely downward and backward. The chief roots come from the ventral branches of the sixth and seventh cervical nerves. The root derived from the fifth nerve is small and inconstant. The root from the seventh cervical comes by way of the brachial plexus. The course of the nerve is not the same on both sides. On the =right side= the nerve enters the thorax by passing between the brachial artery and the anterior vena cava. It then courses backward and somewhat downward over the right face of the anterior vena cava, crosses the pericardium, and continues along the posterior vena cava to the diaphragm. In the latter part of its course it is inclosed in a special fold of the right pleura and inclines gradually to the ventral face of the vein. On the =left side= the nerve enters the thorax between the left brachial and inferior cervical arteries, and then runs its entire course in the mediastinum. In the anterior mediastinum it lies along the brachiocephalic artery ventral to the left vagus and cardiac nerves, and crosses over the dorso-cervical vein. It then passes over the upper part of the pericardium and runs backward in the posterior mediastinum to reach the tendinous center of the diaphragm considerably to the left of the median plane. Each nerve is usually connected near its origin with the first thoracic ganglion of the sympathetic by a ramus communicans, and each terminates by dividing into several branches which are distributed to the corresponding part of the diaphragm.

THE BRACHIAL PLEXUS

The =brachial plexus= (Plexus brachialis) (Fig. 433) results from anastomoses established between the ventral branches of the last three cervical and first two thoracic nerves. It appears as a thick, wide band between the two parts of the scalenus muscle, and is covered by the anterior deep pectoral and subscapularis muscles. The largest root of the plexus is derived from the first thoracic nerve, while the root supplied by the sixth cervical nerve is quite small. Each of the three chief roots, _i. e._, those from the last two cervical and the first thoracic nerve, is connected with the sympathetic by a ramus communicans.

The =branches= emanating from the plexus go for the most part to the thoracic limb, but some are distributed on the chest-wall. The names of the branches, and their arrangement so far as they can be conveniently examined before removal of the fore limb, are as follows:[207]

1. The large =suprascapular nerve= (N. suprascapularis) turns outward and forward and disappears between the supraspinatus and subscapularis.

2. The much smaller =subscapular nerves= (Nn. subscapulares), usually two primary trunks, arise close behind the suprascapular, run backward a short distance, and divide into several branches which enter the lower third of the subscapularis.

3. The =anterior thoracic= or =pectoral nerves= (N. pectorales craniales), three or four in number, arise from the anterior part of the plexus and from the loop formed by the musculo-cutaneous and median nerves. One enters the anterior deep pectoral muscle. Another passes out between the divisions of the deep pectoral to supply the superficial pectoral, giving a twig usually to the posterior deep muscle. The latter receives one or two other nerves.

4. The =musculo-cutaneous nerve= (N. musculocutaneus) arises from the anterior part of the plexus and passes over the outer face of the brachial artery, below which it is connected by a large but short branch with the median nerve, thus forming a loop in which the artery lies. One or two branches to the pectoral muscles are given off from the nerve or the loop.

5. The =median nerve= (N. medianus) is usually the largest branch of the brachial plexus. It lies at first above the brachial artery, then passes between the artery and vein to reach the anterior border of the former. It is easily recognized by its large size and the loop which it forms with the musculo-cutaneous nerve.

6. The =ulnar nerve= (N. ulnaris) arises with the median by a short common trunk. It descends behind the brachial artery and is accompanied a short distance by the radial nerve.

7. The =radial nerve= (N. radialis) arises from the posterior part of the plexus and is sometimes the largest branch. It descends behind the ulnar nerve over the origin of the subscapular artery and the lower part of the teres major, and dips into the interstice between that muscle and the long and internal heads of the triceps.

8. The =axillary= or =circumflex nerve= (N. axillaris) arises behind the musculo-cutaneous. It passes downward and backward on the inner face of the subscapularis and disappears between that muscle and the subscapular artery.

9. The =long thoracic nerve= (N. thoracalis longus) is wide and thin. It passes backward across the surface of the serratus magnus, to which it is distributed. The branches which enter the muscle are given off both upward and downward in fairly regular fashion.

10. The =thoraco-dorsal nerve= (N. thoracodorsalis) passes upward and backward across the subscapularis muscle to ramify in the teres major and the latissimus dorsi.

