A text-book of veterinary anatomy

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).

4. The peroneal or external popliteal nerve (N. peronæus) (Figs. 455, 460, 527) is a large trunk which arises from the great sciatic nerve very shortly after the latter emerges from the pelvic cavity. It descends with the parent trunk to the origin of the gastrocnemius; here the peroneal nerve deviates outward and forward across the external face of the gastrocnemius under cover of the biceps femoris, and divides at the origin of the lateral extensor muscle into superficial and deep branches. The collateral branches include a twig to the biceps femoris, the reinforcing fasciculus detached about the middle of the thigh to the posterior cutaneous nerve, and, lower down, the external cutaneous nerve of the leg (Ramus cutaneus suræ lateralis). The latter, also known as the peroneal cutaneous nerve, emerges between the middle and posterior divisions of the biceps femoris at the level of the stifle joint and ramifies under the skin. The superficial peroneal or musculo-cutaneous nerve (N. peronæus superficialis) furnishes branches to the lateral extensor, and descends in the furrow between that muscle and the anterior extensor, perforates the deep fascia of the leg, and ramifies under the skin on the front and the outer face of the tarsus and metatarsus. The deep peroneal or anterior tibial nerve (N. peronæus profundus) is the direct continuation in point of size of the peroneal trunk. It dips in between the lateral and anterior extensors of the digit, gives branches to these muscles and the tibialis anterior, and descends in front of the intermuscular septum which separates the former. It continues downward behind the tendon of the anterior extensor and divides on the front of the hock into internal and external branches. The internal branch (Ramus medialis) passes down under the skin on the anterior face of the metatarsus and supplies the cutaneous fibers of this region. The external branch (Ramus lateralis) furnishes a twig to the extensor brevis muscle and descends with the great metatarsal artery. It supplies the skin on the outer face of the metatarsus and the fetlock.

In the greater part of its course in the leg the deep peroneal nerve is separated from the anterior tibial vessels by the tibialis anterior (muscular part of the flexor metatarsi). It should be noted that the outer of the two veins which almost always accompany the artery here is frequently very large, and the layer of muscle which intervenes between it and the nerve is often exceedingly thin. In the distal part of the leg the nerve is in direct contact with the vein, and on the front of the hock it lies behind the vessels.

Tibial Nerve

The tibial nerve (N. tibialis), also termed the posterior tibial, is the direct continuation of the great sciatic nerve (Figs. 451, 455, 458, 459, 527). It passes down between the two heads of the gastrocnemius, and accompanies the recurrent tibial vessels to the distal third of the leg, where it divides into internal and external plantar nerves. In the upper third of the leg it lies along the inner side of the superficial flexor under cover of the internal head of the gastrocnemius; lower down it is covered by the common deep fascia and is situated in the space between the deep flexor and the inner border of the tendo Achillis.

At the usual point of election for tibial neurectomy, i. e., about a handbreadth above the level of the tuber calcis, the nerve lies in areolar tissue and fat in a fascial compartment formed by the special fascia of the deep flexor in front and by the common fascia and the accessory or tarsal tendon of the semitendinosus and biceps femoris behind and internally.

At its origin the tibial nerve gives off a muscular branch (Ramus muscularis distalis), the divisions of which pass between the two heads of the gastrocnemius and radiate to supply that muscle, the popliteus, the soleus, and the flexors of the digit. Small cutaneous twigs are also detached along the course of the nerve.

Plantar Nerves

The plantar nerves, internal and external (N. plantaris medialis, lateralis), result from the bifurcation of the tibial nerve in the lower part of the leg (Figs. 458, 459). They continue at first in the same direction and relations as the parent trunk, in direct apposition and inclosed in a common sheath. At the hock they diverge at a very acute angle and descend in the tarsal canal behind the deep flexor tendon in company with the plantar arteries. The internal plantar nerve supplies cutaneous nerves to the inner aspect of the tarsus and metatarsus, descends along the inner border of the flexor tendons behind the superficial plantar metatarsal vessels, and is otherwise arranged like the corresponding volar nerve of the fore limb. The external plantar nerve deviates outward between the two flexor tendons to reach their outer border. It supplies a branch to the suspensory ligament, and in its further course resembles the corresponding nerve of the fore limb.

