Chapter V: Muscles of the Leg and Foot (21)

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.

FIG. 527.—CROSS-SECTION OF DISTAL THIRD OF LEFT LEG OF HORSE.

The deep fascia is shown by red line.
]

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.

FIG. 528.—BRAIN OF COW, DORSAL VIEW.

_1_, Sulcus transversus; _2_, longitudinal fissure; _3_,
antero-internal frontal gyrus; _4_, olfactory bulb; _5_, marginal or
sagittal pole of hemisphere; _6_, lateral fissure (of Sylvius); _7_,
hemisphere of cerebellum; _8_, vermis of same; _9_, accessory nerve;
_10_, dorsal root of first cervical nerve. (Ellenberger-Baum, Anat.
d. Haustiere.)
]

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.

FIG. 529.—CEREBRAL HEMISPHERES OF OX, DORSAL VIEW.

_a_, Transverse fissure; _b_, lateral fissure (of Sylvius); _c_,
coronal fissure; _d_, diagonal fissure; _e_, entomarginal fissure;
_f_, marginal fissure; _g_, ectomarginal fissure; _h_, suprasylvian
fissure. (After Ellenberger, in Leisering’s Atlas.)
]

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.

FIG. 530.—NERVES OF DISTAL PART OF RIGHT FORE LIMB OF OX, ANTERIOR
VIEW (SCHEMATIC).

_p_, Dorsal digital nerves.
]

FIG. 531.—NERVES OF DISTAL PART OF RIGHT FORE LIMB OF OX, POSTERIOR
VIEW (SCHEMATIC).

_p_, Volar digital nerves.
]

The =ulnar nerve= divides at a variable distance down the forearm into two branches. The =dorsal= or =superficial branch= emerges between the tendons of the external and middle flexors of the carpus, and is continued as the =external dorsal digital nerve= on the outer chief digit. The =volar= or =deep branch= descends along the superficial digital flexor, gives a branch to the suspensory ligament below the carpus, and unites with the external branch of the median nerve to form the =external volar digital nerve=.

The =median nerve= is much larger than the ulnar. It passes beneath the pronator teres, descends the forearm as in the horse, and divides in the distal part of the metacarpus into two branches. The =internal branch= gives twigs to the inner small digit and is continued on the inner side of the volar surface of the inner chief digit as the =internal volar digital nerve= (N. digitalis medialis digiti III); it also concurs with the outer branch in forming the two digital nerves which descend along the interdigital aspect of the chief digits. The =external branch= is larger. It bifurcates, and one division unites with the twig from the internal branch to form a common digital trunk. From the latter two digital nerves proceed as mentioned above; these are the =external= and =internal volar digital nerves= of the internal and external chief digits respectively (N. digitalis volaris lateralis digiti III, medialis digiti IV). The other division unites with the volar branch of the ulnar nerve to form the =external volar digital nerve= of the external chief digit (N. digitalis lateralis digiti IV).

FIG. 532.—NERVES OF DISTAL PART OF RIGHT HIND LIMB OF OX, ANTERIOR
VIEW (SCHEMATIC).

_p_, Dorsal digital nerves.
]

FIG. 533.—NERVES OF DISTAL PART OF RIGHT HIND LIMB OF OX, POSTERIOR
VIEW (SCHEMATIC).

_p_, Plantar digital nerves.
]

The =lumbo-sacral plexus= and its branches to the pelvis and thigh present no very striking special features.

The =superficial peroneal nerve= is much larger than in the horse. After crossing beneath the peroneus longus it passes down on the front of the tarsus and metatarsus and divides into three branches. The internal and external branches descend as the =internal= and =external dorsal digital nerves= on the chief digits. The larger middle branch joins a branch of the deep peroneal nerve in the interdigital space, and from this union proceed the =dorsal digital nerves= which descend on the opposed surfaces of the chief digits.

The =deep peroneal nerve= descends in the leg as in the horse and continues down the anterior groove of the metatarsus with the dorsal metatarsal artery. Its terminal branches concur with branches from the superficial peroneal nerve in the formation of the two axial =dorsal digital nerves= (N. dig. dors. lat. dig. III et dig. dors. med. dig. IV pedis), and with a branch of the internal plantar nerve in the formation of corresponding plantar digital nerves.