11. The =external= (or subcutaneous) =thoracic nerve= arises by a common trunk with the ulnar. It runs backward and downward across the inner face of the tensor fasciæ antibrachii, communicates with the anterior pectoral branches, and gives twigs to the deep pectoral muscle. It then runs backward in company with the external thoracic or “spur” vein, gives branches to the latissimus dorsi and the deep pectoral, and ramifies in the panniculus and the skin of the abdominal wall. It communicates with perforating branches of the intercostal nerves. A branch from it, accompanied by a large perforating intercostal branch, winds around the lower border of the latissimus dorsi and ramifies in the panniculus on the outer surface of the arm.

The term posterior thoracic or pectoral nerves (Nn. pectorales
caudales) may be used to include 8, 9, and 10.

SUPRASCAPULAR NERVE

The =suprascapular= (Fig. 441) is a large nerve derived chiefly, if not exclusively, from the sixth and seventh cervical components of the brachial plexus. It passes between the supraspinatus and subscapularis muscles and turns around the distal fourth of the anterior border of the scapula to reach the supraspinous fossa. It gives branches to the supraspinatus and continues backward and upward into the infraspinous fossa, where it supplies the infraspinatus, deltoid, and teres minor muscles.

The direct relation of this nerve to the scapula renders it liable to
injury, the result of which may be paralysis and atrophy of the
muscles supplied by it.

MUSCULO-CUTANEOUS NERVE

The =musculo-cutaneous nerve= (Fig. 441) arises close behind the suprascapular, and is derived chiefly from the part of the brachial plexus which is supplied by the seventh and eighth cervical nerves. It descends across the outer surface of the brachial artery, below which a great part of the nerve unites with the median to form the loop previously mentioned. It gives off a branch which enters the upper part of the belly of the coraco-brachialis, passes downward and forward between the two parts of that muscle or between the muscle and the bone, and divides into branches which enter the biceps brachii. It contributes one of the nerves to the pectoral muscles. In some cases this nerve sends a branch to join the cutaneous branch of the median.

AXILLARY NERVE

The =axillary nerve= (Figs. 441, 520), also termed the circumflex, derives its fibers from the eighth cervical and first thoracic roots of the brachial plexus. It runs downward and backward across the lower part of the subscapularis and dips in between that muscle and the subscapular artery at the level of the shoulder joint. Continuing outward in the interval between the teres minor and the long and external heads of the triceps, it reaches the deep face of the deltoid and divides into several divergent branches. The =muscular branches= supply the teres major, capsularis, teres minor, infraspinatus, deltoid, and mastoido-humeralis. The =cutaneous branch= (N. cutaneus brachii lateralis) runs downward and a little forward across the external head of the triceps and ramifies on the fascia on the front of the forearm and on the superficial pectoral muscle.

RADIAL NERVE

The =radial nerve= (Figs. 441, 443, 520), also called the musculo-spiral, is sometimes the largest branch of the brachial plexus. Its fibers are derived from the seventh and eighth cervical and first thoracic roots of the plexus. It passes downward and backward over the inner surface of the subscapular artery and the teres major. In this part of its course it is related in front to the ulnar nerve, which separates it from the brachial vein. It detaches a branch to the tensor fasciæ antibrachii, passes outward in the interval between the teres major and the long and internal heads of the triceps, and gains the musculo-spiral groove of the humerus. Accompanied by a branch of the deep brachial artery, it runs obliquely downward and outward in the groove, covered externally by the external head of the triceps and the extensor carpi radialis, and reaches the flexion surface of the elbow joint. In this part of its course it gives off a muscular branch which ramifies in the long and external heads of the triceps and in the anconeus, and a =cutaneous nerve= (N. cutaneus antibrachii dorsalis); branches of the latter emerge below or through the external head of the triceps and ramify on the dorso-lateral surface of the forearm. At the elbow the nerve descends with the anterior radial vessels on the joint capsule between the brachialis and extensor carpi radialis, and supplies branches to the extensor carpi and the common or anterior extensor of the digit and (inconstantly) to the brachialis. Below the elbow joint the nerve detaches a large branch which passes back to the flexor carpi externus, and terminates by small branches which descend on the radius to enter the radial and ulnar heads of the common extensor, the lateral extensor, and the oblique extensor of the carpus. Thus the radial nerve innervates the extensors of the elbow, carpal and digital joints, and supplies also the flexor carpi externus.[208]

FIG. 520.—CUTANEOUS NERVES OF RIGHT FORE LIMB OF HORSE, EXTERNAL FACE.