In so-called “tibial” neurectomy the operator really cuts the two plantar nerves which have not yet separated. The anastomotic branch between the two plantar nerves is smaller than that which connects the corresponding nerves of the fore limb, and is absent in 30 per cent. of the cases, according to Rudert.

Sacral Nerves

Five pairs of sacral nerves (Nn. sacrales) are present in the horse.

The small dorsal branches emerge through the dorsal sacral foramina and the space between the sacrum and the first coccygeal vertebra, and ramify in the muscles and skin of the sacral region and the adjacent part of the tail (Fig. 525). The fifth anastomoses with the dorsal branch of the first coccygeal nerve.

The ventral branches leave the vertebral canal through the ventral sacral foramina and the interval between the sacrum and first coccygeal vertebra. They are connected with the sympathetic by rami communicantes, and contribute branches to the pelvic plexus. The first and second are the largest, and unite with each other and with those of the last three lumbar nerves to form the lumbo-sacral plexus. The third and fourth are connected with each other, and the majority of their fibers go to form the pudic and posterior hæmorrhoidal nerves.

The pudic nerve (N. pudendus) (Figs. 451, 455, 526) accompanies the internal pudic artery to the ischial arch, turns around the latter, parting company with the artery, and pursues a flexuous course along the dorsum penis as the nervus dorsalis penis and ramifies in the glans and the penile layer of the prepuce. Within the pelvis it anastomoses with the posterior hæmorrhoidal nerve, and gives branches to the bladder and urethra, the terminal part of the rectum, and the skin and muscles of the anus (Fig. 452). It also supplies the nerve to the ischio-cavernosus muscle and numerous branches to the corpus cavernosum and corpus spongiosum. In the female it terminates in the clitoris and vulva (Fig. 453).

The posterior hæmorrhoidal nerve (N. hæmorrhoidalis caudalis) passes downward and backward above the pudic nerve, with which it anastomoses. It gives twigs to the terminal part of the rectum, the sphincter ani externus, and the surrounding skin (Fig. 452). In the female it supplies twigs to the vulva also (Fig. 453).

The ventral branch of the fifth nerve is small. It gives twigs to the sacro-coccygeus ventralis lateralis and the skin of the root of the tail and joins the first coccygeal nerve.

Coccygeal Nerves

The coccygeal nerves (Nn. coccygei) commonly number five pairs. Their dorsal and ventral branches anastomose to form respectively two trunks on either side, which extend to the tip of the tail and supply its muscular and cutaneous nerves. The dorsal trunk runs with the dorso-lateral artery between the sacro-coccygeus dorsalis and intertransversales muscles (Fig. 454). The ventral trunk accompanies the ventro-lateral artery below the intertransversales.

The Sympathetic Nervous System^[210]

The sympathetic nervous system (Systema nervorum sympathicum) is that part of the nervous system which serves (1) to transmit stimuli to the heart-muscle, unstriped muscle, and glands; (2) to conduct impulses from the viscera to the cerebrospinal system.

Many of the fibers are derived from the cerebrospinal system and are rearranged and distributed in the sympathetic system. Numerous sympathetic fibers are contributed to the cerebrospinal nerves, through which they are distributed to the unstriped muscular tissue and glands, as vasomotor, pilomotor, and secretory nerves. The sympathetic, like the cerebrospinal system, consists of neurones, each of which comprises the cell-body, an axone, and numerous branched dendrites. The cell-bodies are aggregated into ganglia, some of which are large and more or less constant in position and form, while others are microscopic; and are scattered in an irregular manner through the peripheral part of the system. It is probable that simple visceral reflexes may be mediated by sympathetic neurones alone.

In descriptive anatomy the sympathetic system is regarded as consisting of (1) a chain of ganglia extending along either side of the vertebral column and connected by association fibers to form the sympathetic trunk; (2) central branches to and from the cerebrospinal nerves; (3) peripheral branches, which form plexuses with each other and the cerebrospinal nerves; (4) the peripheral ganglia which are interposed in the plexuses.

The sympathetic trunk (Truncus sympathicus) extends on either side from the base of the cranium to the tail. In it are interposed, at intervals of varying regularity, the ganglia of the sympathetic trunk (Ganglia trunci sympathici). These are connected with the cerebrospinal nerves by central branches, the rami communicantes.

