Part 86

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. ]

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.

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).

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.

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.

2. The =ectosylvian fissure= has approximately the shape of an inverted u, and curves over the lateral fissure. It is regarded as consisting of three parts—anterior, middle, and posterior (F. ectosylvia anterior, media, posterior).

3. The =suprasylvian fissure= lies above and approximately concentric with the preceding. It also consists of three parts—anterior, middle, and posterior.

4. The =ectomarginal= or =collateral fissure= runs forward on the dorsal aspect of the hemisphere almost parallel with the dorso-medial border. A little in front of the middle of the surface it gives off a short =ansate fissure= (F. ansata) which runs obliquely inward and forward, and is continued by the coronal fissure.

5. The =coronal fissure= is the continuation of the ectomarginal. It runs forward and downward, curving around the cruciate fissure, and ends behind the middle of the presylvian fissure.

6. The =ectolateral fissure= lies between the posterior part of the ectomarginal fissure and the suprasylvian fissure.

7. The =entomarginal fissure= lies between the anterior part of the ectomarginal fissure and the dorso-medial border. It is often indistinct and not rarely absent.

8. The =presylvian fissure= begins at the sulcus rhinalis, a little in front of its middle, and curves over the anterior third of the lateral surface of the hemisphere in front of the coronal fissure.

9. The =cruciate fissure= is the deepest and most characteristic. It cuts deeply into the anterior third of the dorso-medial border and runs almost straight outward. It is continuous on the inner surface of the hemisphere with the calloso-marginal or splenial fissure.

10. The =sulcus rhinalis= is continued on the tentorial surface of the hemisphere by the occipito-temporal fissure. Anteriorly it is prolonged as the olfactory fissure, which is concealed by the olfactory bulb and tract.

11. The =calloso-marginal= or =splenial fissure= is parallel with the splenium and middle part of the corpus callosum on the inner surface of the hemisphere. It is continuous in front with the cruciate fissure and runs downward and outward across the tentorial surface to join the occipito-temporal fissure and sulcus rhinalis. It gives off from its highest part the short lesser or accessory cruciate fissure.

12. The =suprasplenial fissure= runs almost parallel with the posterior part of the calloso-marginal fissure on the inner and tentorial surfaces of the hemisphere.

The =olfactory bulb= is large and is strongly compressed laterally; its anterior end projects beyond the frontal pole of the hemisphere. The tract is short and is also flattened laterally. Both contain a narrow cavity which communicates with the lateral ventricle. The =pyriform lobe= is large and rounded.

The =cranial nerves= present the following special features which are worthy of notice:

The =ophthalmic nerve= gives off lacrimal, frontal, long ciliary, ethmoidal, and infratrochlear branches. The =lacrimal nerve= arises from the ophthalmic at its origin;[215] its recurrent branch emerges at the orbital ligament and concurs with the zygomatic and frontal nerves in the formation of the anterior auricular plexus. The =frontal nerve= emerges from the orbit in front of the upper end of the orbital ligament, ramifies in the upper lid and the adjacent skin of the forehead, and sends branches backward to the anterior auricular plexus. The =long ciliary nerve= accompanies the optic nerve and divides into several branches which pierce the posterior part of the sclera. The =ethmoidal nerve= gives off internal nasal branches and ends in the muzzle. The =infratrochlear nerve= runs forward between the internal straight and the superior oblique muscles of the eye and ramifies on the face in the vicinity of the inner canthus.

The =maxillary nerve=, after giving off the zygomatic, divides into infraorbital and sphenopalatine branches. The =zygomatic nerve= emerges through an opening in the upper part of the orbital ligament and ramifies in the lower lid and on the adjacent surface of the face. The =infraorbital nerves=, two in number, divide within the infraorbital canal and after their emergence upon the face, thus forming seven or eight =external nasal= and =superior labial branches=. The =sphenopalatine nerve= gives off =lesser= and =greater palatine= and =posterior nasal nerves=.

The =mandibular nerve= passes out through the foramen ovale. The =superficial temporal nerve= divides into auricular, temporal, and malar branches; the last-named crosses the masseter and ramifies with the buccal nerves. The =mylo-hyoid nerve= is given off from the mandibular nerve almost immediately after the latter emerges from the cranium; it innervates the mylo-hyoideus and digastricus and gives off a branch which turns around the lower jaw, joins the inferior buccal nerve, and ramifies on the lateral surface of the face. The =inferior alveolar nerve= arises by a common trunk with the mylo-hyoid; it gives off dental branches and terminates in mental and inferior labial branches. The =lingual nerve= supplies vasodilator and excito-secretory filaments to the submaxillary and sublingual salivary glands; these fibers are derived from the chorda tympani.