_a_, Cutaneous branch of axillary nerve; _b_, cutaneous branches of
radial nerve; _c_, posterior cutaneous branch of ulnar nerve; _d_,
cutaneous branch of median nerve; _e_, superficial branch of ulnar
nerve; _f_, external metacarpal nerve; _g_, anastomotic branch
connecting internal and external metacarpal nerves; _1_, external
head of triceps; _2_, extensor carpi radialis; _3_, anterior
extensor; _4_, flexor carpi externus. (After Ellenberger, in
Leisering’s Atlas.)
]

ULNAR NERVE

The =ulnar nerve= (Figs. 441, 442, 443, 520) arises with the median from the thoracic components of the brachial plexus. It descends between the brachial artery and vein, accompanied for a short distance by the radial nerve. It then crosses the vein and continues behind the latter along the anterior border of the tensor fasciæ antibrachii and dips under that muscle near the elbow. Here it is joined by the ulnar vessels and passes downward and backward over the internal epicondyle of the humerus. In the forearm it crosses obliquely the deep face of the ulnar head of the flexor carpi medius and descends under the deep fascia with the vessels, at first on the ulnar head of the perforans, and then between the middle and external flexors of the carpus. Near the level of the accessory carpal bone it divides into two terminal branches, superficial and deep. It gives off two principal collateral branches. The =cutaneous branch= (Ramus cutaneus palmaris) is detached just before the nerve passes under the tensor fasciæ antibrachii; it runs downward and backward on that muscle under cover of the posterior superficial pectoral, becomes superficial below the elbow and ramifies on the posterior surface and both sides of the forearm. The =muscular branch= is given off at the elbow joint and divides to supply the flexors of the digit and the middle flexor of the carpus. Of the two terminals, the =superficial branch= (Ramus superficialis) emerges between the tendons of insertion of the external and middle flexors of the carpus and ramifies on the dorso-lateral aspect of the carpus and metacarpus. The =deep branch= (Ramus profundus), after a very short course, unites under cover of the tendon of the flexor carpi medius with the outer branch of the median nerve to form the external metacarpal or volar nerve.

FIG. 521.—CROSS-SECTION OF FOREARM OF HORSE ABOUT THREE INCHES (CA. 8
CM.) BELOW THE ELBOW JOINT.

The deep fascia is designated by red line. _N_, _A_, _V_, Interosseous
vessels and nerve.
]

MEDIAN NERVE

The =median nerve= (Figs. 441, 442, 447, 520, 521) derives its fibers chiefly from the first thoracic root of the brachial plexus. It is usually the largest branch and it accompanies the chief arterial trunks to the distal part of the limb. It descends over the inner face of the brachial artery, which it crosses obliquely, and continues down the arm in front of the artery. Near its origin it is joined by a large branch with the musculo-cutaneous nerve, thus forming a loop in which the artery appears to be suspended. Near the elbow it crosses obliquely over the artery (posterior radial) and lies behind it on the internal lateral ligament. Below the joint it again crosses the artery and lies behind the radius and the lower part of the long internal lateral ligament.

FIG. 522.—DISSECTION OF DISTAL PART OF RIGHT FORE LIMB OF HORSE,
VIEWED FROM BEHIND.

_1_, Lateral cartilage; _2_, _3_, _4_, ligaments from first phalanx to
third sesamoid, bulb of heel, and lateral cartilage. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

At the elbow the nerve is covered by the posterior superficial
pectoral muscle and is crossed by the large oblique connection between
the cephalic and brachial veins. The variable and often plexiform
venous arrangement renders approach to the nerve here undesirable. It
can be reached best by an incision just behind the lower part of the
long internal lateral ligament; here the nerve lies under the deep
fascia in the furrow between the radius and the anterior border of the
flexor carpi internus, overlapped by the latter. In some cases the
nerve retains its position in front of the artery at the elbow, and in
others it crosses the deep face of the artery.

Passing beneath the internal flexor of the carpus, the nerve continues downward in the forearm with the posterior radial vessels, and divides at a variable distance above the carpus into two branches; these are the internal volar or metacarpal nerve and the branch which unites with the ulnar to form the corresponding external nerve.