Two kinds of rami communicantes occur. Of these, one type consists largely of medullated fibers derived from the spinal nerves and ganglia; they have therefore a white appearance, and are termed white rami. They contain both efferent and afferent fibers. The efferent splanchnic fibers are derived from the ventral roots of the spinal nerves and terminate in great part about the cells of the nearest sympathetic ganglion; others end in more distant or in peripheral ganglia. The afferent splanchnic fibers are chiefly peripheral processes of the cells of the spinal ganglia, but some are sympathetic fibers which enter the spinal nerve-trunk and terminate about cells of the spinal ganglion. The gray rami consist mainly of non-medullated fibers derived from the sympathetic ganglia directly or through the trunk, which proceed centrally to the spinal nerves and are distributed along the somatic divisions of the latter to unstriped muscle and glands as vasomotor, pilomotor, and secretory fibers. Some go to the membranes of the spinal cord, and a few terminate about cells of the spinal ganglia as sensory sympathetic fibers.

Similar but more complex and irregular communications which exist between the sympathetic system and the cranial nerves—with the exception of the first and second—have been referred to in the accounts of the latter.

It is convenient for descriptive purposes to divide the sympathetic system into cephalic, cervical, thoracic, abdominal, and pelvic parts.

1. The cephalic part (Pars cephalica systematis sympathici) comprises the otic, sphenopalatine, and ciliary ganglia, which may be regarded as homologues of the ganglia of the trunk of other regions. It also includes three plexuses formed by branches derived from the superior cervical ganglion. The ganglion gives off two or three filaments which subdivide to form the internal carotid plexus around the artery of like name. The cavernous plexus surrounds the artery within the cavernous sinus and communicates with that of the opposite side. The external carotid plexus is formed around the homonymous artery, and filaments from it go to the vessel and its branches and to the salivary glands.

2. The cervical part (Pars cervicalis s. sympathici) includes two ganglia and the trunk which connects them.

The superior or anterior cervical ganglion (G. cervicale craniale) lies on the guttural pouch below the occipito-atlantal articulation (Fig. 437). It is reddish-gray in color, fusiform, and about an inch (ca. 2 to 3 cm.) in length. It is connected by rami communicantes with the last four cranial and first cervical nerves, sends branches to form the plexuses mentioned above, and contributes to the pharyngeal plexus.

The cervical trunk of the sympathetic connects the superior and inferior cervical ganglia. On leaving the former it is associated in a common sheath with the vagus along the dorsal face of the common carotid artery. At the root of the neck it separates from the vagus and joins the inferior ganglion.

The inferior or posterior cervical ganglion (G. cervicale caudale) is situated under the first rib and the insertion of the scalenus (Figs. 428, 429). On the right side it lies upon the longus colli and the trachea, on the left side upon the same muscle and the œsophagus. It is flattened, very irregular and variable in outline, and is blended more or less with the first thoracic ganglion. It is connected by large gray rami with the ventral branches of the last two cervical nerves, and by small filaments with the recurrent nerve. It receives a nerve—termed by Schmaltz the nervus transversarius—which accompanies the vertebral artery; through it rami come from the second to the sixth cervical nerves inclusive.

A middle cervical ganglion may be present, and occurs oftenest on the right side. It lies on the carotid artery an inch or two in front of the first rib and is intercalated in the vago-sympathetic trunk.

The cardiac nerves (Nn. cardiaci) are formed by branches from the inferior cervical and first thoracic ganglia, together with twigs from the sympathetic trunks and the vagi. They form a plexus (P. cardiacus) on the ventral face of the trachea with each other and with branches of the vagus and recurrent nerves. They are subject to considerable variation, but their general arrangement may be stated as follows:

(a) On the right side there are usually two cardiac nerves. Of these one passes back with the vagus in the angle between the right brachial artery and the common carotid trunk, pierces the pericardium, crosses the aortic arch, and divides into branches which are mingled with those of the left nerves. The second crosses obliquely over the right face of the trachea and joins the vagus, where the latter gives off the right recurrent nerve; a small plexus is formed here, from which two or three cardiac branches are detached. These pass back beneath the trachea and ramify on the atria and ventricles.