The =tibial nerve= divides at the back of the hock into internal and external plantar nerves. The =internal plantar nerve= descends between the superficial flexor tendon and the suspensory ligament and divides into two branches; the inner branch descends as the =internal plantar digital nerve= (N. dig. plant. med. dig. III) along the inner side of the flexor tendons of the inner chief digit; the outer branch turns around the flexor tendons to reach the interdigital space, where it concurs with a branch of the deep peroneal nerve in the formation of two =axial plantar digital nerves= (N. dig. plant. lat. dig. III et dig. plant. med. dig. IV) which descend on the opposed surfaces of the chief digits. The =external plantar nerve= descends along the outer border of the flexor tendons, gives a branch to the suspensory ligament and to the external small digit, and continues along the outer face of the external digit as the =external plantar digital nerve= (N. dig. plant. lat. dig. IV).

The =sympathetic system= closely resembles that of the horse in its general arrangement, and only a few differential features will receive attention. The =superior cervical ganglion= is closer to the cranial base and is thicker than in the horse; its branches to the carotid and cavernous plexuses are large, and no connection is formed with the spinal accessory nerve. The =cervical trunk= is smaller than in the horse, and arises by two or three bundles from the superior cervical ganglion behind its middle. The =inferior cervical ganglion= is distinctly separable from the first thoracic; the latter is large. There are thirteen pairs of =thoracic ganglia=. The =cœliaco-mesenteric ganglia= are plexiform, not compact. There are five pairs of =sacral ganglia=, and the right and left trunks are connected here by transverse anastomoses.

THE NERVOUS SYSTEM OF THE PIG

The =spinal cord= weighs about 42 grams (Ellenberger-Baum). It is almost circular in cross-section, except at the enlargements, where it is somewhat flattened dorso-ventrally. The conus medullaris extends to the anterior part of the third sacral segment. The epidural space is occupied by a large quantity of fat.

The =brain= in adults of medium size weighs about 125 grams (ca. 4 to 4½ ounces). When viewed from above, it is seen to have an elongated oval form. The occipital poles overlie the cerebellum to a small extent only; the frontal poles are rounded and relatively large, but do not conceal the ends of the olfactory bulbs. The =medulla= is relatively broad. The cuneate tubercle is very large and is limited externally by a groove. The corpus trapezoideum is very wide laterally. The =pons= is less prominent than in the ox. The =cerebellum= is very wide and short. Its anterior face is flattened and presents a depression for the corpora quadrigemina. The vermis is large. The hemispheres consist of an inner large and an outer small part. The =cerebral peduncles= are very short. The posterior corpora quadrigemina are wide apart, rounded, and relatively large. The internal geniculate body is prominent. The =cerebral hemispheres= are somewhat bean-shaped in lateral profile, the convex border being dorsal. The arrangement of the gyri and sulci is simpler than in the horse or ox. The principal fissures are as follows:

1. The =lateral fissure= (of Sylvius) begins at the sulcus rhinalis and runs upward and somewhat backward on the depressed part of the lateral surface of the hemisphere; it appears unbranched on superficial examination, but when the gyrus which almost completely conceals the insula is raised, an anterior branch is exposed which forms the dorsal boundary of the insula.

2. The =suprasylvian fissure= pursues a curved course approximately parallel with the dorso-medial border of the hemisphere. Its posterior end is separated by a short interval from the sulcus rhinalis posterior. From its highest point a branch proceeds obliquely upward and forward, crosses the dorso-medial border, and joins the splenial fissure.

3. The =diagonal fissure= crosses the anterior part of the lateral surface. It is directed obliquely downward and backward.

4. The =coronal fissure= is constant and deep. It is directed upward and inward over the frontal pole, continues backward near the dorso-medial border, and usually ends by passing obliquely over the border to the inner surface of the hemisphere.