At the proximal part of the forearm the nerve runs almost straight
downward along the posterior border of the long internal lateral
ligament, while the artery here inclines somewhat backward. Thus the
nerve is superficial to the artery for a short distance, then lies in
front of the latter to about the middle of the region, where it
inclines a little backward and arrives at the interval between the
internal and middle flexors of the carpus. The division may occur
about the middle of the region, but commonly takes place in the distal
third or fourth.

The collateral branches are as follows:

1. The =musculo-cutaneous branch= is in reality the continuation of the nerve of that name. It is given off about the middle of the arm, passes beneath the lower part of the biceps, and divides into muscular and cutaneous branches. The =muscular branch= enters the brachialis. The =cutaneous branch= emerges between the mastoido-humeralis and the biceps and divides into two branches; these descend on the fascia of the forearm with the cephalic vein and its accessory, and ramify on the front and inner face of the forearm, carpus, and metacarpus.

2. =Muscular branches= to the internal flexor of the carpus and the flexors of the digit are given off just as the nerve passes under the first named muscle.

3. The very small =interosseous nerve= passes through the interosseous space and is distributed chiefly to the periosteum, but in some cases twigs may go to the extensor muscles.

The =metacarpal= or =volar nerves=,[209] internal and external, are the continuations of the median and ulnar nerves in the distal part of the limb.

The =internal metacarpal= or =volar nerve= (N. volaris medialis) arises as the inner terminal branch of the median nerve at a variable distance above the carpus (Fig. 442). It descends through the carpal canal along the inner border of the superficial flexor tendon, and lies at first in front of the large metacarpal or common digital artery. It then passes behind the artery to the distal third of the metacarpus, where it lies behind the vein, the artery here becoming deeper in position. In addition to cutaneous twigs, the nerve gives off about the middle of the metacarpus a considerable =anastomotic branch= which winds obliquely downward and outward over the flexor tendons and joins the external nerve at the lower third of the metacarpus (Fig. 447). Near the fetlock the nerve divides into two digital branches.

1. The =dorsal= or =anterior digital branch= (Ramus dorsalis) descends at first between the digital artery and vein, then crosses over the vein and ramifies in the skin and the matrix of the hoof on the dorsal (anterior) face of the digit.

2. The =volar= or =posterior digital branch= (Ramus volaris) is the direct continuation of the trunk. It descends behind the digital artery, which it accompanies in its ramification. A =middle digital branch= is sometimes described as descending behind the vein. In some cases this branch is distinct, but usually there are instead several small twigs derived from the posterior branch, which cross very obliquely over the artery and anastomose in a variable manner with each other and with the anterior branch.

The =external metacarpal= or =velar nerve= (N. volaris lateralis) is formed by the union of the external terminal branch of the median with the deep branch of the ulnar nerve (Fig. 442). It descends with the internal volar or small metacarpal artery in the texture of the posterior annular ligament of the carpus. In the metacarpus it descends along the outer border of the deep flexor tendon behind the external metacarpal vein, and is accompanied by a small artery from the volar subcarpal arch. Toward the distal end of the metacarpus it is joined by the oblique branch from the internal nerve, and beyond this is arranged like the latter. Below the carpus it detaches a deep branch to the suspensory ligament and the interossei, and also supplies twigs to the skin.

Anastomoses are established between the digital branches, and the
areas innervated by them are not well defined, but really overlap each
other. In certain diseased conditions, however, in which the lesions
are confined to the volar structures, relief from pain may be afforded
by section of the volar branches only.

THE THORACIC NERVES

The =thoracic nerves= (Nn. thoracales) number eighteen on either side in the horse. They are designated numerically according to the vertebræ behind which they emerge. Most of them are arranged in a very similar manner and therefore do not require separate description. Each divides into a dorsal and a ventral branch, the latter being the larger.

The =dorsal branches= (Rami dorsales) emerge behind the levatores costarum and divide into internal and external branches. The internal branches ascend on the multifidus and supply the dorsal spinal muscles. The external branches run outward under the longissimus and emerge between that muscle and the transversalis costarum; after giving twigs to these muscles they pass through the latissimus dorsi and the lumbo-dorsal fascia and ramify as dorsal cutaneous nerves under the skin of the back (Fig. 523). In the region of the withers they give branches to the serratus anticus and rhomboideus, and their cutaneous terminals pass through these muscles and the dorso-scapular ligament to supply the skin over the ligamentum nuchæ and the scapular cartilage.