(b) On the left side there are commonly three cardiac nerves. One of these is distributed to the great vessels in the anterior mediastinum. The largest passes back at first with the vagus beneath the arch of the left brachial artery, inclines downward, perforates the pericardium, and divides into two branches. One branch passes beneath the bifurcation of the pulmonary artery and is distributed to the left atrium. The larger branch dips in between the aorta and the pulmonary artery, gives twigs to these vessels, and ramifies on the ventricles, especially along the course of the right coronary artery. The third nerve crosses the deep face of the left brachial artery, passes back below the trachea, and unites with filaments of a right cardiac nerve. It passes around the aorta and ramifies chiefly along the course of the left coronary artery on the left face of the ventricles.

3. The thoracic part (Pars thoracalis systematis sympathici) extends below the costo-vertebral joints from the inferior cervical ganglion to the upper border of the diaphragm, and passes between the latter and the psoas minor to be continued by the abdominal part.

The trunk is concealed in the first part of its course by the outer border of the longus colli, but further back it is visible under the pleura.

The ganglia are arranged segmentally at each intercostal space and partly on the heads of the ribs. They are flattened and are small and fusiform, with the exception of the first. This (G. thoracale primum) is extensive, irregularly quadrilateral in outline, and is united with the inferior cervical ganglion. The ganglia are connected with the thoracic nerves by white and gray rami communicantes.

The visceral branches comprise aortic, cardiac, pulmonary, and œsophageal branches, and the splanchnic nerves. The aortic branches ramify on the thoracic artery, forming around that vessel the thoracic aortic plexus (P. aorticus thoracalis). The cardiac branches concur with those of the vagus in forming the cardiac plexus (P. cardiacus). From this branches go to form the coronary plexuses (P. coronarii) along the course of the vessels of like name. The pulmonary branches join with corresponding branches of the vagus and filaments from the cardiac plexus in forming the pulmonary plexus (P. pulmonalis) at the root of the lung. Branches of the plexus, on which are minute ganglia, ramify with the bronchi in the substance of the lung.^[211] The œsophageal branches join with those of the vagus in the formation of the œsophageal plexus (P. œsophageus).

The great splanchnic nerve (N. splanchnicus major) arises by a series of roots derived from the sixth or seventh to the fifteenth or sixteenth thoracic ganglia inclusive. It extends at first along the inner side of the thoracic trunk, then crosses the latter ventrally, passes back between the crus of the diaphragm and the outer border of the psoas minor, and joins the cœliac ganglion. It is small at its origin, but becomes considerably larger than the sympathetic trunk. Near its termination it bears the small splanchnic ganglion, from which and from the nerve, filaments go to the aorta, the œsophagus, and the vertebræ.

The small splanchnic nerve (N. splanchnicus minor) is formed by roots derived from the last two or three thoracic ganglia. It runs back with the great splanchnic nerve, but ends in the cœliac or in the renal and adrenal plexuses.

This nerve is not always present and is variable in arrangement. The roots communicate by filaments with the great splanchnic.

4. The abdominal part of the sympathetic trunk (Pars abdominalis systematis sympathici) lies along the inner border of the psoas minor, above the aorta on the left side and the posterior vena cava on the right (Fig. 524). The trunk is smaller than the thoracic part, and presents usually six small fusiform lumbar ganglia (G. lumbalia), which are connected by rami communicantes with the ventral divisions of the lumbar nerves. Visceral branches go to the aortic and hypogastric plexuses, and to the cœliac and mesenteric ganglia.

5. The pelvic part of the sympathetic trunk (Pars pelvina systematis sympathici) begins at the last lumbar ganglion and extends along the pelvic surface of the sacrum internal to the emergence of the ventral branches of the sacral nerves (Fig. 451). At the third segment of the sacrum the trunk divides into internal and external branches. The internal branch inclines to the median plane and unites with the opposite branch. At the junction there is often found the small coccygeal ganglion (G. impar), which lies on the coccygeal artery at the joint between the first and second coccygeal vertebræ. A filament from the ganglion accompanies the artery. The external branch communicates with the last two sacral nerves and joins the ventral coccygeal nerves. Ganglia occur near each of the first three sacral foramina, and are connected by gray rami communicantes with the adjacent spinal nerves. The visceral branches are distributed through the hypogastric plexus. They supply motor fibers to the longitudinal and inhibitory fibers to the circular muscular coat of the rectum, motor fibers to the bladder and uterus, and the vasodilator fibers (Nn. erigentes) to the penis.