5. The =presylvian fissure= begins—apparently as a branch of the sulcus rhinalis anterior—about 2 cm. in front of the lateral fissure, and curves over the lateral aspect of the frontal pole.

FIG. 534.—CEREBRAL HEMISPHERES OF PIG, DORSAL VIEW.

_a_, Upper part of lateral fissure (of Sylvius); _b_, cruciate
fissure; _c_, suprasylvian fissure, with its anterior branch (_c′_)
and upper branch (_c″_); _d_, diagonal fissure; _e_, coronal
fissure; _f_, presylvian fissure; _g_, marginal fissure; _h_,
ectomarginal fissure; _i_, entomarginal fissure. (After Ellenberger,
in Leisering’s Atlas.)
]

6. The =marginal fissure= begins behind the cruciate fissure close to the dorso-medial border and extends in a gentle curve backward almost to the occipital pole.

7. The =ectomarginal fissure= lies above and nearly parallel to the posterior part of the suprasylvian fissure.

8. The =entomarginal fissure= lies along the posterior part of the dorso-medial border.

9. The =calloso-marginal fissure= consists of two separate parts. The more extensive posterior part is termed the =splenial fissure=. It extends from the tentorial aspect of the hemisphere in a direction parallel with the corpus callosum nearly to the middle of the medial surface; it is continued by the cruciate fissure and is also connected with the suprasylvian and coronal fissures by branches which pass obliquely upward and forward. The anterior part, the =genual fissure=, lies about midway between the anterior part of the corpus callosum and the dorso-medial border.

10. The =cruciate fissure= is short and shallow. It runs obliquely upward and forward from the anterior end of the splenial fissure on the inner surface of the hemisphere.

11. The =sulcus rhinalis= extends in an undulating manner along the entire lower part of the lateral surface of the hemisphere and forms the upper limit of the rhinencephalon.

The =olfactory bulbs= are very large and the =tracts= extremely broad and short. The trigonum olfactorium is so prominent as to be termed the =tuberculum olfactorium=.

The =cranial nerves= present the following special features:

The =third=, =fourth=, =sixth=, and the =ophthalmic= and =maxillary divisions= of the fifth nerve, emerge together as in the ox.

The =lacrimal nerve= resembles that of the horse, the =frontal= that of the ox. The =naso-ciliary nerve= is relatively large and sends numerous filaments to the ocular muscles. The =maxillary nerve= has a very short course in the pterygo-palatine fossa. The =infraorbital nerve= is large in correlation with the development of the snout which receives numerous branches. The =mandibular nerve= emerges through the foramen lacerum anterius. The =superficial temporal nerve= is small; according to Moussu it furnishes the excito-secretory fibers to the upper part of the parotid gland, while those going to the lower part are derived from the mylo-hyoid nerve.

FIG. 535.—NERVES OF DISTAL PART OF RIGHT FORE LIMB OF PIG, ANTERIOR
VIEW (SCHEMATIC).

_c_, Dorsal common digital nerves; _p_, dorsal proper digital nerves.
]

FIG. 536.—NERVES OF DISTAL PART OF RIGHT FORE LIMB OF PIG, POSTERIOR
VIEW (SCHEMATIC).

_p_, Volar proper digital nerves.
]

The =inferior buccal nerve= passes downward and forward under cover of the parotid gland and accompanies the parotid duct, with which it turns around the lower border of the jaw in front of the masseter.

The =vagus= bears a =jugular ganglion= and a =ganglion nodosum=; the latter occurs at the point of origin of the superior laryngeal nerve, and may be as large as a small pea. Proximal to this the vagus is easily divided into two strands, one of which is the accessory component. The œsophageal trunks form a posterior œsophageal plexus, as in man, from which two nerves issue. The inferior nerve is small and ramifies on the parietal surface of the stomach. The superior trunk is much larger; it gives branches to the stomach, crosses the lesser curvature of that organ, and joins the solar plexus.

The =hypoglossal nerve= may present a small dorsal root, on which there is a minute hypoglossal ganglion (of Froriep).

