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.

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.

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.

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.