THE ABDOMINAL AND PELVIC PLEXUSES

The chief plexuses which distribute nerves to the viscera and vessels of the abdominal and pelvic cavities are two in number, the solar and the pelvic. From them fibers proceed to form numerous subsidiary plexuses which are named according to the organs which they supply or the vessels which they enlace.

The Solar Plexus

The solar plexus (Fig. 450) is situated on the dorsal wall of the abdominal cavity in relation to the aorta and the origin of its chief visceral branches. It comprises the cœliaco-mesenteric ganglia and the cœliac and anterior mesenteric plexuses.

The cœliaco-mesenteric ganglia^[212] are two in number, right and left. They are situated on each side of the aorta, in relation to the origin of the cœliac and anterior mesenteric arteries. The right ganglion is concealed by the posterior vena cava; it is irregularly quadrilateral and is about two inches (4 to 6 cm.) in length. The left ganglion is largely covered by the left adrenal; it is narrower than the right one and is three or four inches (ca. 8 to 10 cm.) long. The two ganglia are united by irregular connecting branches in front of and behind the anterior mesenteric artery.^[213] Each receives the splanchnic nerve of its own side and branches from the dorsal œsophageal continuation of the vagus nerves. Branches from the ganglia and the cords which connect them pass back to the small stellate posterior mesenteric ganglia, which are situated at the origin of the posterior mesenteric artery.

The cœliac plexus (P. cœliacus) consists of a feltwork of nerve-fibers in which are small ganglia. It surrounds the cœliac artery and from it proceed the following unpaired secondary plexuses which accompany the arterial branches:

1. The gastric plexus (P. gastricus) enlaces the gastric artery and divides like the artery into two parts, forming the anterior and posterior gastric plexuses. These receive branches of the vagus nerves.

2. The hepatic plexus (P. hepaticus) is formed by several nerves of considerable size which accompany the hepatic artery and the portal vein. It receives fibers from the left vagus, ramifies in the liver, and gives off branches which accompany the collateral branches of the hepatic artery and supply the areas in which these vessels are distributed.

3. The splenic plexus (P. lienalis) resembles the preceding in its arrangement. In addition to its terminal branches to the spleen, it gives collateral twigs to the pancreas and the left part of the greater curvature of the stomach.

The anterior mesenteric plexus (P. mesentericus cranialis) is formed mainly by branches from the posterior part of the cœliaco-mesenteric ganglia; it is continuous with the cœliac plexus in front and the posterior mesenteric behind. It surrounds the anterior mesenteric trunk and its branches, and supplies the viscera to which these vessels are distributed.

The posterior mesenteric plexus (P. mesentericus caudalis) accompanies the artery of like name in its distribution.

The secondary plexuses which accompany the branches of the mesenteric arteries give off branches which form two fine peripheral plexuses in the wall of the intestine. One of these, the myenteric plexus (P. myentericus) or plexus of Auerbach, lies between the layers of the muscular coat, and is provided with microscopic ganglia. The other is in the submucous tissue, and is therefore termed the submucous plexus (P. submucosus) or plexus of Meissner.

The following paired plexuses are derived mainly from branches of the cœliac and aortic plexuses:

1. The renal plexuses (P. renales) proceed largely from the cœliaco-mesenteric ganglia, but receive fibers also from the small splanchnic nerves. They enlace the renal arteries and supply the kidneys. Minute renal ganglia occur on the course of the nerves along the renal vessels.

2. The adrenal plexuses (P. suprarenales) are formed by a relatively very large number of fine fibers derived in great part directly from the cœliaco-mesenteric ganglia. Numerous minute ganglia occur in their meshes.

3 a. The spermatic plexuses (P. spermatid) proceed largely from the aortic and renal plexuses. Each accompanies the corresponding (internal) spermatic artery to the testicle.

3 b. The utero-ovarian plexuses (P. ovarici) are the homologues in the female of the preceding. They accompany the utero-ovarian arteries to the ovary and the cornua of the uterus.

The hypogastric or pelvic plexus (P. hypogastricus) is the pelvic continuation of the aortic and posterior mesenteric plexuses. The chief nerves which form it enter the pelvic cavity ventral to the large vessels, anastomose with each other and with branches from the third and fourth sacral nerves, and ramify on the pelvic viscera. The peripheral plexuses derived therefrom are named according to the organs which they supply; the chief of these are the hæmorrhoidal, vesical, uterovaginal, prostatic, cavernous (of the penis or clitoris).