The =spinal nerves= number on each side eight in the cervical region, fourteen (commonly) in the thoracic, seven in the lumbar, and four in the sacral. Some of the special features of the nerves of the limbs are as follows:

The =brachial plexus= is derived from the same nerves as in the ox, but the root furnished by the sixth cervical is relatively smaller. The plexus consists of two parts, the upper of which emerges above the scalenus, the lower between the two parts of that muscle. The more important differences in the nerves emanating from the plexus from the arrangement in the ox occur in the lower part of the limb. The cutaneous branch of the =radial nerve= divides at the carpus into branches which concur with the dorsal branch of the ulnar nerve in supplying the =dorsal digital nerves=, two for each digit. The =volar digital nerves=, also two for each digit, are formed by the terminal branches of the =median nerve= and the volar or deep branch of the =ulnar nerve=. The formation and arrangement of the digital nerves are indicated in the annexed schematic figures.

FIG. 537.—NERVES OF DISTAL PART OF RIGHT HIND LIMB OF PIG, ANTERIOR
VIEW (SCHEMATIC).

_p_, Dorsal proper digital nerves.
]

FIG. 538.—NERVES OF DISTAL PART OF RIGHT HIND LIMB OF PIG, POSTERIOR
VIEW (SCHEMATIC).

_p_, Plantar proper digital nerves.
]

The =lumbo-sacral plexus= is derived from the ventral branches of the last three lumbar and first sacral nerves. The distribution of the branches of the plexus differs chiefly in the distal part of the limb. The =superficial peroneal nerve= is large and terminates by forming the greater part of the =dorsal digital nerves=. The latter, two for each digit, are also in part formed by the terminal branches of the =deep peroneal nerve=.

The =tibial nerve= divides at the tarsus into =internal= and =external plantar nerves=. The latter divide to form the =plantar digital nerves=, two for each digit. In addition, the external plantar nerve supplies a branch to the dorsal aspect of the outer (fifth) digit. The arrangement of the digital nerves is indicated by the annexed schematic figures.

The =sympathetic system= of the pig has received very little attention from anatomists. The =superior cervical ganglion= is long and fusiform. It gives off filaments which join the vagus near the ganglion nodosum. The =cervical trunk= is short and relatively larger than in the ox; it is inclosed in a common sheath with the vagus in the neck and separates from that nerve to join the =middle cervical ganglion= at the thoracic inlet.

THE NERVOUS SYSTEM OF THE DOG

The =spinal cord= is almost circular in cross-section except at the well-marked cervical and lumbar enlargements, where it is compressed dorso-ventrally. The conus medullaris lies over the junction of the sixth and seventh lumbar vertebræ. The length of the cord of a rather large dog was found to be about 38 cm.; of this, the cervical part was 11 cm., the thoracic 17.4 cm., the lumbar about 7 cm., and the sacro-coccygeal about 2.6 cm. (Flatau-Jacobson).

FIG. 539.—BASE OF BRAIN OF DOG.

_a_, Olfactory bulb; _a′_, _a″_, external and internal olfactory
tracts; _b_, optic nerve; _c_, oculomotor nerve; _d_, trochlear
nerve; _e_, trigeminus; _f_, abducens; _g_, facial; _h_, auditory;
_i_, glosso-pharyngeal; _k_, vagus; _l_, spinal accessory; _m_,
hypoglossal; _2_, trigonum olfactorium; _3_, anterior perforated
substance; _4_, tuber cinereum and infundibulum; _4′_, mammillary
body; _5_, pyriform lobe; _6_, temporal lobe; _7_, parietal lobe;
_8_, frontal lobe; _9_, pons; _10_, medulla oblongata; _11_,
cerebellum; _12_, cerebral peduncle; _13_, occipital lobe.
(Ellenberger-Baum, Anat. d. Hundes.)
]

FIG. 540.—BRAIN STEM OF DOG, DORSAL VIEW.