The =ventral branches= or =intercostal nerves= (Nn. intercostales) are much larger than the preceding, and are connected with the sympathetic by rami communicantes. The first goes almost entirely to the brachial plexus, but sends a fine branch downward in the first intercostal space which is expended in the muscle there without reaching the lower end of the space. The second ventral branch furnishes a considerable root to the brachial plexus, but its intercostal continuation is typical. The intercostal nerves (Fig. 184) descend in the intercostal spaces with the vessels of like name, at first between the intercostal muscles, and lower down chiefly between the pleura and the internal intercostal muscle. In the anterior spaces the artery lies along the posterior border of the rib, with the nerve in front of it; further back the nerve lies behind the border of the rib, with the artery in front of it. They supply the intercostal muscles, give off lateral perforating branches, and terminate in the following manner: The second to the sixth inclusive emerge through the spaces between the costal cartilages and concur in supplying the pectoral muscles. The second to the eighth give branches to the transversus thoracis. The succeeding ones give branches to the diaphragm, pass between the transverse and internal oblique muscles, give twigs to these, and end in the rectus abdominis. There are three series of cutaneous nerves given off by the intercostal nerves. The upper nerves emerge through the serratus magnus and the external intercostals about parallel with the digitations of the external oblique. The middle set perforate the origin of the latter muscle. The lower ones appear through the abdominal tunic. They supply branches to the abdominal muscles, the panniculus, and the skin. Some of the anterior ones anastomose with the posterior thoracic branches of the brachial plexus. The posterior three supply in part the skin of the flank. The ventral branch of the last thoracic nerve runs outward behind the last rib across the dorsal surface of the psoas major and divides into superficial and deep branches. The superficial branch passes between the obliquus internus and transversus abdominis, perforates the obliquus externus, and ramifies under the skin of the flank (Fig. 525). The deep branch descends on the inner face of the internal oblique to the rectus abdominis, in which it ends.

THE LUMBAR NERVES

There are six pairs of =lumbar nerves= (Nn. lumbales) in the horse, the last of which emerge between the last lumbar vertebra and the sacrum. The anterior two or three are about the same size as the thoracic nerves, but the others are much larger.

FIG. 523.—SUPERFICIAL NERVES OF NECK AND TRUNK OF HORSE. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]

Their =dorsal branches= are small in comparison with the ventral ones. They are distributed to the muscles and skin of the loins and croup in a fashion similar to those of the thoracic nerves.

The =ventral branches= are connected with the sympathetic by small rami communicantes, and give branches to the sublumbar muscles. Those of the first two nerves are arranged in a manner analogous to the corresponding branch of the last thoracic nerve.

The ventral branch of the =first lumbar nerve= is termed the =ilio-hypogastric nerve= (N. iliohypogastricus). It passes outward between the quadratus lumborum and the psoas major, and divides at the outer border of the latter into an anterior or superficial and a posterior or deep branch. The =anterior= or =superficial branch= passes over the upper edge of the internal oblique, descends between that muscle and the external oblique, perforates the latter, and runs downward and backward and ramifies under the skin of the posterior part of the flank and the outer surface of the thigh. It gives branches to the transversus and obliquus externus abdominis. The =posterior= or =deep branch= is smaller; it runs downward and backward beneath the peritoneum to the outer border of the rectus abdominis, gives branches to the internal oblique, and terminates in the rectus abdominis.

FIG. 524.—LUMBAR NERVES OF HORSE, VENTRAL VIEW. (After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]

The ventral branch of the =second lumbar nerve= is connected by an anastomotic branch with that of the third nerve. It gives off a large branch to the psoas muscle and an inguinal branch, and is continued as the ilio-inguinal nerve. The =inguinal branch= (N. spermaticus externus) runs backward in the substance of the psoas minor, emerges a short distance in front of the circumflex iliac vessels, and runs backward and downward under the peritoneum to the internal inguinal ring. It gives branches to the internal oblique muscle and descends in the inguinal canal along the outer border of the cremaster (to which it detaches filaments) and ends subcutaneously in the scrotum and prepuce in the male, the mammary gland in the female. The =ilio-inguinal nerve= (N. ilioinguinalis) may be regarded as the continuation of the ventral branch. It runs parallel with the ilio-hypogastric nerve and has a similar arrangement. Its =anterior= or =superficial branch= perforates the external oblique muscle a little in front of the point of the hip, runs downward on the front of the thigh and the outer surface of the stifle, and gives off cutaneous branches. The =posterior= or =deep branch= runs behind and parallel with that of the iliohypogastricus, detaches branches to the internal oblique muscle, and descends the inguinal canal with the inguinal branch, to be distributed to the external genital organs and the surrounding skin in the inguinal region.

The ventral branch of the =third lumbar nerve= is connected by a small anastomotic branch with the second nerve and furnishes a root of the lumbo-sacral plexus. It gives off a branch to the psoas muscles, an inguinal branch, and is continued as the external cutaneous nerve. The =inguinal branch= (N. spermaticus externus) passes backward in the substance of the psoas minor, from which it emerges under cover of or near the circumflex iliac vessels. It then runs external to and parallel with the external iliac artery and descends in the inner part of the inguinal canal. It emerges at the external ring with the external pudic artery and ramifies in the external genital organs and the skin of the inguinal region. The =external cutaneous nerve= of the thigh (N. cutaneus femoris lateralis) runs backward in the substance of the psoas muscles and emerges at the outer border of the psoas minor. It then passes outward and backward on the iliac fascia and accompanies the posterior branch of the circumflex iliac artery. With this vessel it perforates the abdominal wall by passing between the external oblique and the iliacus a short distance below the point of the hip, and descends on the inner face of the tensor fasciæ latæ (near its anterior border) and ramifies subcutaneously in the region of the stifle.

FIG. 525.—SUPERFICIAL NERVES OF PELVIC LIMB AND POSTERIOR PART OF
TRUNK OF HORSE.

_a_, Cutaneous branches of sixteenth and seventeenth thoracic nerves;
_b_, cutaneous branches of lumbar nerves; _c_, cutaneous branches of
sacral nerves; _d_, cutaneous branches of coccygeal nerves; _e_,
_f_, _g_, cutaneous branches of last intercostal, ilio-hypogastric,
and ilio-inguinal nerves; _g′_, end of external cutaneous nerve of
thigh; _h_, posterior cutaneous nerve of thigh; _i_, _i_, cutaneous
branches of great sciatic nerve; _k_, posterior cutaneous nerve of
the leg; _l_, superficial peroneal nerve; _m_, terminal part of deep
peroneal nerve; _n_, external plantar nerve; _1_, obliquus abdominis
externus; _2_, tensor fasciæ latæ; _3_, gluteus superficialis; _4_,
biceps femoris; _5_, semitendinosus; _6_, anterior extensor; _7_,
lateral extensor; _8_, flexor tendons; _9_, great metatarsal artery.
(After Ellenberger, in Leisering’s Atlas.)
]

The origin and disposition of some of the foregoing nerves are
variable. In some cases the ilio-inguinal nerve ends in the psoas
major, and appears then to be absent. The mode of formation of the
inguinal nerves is very inconstant. The inner nerve may arise with one
of the outer ones from a common trunk, or they may anastomose. The
inguinal branch of the ilio-inguinal nerve often receives a twig from
the ilio-hypogastric nerve.

The ventral branches of the =fourth=, =fifth=, and =sixth lumbar= nerves concur in the formation of the lumbo-sacral plexus.

LUMBO-SACRAL PLEXUS

This plexus (Fig. 451) results substantially from the union of the ventral branches of the last three lumbar and the first two sacral nerves, but it derives a small root from the third lumbar nerve also. The anterior part of the plexus lies in front of the internal iliac artery between the lumbar transverse processes and the psoas minor. The posterior part lies partly upon and partly in the texture of the sacro-sciatic ligament. From the plexus are derived the nerves of the pelvic limb, which are now to be described.

FEMORAL NERVE

The =femoral= (or anterior crural) nerve (N. femoralis) is derived chiefly from the fourth and fifth lumbar nerves, but commonly receives a fasciculus from the third nerve also (Fig. 524). It is the larger of the two nerves which are given off from the anterior part of the lumbo-sacral plexus. It runs backward at first between the psoas major and minor, then crosses the deep face of the tendon of insertion of the latter, and descends under cover of the sartorius in the furrow between the two heads of the iliacus. It gives off the saphenous nerve, crosses the lower part of the outer head of the iliacus, and divides into several branches which dip into the interstice between the rectus femoris and the vastus internus (Fig. 450). These branches are accompanied by the anterior femoral vessels and innervate the quadriceps femoris. Collateral muscular branches are supplied to the ilio-psoas and sartorius.

The =saphenous nerve= (N. saphenus) (Figs. 450, 451, 457) is given off as the parent trunk crosses the furrow between the psoas major and the outer head of the iliacus. It descends with the femoral vessels in the femoral canal, and gives branches to the sartorius, gracilis, and pectineus. About the middle of the thigh it divides into several branches which emerge from between the sartorius and gracilis, perforate the deep fascia and ramify on the inner surface and the front of the limb as far downward as the hock. The longer posterior branches accompany the saphenous vessels, while the anterior branches deviate forward toward the stifle and the anterior surface of the leg.

OBTURATOR NERVE

This nerve (N. obturatorius) is derived from the ventral branches of the last three lumbar nerves (Fig. 524). It runs downward and backward, at first above and then upon the external iliac vein, inclines inward across the obturator vein, and passes through the anterior part of the obturator foramen in front of that vessel (Fig. 451). It continues downward through the obturator externus, and divides into several branches which innervate the obturator externus, pectineus, adductor, and gracilis muscles (Fig. 456).

ANTERIOR GLUTEAL NERVE

This nerve (N. glutæus cranialis) is derived chiefly from the last lumbar and first sacral nerves. It divides into four or five branches which emerge through the great sacro-sciatic foramen with the divisions of the anterior gluteal artery and supply the gluteal and tensor fasciæ latæ muscles. The nerve to the latter and the anterior part of the superficial gluteus passes between the deep part of the gluteus medius and the deep gluteal muscle, and is accompanied on the iliacus by branches of the external circumflex vessels.

POSTERIOR GLUTEAL NERVE

The =posterior gluteal nerve= (N. glutæus caudalis) is derived mainly from the sacral roots of the lumbo-sacral plexus (Figs. 451, 526). It divides into two trunks which emerge above the great sciatic nerve. The upper nerve passes backward on the sacro-sciatic ligament and divides into branches which enter the two heads of the biceps femoris; it supplies a branch to the posterior part of the middle gluteus, and a nerve which turns around the posterior border of the latter and enters the posterior head of the superficial gluteus. The inferior nerve runs downward and backward on the sacro-sciatic ligament and divides into the =posterior cutaneous nerve= and muscular branches which supply the semitendinosus. The former (N. cutaneus femoris caudalis) passes through the biceps femoris, emerges between that muscle and the semitendinosus a little below the level of the tuber ischii, and ramifies subcutaneously on the outer and posterior surfaces of the hip and thigh (Fig. 525).

FIG. 526.—VESSELS AND NERVES OF PELVIC WALL OF HORSE.

Nervus ischiadicus = great sciatic nerve; n. glut. inf = anterior
gluteal nerve; n. pudendus = internal pudic nerve. (After Schmaltz,
Atlas d. Anat. D. Pferdes.)
]

GREAT SCIATIC NERVE

The =great sciatic nerve= (N. ischiadicus) (Figs. 451, 455, 526), the largest in the body, is derived chiefly from the last lumbar and the sacral roots of the lumbo-sacral plexus, but may receive a fasciculus from the third sacral nerve also. It emerges through the great sacro-sciatic foramen as a broad flat band—blended at first with the posterior gluteal nerve—which passes downward and backward on the lower part of the sacro-sciatic ligament and on the origin of the deep gluteus muscle. It turns downward in the hollow between the trochanter major and the tuber ischii over the gemellus, the tendon of the obturator internus, and the quadratus femoris. In its descent in the thigh it lies between the biceps femoris externally and the adductor, semimembranosus, and semitendinosus internally, and it is continued between the two heads of the gastrocnemius as the tibial nerve. Its chief branches are as follows:

1. In the pelvic part of its course the sciatic nerve supplies small branches to the obturator internus, gemellus, and quadratus femoris; the branch to the obturator internus reaches the muscle by passing through the anterior end of the lesser sacro-sciatic foramen.

2. As the sciatic nerve turns down behind the hip joint it gives off a large branch (Ramus muscularis proximalis) which divides to supply the semimembranosus and the short heads of the biceps femoris and semitendinosus.

3. The =posterior cutaneous nerve= of the leg or =external saphenous nerve= (N. cutaneus suræ posterior) is detached from the sciatic about the middle of the thigh. It receives a fasciculus from the peroneal nerve and descends with the recurrent tarsal vein on the outer face of the gastrocnemius to the distal third of the leg. Here it perforates the deep fascia and ramifies under the skin on the outer surface of the tarsus and metatarsus (Fig. 525).