THE NERVOUS SYSTEM OF THE OX^[214]

The spinal cord resembles that of the horse in conformation and structure. In cattle of medium size its length is about 165 to 170 cm. (ca. 65 to 67 inches) and its weight about 240 to 250 grains (ca. 8 ounces).

In a cow 140 cm. in height Dexler found the weight of the cord (including the intradural nerve-roots) to be 260 g. and the length 162 cm. The lengths of the regions were: cervical 41 cm., thoracic 72 cm., lumbar 32 cm., sacral 7 cm.

The brain has an average weight of about 500 grams (ca. 16 to 17 ounces). Its differences in general form are correlated with those noted in the consideration of the cranial cavity.

The medulla is short, wide, and thick. Its ventral face is strongly convex. The pyramids are narrow, short, and close together in front. Near the decussation there is a well defined oval prominence on either side which indicates the position of the posterior olive. The corpus trapezoideum is large; it has no central part between the pyramids. The external arcuate fibers are distinct. The restiform bodies are short and thick and diverge more strongly than in the horse. The floor of the fourth ventricle is only about two-thirds as long as in the horse. The area acustica is a well-marked oval prominence lateral to the middle part of the limiting sulcus. The tuberculum acusticum at the origin of the auditory nerve is very large. The anterior fovea is distinct. The posterior recess of the fourth ventricle communicates through the foramen of Magendie (Apertura mediana) with the subarachnoid space.

The pons is smaller, both transversely and longitudinally, than that of the horse. It is strongly convex and has a distinct central depression.

The cerebellum is smaller and more angular in form than in the horse. The vermis is large and has a distinct depression on its anterior face for the posterior corpora quadrigemina. The hemispheres are relatively small and are not clearly divided into tabulations. The anterior peduncles are very short.

The cerebral peduncles are short. A small eminence in the locus perforatus posticus is caused by the presence of the interpeduncular ganglion. The internal geniculate body is prominent. The pineal body is long and fusiform, and is often pigmented in spots. The third ventricle forms two considerable recesses in relation to the pineal body: one (R. pinealis) extends up into the body; the other (R. suprapinealis) is a long tubular prolongation in front of it.

The optic tracts cross the cerebral peduncles almost at a right angle.

The pituitary body is situated in a deep fossa and is surrounded by a plexus of vessels.

The cerebral hemispheres are shorter, higher, and relatively wider than in the horse. The frontal poles are small, the occipital large. Viewed from the side, the dorsal surface is strongly convex. The highest point of the dorso-medial border is a little in front of its middle and forms a marked prominence termed the sagittal or marginal pole (Polus sagittalis s. prominentia marginalis dorsalis). Anterior to this the border drops abruptly, being cut into by the deep transverse fissure. The arachnoid on the basal and anterior parts of the hemispheres is usually pigmented. The corpus callosum extends through a little more than a third of the length of the hemisphere. The pattern of the fissures and gyri of the pallium is somewhat simpler than in the horse.

1. The lateral fissure (of Sylvius) is very deep. Its middle branch extends almost vertically upward on the middle of the lateral surface of the hemisphere, and is separated by a gyrus of variable width from the suprasylvian fissure. The anterior branch runs forward about parallel with the sulcus rhinalis anterior, from which it is separated by the short gyri of the insula. The latter are covered to a small extent only by the overhanging gyri (operculum). The posterior branch may run back a distance of only about 1.5 cm. and end in T-shaped manner, or it may join the sulcus rhinalis posterior.

2. The suprasylvian fissure is deep and very distinct. It extends in an undulating manner from the lateral surface of the occipital pole to the outer side of the sagittal pole. Here it may be interrupted or may be continued by the coronal fissure, which descends to the frontal pole and divides into two short branches.

3. The diagonal fissure (S. diagonalis) begins in front of the stem of the lateral fissure and runs upward and forward. Its form is very variable.

4. The transverse fissure cuts obliquely into the dorsal border in front of the sagittal pole. It is short and deep and commonly communicates with the suprasylvian and coronal fissures.

5. The marginal fissure is distinct. It extends from the sagittal pole to the occipital pole. It is flexuous and deviates outward somewhat in its posterior two-thirds.

6. The entomarginal fissure lies medial and parallel to the posterior part of the preceding.

7. The ectomarginal fissure is lateral and parallel to the marginal fissure. It does not extend so far in either direction as the latter and is often more or less broken up by annectent gyri.

8. The sulcus rhinalis is more open than in the horse, since it is not overlapped by the gyri above it.

9. The calloso-marginal fissure is flexuous and not so regular as in the horse. It is often interrupted at or near the level of the genu of the corpus callosum, in which case the genual fissure continues it downward.

10. The crucial fissure is faint and is confined to the medial surface. It lies about one centimeter in front of the transverse fissure and commonly joins the calloso-marginal fissure below.

11. Short and variable fissures occur in front of and behind the middle branch of the lateral fissure (F. ectosylvia anterior, posterior).

The olfactory bulb is much smaller than in the horse. The external olfactory tract is large, the inner one small and not distinct.

The cranial nerves have in general the same superficial origin as in the horse. The more important differences in other respects are as follows:

The oculomotor nerve is larger than in the horse. It emerges with the fourth, sixth, ophthalmic, and maxillary nerves through the large foramen, which is equivalent to the foramen lacerum orbitale and foramen rotundum of the horse.

The internal branch of the lacrimal nerve supplies twigs to the mucous membrane of the frontal sinus; the large outer branch runs backward and supplies the matrix of the horn. The frontal nerve emerges from the orbit below the supraorbital process. The naso-ciliary nerve is large and sends filaments to the ocular muscles. The ciliary ganglion is somewhat larger than in the horse and is connected with the lower division of the oculomotor nerve by several short filaments. In consequence of the absence of the canine and upper incisor teeth the corresponding branch of the infraorbital nerve is naturally wanting. The mandibular nerve emerges through the foramen ovale. The superficial temporal nerve gives off a large branch which joins the superior buccal division of the facial on the masseter, about half-way between the zygomatic arch and the angle of the jaw. The otic ganglion is larger than in the horse. According to Moussu the buccinator nerve furnishes excito-secretory twigs to the parotid and inferior buccal glands.

The facial nerve divides into its two terminal branches before reaching the border of the jaw. The superior buccal nerve is the larger of the two; it crosses the masseter much lower than in the horse. The relatively small inferior buccal nerve runs beneath the parotid or in the gland substance parallel with the border of the lower jaw, crosses under the insertion of the sterno-cephalicus, and runs forward along the depressor labii inferioris. At the point where it crosses the facial vein and parotid duct it gives off an anastomotic branch to the superior nerve. The auriculo-palpebral nerve is large.

The vagus bears—in addition to the relatively large jugular ganglion—a ganglion nodosum at the point of origin of the superior laryngeal nerve. The trunk is large. The pharyngeal branch is large and anastomoses with the superior and external laryngeal nerves. The latter commonly arises directly from the trunk. The superior œsophageal trunk communicates with the splanchnic nerve, contributes twigs to the hepatic plexus, and ramifies chiefly on the right surface of the rumen and the adjacent surface of the abomasum. The inferior œsophageal trunk supplies branches to the hepatic plexus and to all the divisions of the stomach.

The spinal accessory nerve presents two special features. The part which joins the jugular ganglion bears a small ganglion. The ventral branch supplies both parts of the sterno-cephalicus.

The hypoglossal nerve is large. It is connected with the ventral division of the first cervical nerve by a branch of considerable size, and detaches a long branch which runs backward along the carotid artery.

The spinal nerves resemble those of the horse in origin and general arrangement. The thoracic nerves number thirteen pairs. The more important differences in the limb-plexuses and their branches are as follows:

The brachial plexus is formed by the ventral branches of the last three cervical and first thoracic nerves; the second thoracic nerve furnishes no root, but the ventral branch of the sixth cervical goes almost entirely to the plexus after giving off the nerve to the rhomboideus and the cervical part of the serratus magnus.

The differences in the nerves above the elbow are not of sufficient moment to receive notice in this brief account, but below this point there are naturally important special features correlated chiefly with the arrangement of the digits.

The radial nerve is continued below the elbow by a large cutaneous branch (N. cutaneus antibrachii dorsalis) which emerges at the lower border of the external head of the triceps and descends on the dorsal aspect of the limb. It communicates above the carpus with the cutaneous branch of the median nerve and terminates in three dorsal digital nerves; two of these descend along the axial or interdigital side of the dorsal surface of the chief digits, and the third along the inner (abaxial) side of the inner chief digit.