Part 216

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.