The =facial nerve= divides near the posterior border of the jaw into four branches. The upper branch is the =auriculo-palpebral nerve=, which divides after a very short course into anterior auricular and zygomatic branches. The latter curves upward and forward across the zygomatic arch toward the eye, and divides into branches which supply the eyelids and nasal region and concur with the frontal and lacrimal nerves in forming the anterior auricular plexus. The =superior buccal nerve= accompanies the parotid duct across the masseter. The =inferior buccal nerve= runs forward along the lower border of the masseter and the mandible. The two nerves ramify on the cheek and anastomose with each other and the infraorbital nerves to form a plexus from which branches go to the muscles of the lips and nostrils. The =cervical branch= runs downward and backward over the submaxillary gland and ramifies in the cervical panniculus; it communicates with the inferior buccal nerve and sends twigs to the parotido-auricularis muscle and the submaxillary space.

The =vagus= bears a =jugular ganglion= and a =ganglion nodosum=. The latter is situated near the superior cervical ganglion on the rectus capitis anterior major and dorsal to the carotid artery; it is fusiform and may be about half an inch (ca. 1 to 1.5 cm.) long in a large dog. In its course in the neck the nerve is inclosed with the sympathetic trunk in a common sheath and is related ventrally to the common carotid artery. The formation of dorsal and ventral =œsophageal trunks= by the union of the dorsal and ventral divisions of the two nerves takes place at the hiatus œsophageus. The =dorsal trunk= concurs with branches of the solar plexus in forming the =posterior gastric plexus= on the visceral surface of the stomach. The =ventral trunk= ramifies on the parietal surface of the stomach, forming the =anterior gastric plexus=, from which a considerable branch passes along the lesser curvature to the pylorus. Some special features of the collateral branches are: Two pharyngeal branches are present. The =superior pharyngeal branch= arises from the vagus above the ganglion nodosum. It passes under the deep face of the carotid artery and descends on the lateral surface of the pharynx to end in the crico-thyroid muscle. It furnishes a root of the inferior pharyngeal branch, and communicates with the pharyngeal branch of the ninth and with the superior laryngeal and hypoglossal nerves. The =inferior pharyngeal branch= is formed by the union of roots derived from the superior pharyngeal nerve and the ganglion nodosum, together with filaments from the superior cervical ganglion. It crosses the side of the pharynx behind the superior pharyngeal and ramifies on the posterior constrictor of the pharynx and the origin of the œsophagus. It contributes filaments to the pharyngeal plexus, communicates with the recurrent nerve, and supplies twigs to the thyroid gland. The =pharyngeal plexus= is formed on the lateral surface of the pharynx by branches of the pharyngeal nerves and the communications above described; branches from it innervate the muscles and mucous membrane of the pharynx. The =superior laryngeal nerve= arises from the ganglion nodosum and descends over the side of the pharynx, crossing beneath the carotid artery and the superior laryngeal nerve. It passes through the thyroid notch and ramifies in the mucous membrane of the larynx. It communicates with the superior cervical ganglion and the superior pharyngeal branch of the vagus, and gives twigs to the hyo-pharyngeus muscle. Immediately after its entrance into the larynx it gives off a large branch which, instead of uniting with the recurrent as in the other animals, has a peculiar arrangement. It runs back near the dorsal border of the thyroid cartilage, gives a branch to the crico-arytenoideus posterior as it passes over that muscle, and continues along the trachea internal to the recurrent nerve. At the thoracic inlet it communicates with the inferior cervical ganglion and continues backward to unite with the vagus at or near the point of origin of the recurrent nerve. Its collateral filaments supply the trachea and concur with the inferior pharyngeal branch in forming a plexus on the cervical part of the œsophagus which innervates that tube.[216] The =depressor nerve= is a very delicate filament which arises usually from the superior laryngeal nerve and accompanies the vago-sympathetic trunk to the thorax; its fibers reach the heart through the cardiac nerves. The =pulmonary= and =posterior œsophageal plexuses= are highly developed.

The =hypoglossal nerve= gives off a long branch (R. descendens) which runs downward and backward across the pharynx and larynx, communicates with the ventral branch of the first cervical nerve, and supplies the thyro-hyoid, sterno-hyoid, and sterno-thyroid muscles.

The =spinal nerves= number thirty-six or thirty-seven on either side, and comprise eight cervical, thirteen thoracic, seven lumbar, and five or six coccygeal.

The =brachial plexus= (Fig. 487) is derived from the ventral branches of the last four cervical and first thoracic nerves; the root supplied by the fifth cervical nerve is very small. The roots unite at the lower border of the scalenus. The more important special features in the arrangement of the nerves which emanate from the plexus are as follows: