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; 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. 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. ) 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:

The musculo-cutaneous nerve passes between the coraco-brachialis and the brachial artery and descends in the arm in front of the artery. At the shoulder joint it gives off branches to the biceps and coraco-brachialis, and in the lower third of the arm is connected with the median nerve by an oblique branch. It terminates near the elbow by dividing into a branch for the brachialis and a small cutaneous nerve which passes down over the inner face of the elbow and, inclining a little forward, descends over the deep fascia of the forearm to the carpus.

The radial nerve descends behind the ulnar nerve, gives branches to the extensors of the elbow, dips in between the internal head of the triceps and the accessory head of the anconeus, winds around the arm, and divides between the brachialis and the external head of the triceps into two branches. The deep branch (R. profundus) supplies the extensor and supinator muscles on the forearm. The superficial branch (R. superficialis) emerges upon the flexor surface of the elbow and divides into two branches which terminate by supplying two dorsal digital nerves to each digit, except the fifth, which receives its outer dorsal nerve from the ulnar. The inner branch descends along the inner side of the cephalic vein to the carpus, where it divides into dorsal digital nerves for the first digit and the inner side of the second. The outer branch is much larger. It runs downward on the middle of the front of the forearm and supplies the remaining dorsal digital nerves except that to the outer side of the fifth digit.

The ulnar nerve is as large as or larger than the median, with which it is united for some distance. At the distal third of the arm it separates from the median and passes over the internal epicondyle of the humerus. At the proximal part of the forearm it gives off the dorsal branch (R. dorsalis), which supplies cutaneous twigs to the dorso-lateral surface of the lower part of the forearm and carpus and terminates as the external dorsal digital nerve of the fifth digit. Descending under cover of the flexor carpi ulnaris, the ulnar nerve inclines inward under the tendon of insertion of that muscle and divides into superficial and deep branches. The superficial branch (R. superficialis) runs downward along the outer border of the flexor tendons, gives off the external volar digital nerve of the fifth digit (N. dig. vol. lat. dig. V), and a branch (N. met. vol. IV) which descends in the space between the fourth and fifth metacarpal bones and unites with the deep branch. The deep branch (R. profundus) descends in the carpal canal and divides under the deep flexor tendon into its terminal branches. The smaller of these supply the volar metacarpal muscles. The larger terminals are the three volar common digital nerves (Nn. dig. vol. comm. II, III, IV), which descend along the second, third, and fourth intermetacarpal spaces, subdivide, and concur with the volar metacarpal branches of the median nerve in forming the volar proper digital nerves (Nn. dig. vol. proprii).

The median nerve descends behind the brachial artery, passes over the internal epicondyle of the humerus, then under the pronator teres, and continues in the forearm under cover of the flexor carpi radialis. It gives branches below the elbow to the flexor and pronator muscles, and lower down a palmar branch to the skin on the inner and posterior aspect of the carpus, and terminates between the superficial and deep flexor tendons by dividing into three volar metacarpal nerves (N. met. vol. I, II, III). These descend in the first, second, and third intermetacarpal spaces and unite with the volar common digital nerves in forming volar proper digital nerves. The arrangement of the digital nerves is indicated in the annexed schematic figures.

The lumbo-sacral plexus is formed from the ventral branches of the last five lumbar and first sacral nerves. The more important special facts in regard to the nerves of the pelvic limb are as follows:

The saphenous nerve (Fig. ) is relatively large, and, in fact, might be considered as the continuation of the femoral nerve. It descends at first in front of the femoral artery, then passes over the lower part of the inner surface of the thigh with the saphenous artery, continues down the leg with the dorsal branch of that vessel, and anastomoses with branches of the superficial peroneal nerve. It supplies cutaneous twigs from the stifle to the metatarsus.

The peroneal nerve separates from the tibial above the origin of the gastrocnemius, runs downward and forward across the outer head of that muscle, passes between the deep flexor of the digit and the peroneus longus, and divides into superficial and deep branches. The superficial peroneal nerve descends the leg along the peroneus tertius and longus, gives off a dorsal branch to the first digit (N. cutaneus dorsalis medialis), and divides at the proximal part of the metatarsus into three dorsal common digital nerves (Nn. dig. pedis dors. comm. II, III, IV). These descend with the superficial dorsal metatarsal arteries and concur with branches of the deep peroneal nerve in forming dorsal proper digital nerves (Nn. dig. ped. prop. dorsales). The deep peroneal nerve passes down the leg with the anterior tibial vessels. It gives twigs to the hock joint and the extensor brevis muscle, and divides into three dorsal metatarsal nerves (Nn. met. dors. II, III, IV); these descend with the corresponding arteries along the intermetatarsal spaces and unite with the dorsal common digital nerves in supplying the dorsal proper digital nerves.

The tibial nerve divides at the tarsus into two plantar branches. The internal plantar nerve descends along the inner border of the superficial flexor tendon and divides near the middle of the metatarsus into two branches. Of these, the internal branch constitutes the first plantar common digital nerve (N. dig. comm. plant. I). The outer branch descends on the superficial flexor tendon and divides into three plantar metatarsal nerves (Nn. met. plant. II, III, IV), which unite with the plantar common digital nerves. The external plantar nerve passes down between the flexor tendons, sends twigs to the muscles on the plantar surface of the metatarsus, and divides into three plantar common digital nerves (Nn. dig. comm, plant. II, III, IV). These descend with the deep plantar metatarsal arteries in the second, third, and fourth intermetatarsal spaces, receive the plantar metatarsal nerves, and divide into plantar proper digital nerves, which pass down the opposed surfaces of the second to the fifth digits.

The sympathetic system presents few special features worthy of mention. The superior cervical ganglion lies above the ganglion nodosum close to the cranial base and internal to the origin of the internal carotid and occipital arteries. It forms connections directly or through the carotid plexus with the last seven cranial and first cervical nerves. The cervical trunk unites with the vagus so intimately as to form a vago-sympathetic trunk. There may be a small middle cervical ganglion in front of the inferior cervical ganglion. The latter is usually distinct from the first thoracic ganglion, which is stellate and lies on the longus colli opposite the second rib and intercostal space.