_a_, Spinal cord; _b_, dorsal median groove; _c_, _c′_, funiculus
gracilis; _c″_, clava; _d_, _d′_, funiculus cuneatus; _d″_,
tuberculum cuneatum; _e_, funiculus lateralis; _e′_, tuberculum
Rolandi; _f_, medulla oblongata; _I_, _II_, _III_, floor of fourth
ventricle (fossa rhomboidea); _g_, limiting groove; _h_, median
fissure; _i_, eminentia medialis; _k_, ala cinerea; _l_, _l′_, pons;
_m_, posterior peduncle, and _n_, middle peduncle of cerebellum
(cut); _o_, anterior peduncle of cerebellum; _p_, _p′_, corpora
quadrigemina, anterior and posterior; _q_, _q′_, peduncles (brachia)
of corpora quadrigemina; _r_, thalamus; _s_, massa intermedia; _t_,
stria medullaris; _u_, anterior tubercle of thalamus; _v_, pulvinar;
_w_, corpus geniculatum laterale; _x_, corpus geniculatum mediale.
(After Ellenberger, in Leisering’s Atlas.)
]

The =brain= weighs about 60 to 70 g. in dogs of medium size, but there is, of course, a wide range of weight in the different breeds. Thus in small terriers the weight is 30 g. or even less, while in very large dogs it may exceed 150 g. It corresponds much more closely with the cranium in size and general form than in the animals previously described; this is specially true of the small breeds, in which the bony crests and frontal sinuses are little developed.

The =medulla= is broad and thick. Its ventral surface is strongly convex from side to side. The pyramids are large and prominent and are limited by distinct median and lateral grooves. The olivary eminence is a well defined oval elevation situated between the pyramid and the superficial origin of the hypoglossal nerve. The external arcuate fibers form a wide band which crosses the lateral surface obliquely upward and forward, and obscures the groove which limits the restiform body externally. The cuneate tubercle is distinct. The rhomboid fossa is deep and narrow.

FIG. 541.—PART OF CROSS-SECTION OF BRAIN OF DOG, PASSING THROUGH
GANGLION HABENULÆ.

_Al_, Ansa lenticularis of thalamic radiation; _C a d_, dorsal end of
hippocampus; _C a v_, ventral end of same; _C c_, corpus callosum;
_C g m_, corpus geniculatum mediale; _C m_, corpus mammillare; _F i
d_, fimbria; _F i v_, ventral part of fimbria; _G h_, ganglion
habenulæ; _M B_, fasciculus retroflexus; _P p_, pes pedunculi; _T
h_, thalamus; _T o_, tractus opticus; _T t_, habenula; _V A_,
fasciculus thalamo-mammillaris; _V F_, fornix ventralis; _II_,
lateral ventricle; _III_, third ventricle. (After Probst.)
]

The =pons= is relatively small. The =corpus trapezoideum= is very wide and is divided by the pyramids into two lateral parts. The =cerebellum= is very broad, but is low and also compressed from before backward. Rather more than half of it is overlapped by the cerebral hemispheres. The anterior surface is accurately adapted to the concave tentorial surfaces of the hemispheres and to the posterior corpora quadrigemina and their commissure. The posterior surface is almost vertical and is convex centrally and flattened laterally. The vermis is prominent and in general well defined, although it is connected in its middle part with the hemispheres. The latter are three-sided and consist of four lobules. The anterior peduncles are very short.

The =posterior corpora quadrigemina= are large, very wide apart, and prominent, and are connected by a curved commissure. The =internal geniculate body= is large. In the deep interpeduncular fossa are two small bands which indicate the course of the fasciculi retroflexi, tracts which connect the habenular and interpeduncular ganglia. The =mammillary body= is double. The =tuber cinereum= is relatively large. The =pituitary body= is circular and rather small.

The =cerebral hemispheres= are very broad behind and diminish in width anteriorly; there is a sudden narrowing at the frontal poles, which are flattened laterally. The tentorial surfaces are concave and form a deep cavity which receives the anterior part of the cerebellum. The arrangement of the fissures and gyri is simpler than in the animals previously examined. The chief fissures are as follows:

1. The =lateral fissure= (of Sylvius) extends upward and backward from the sulcus rhinalis a little behind the middle of the lateral surface of the hemisphere.