Part 79

The trifacialis fuses with the other cranial nerves and with the sympathetic nervous system. Some of these fusions are as follows: The recurrent ophthalmica fuses with the orbito-nasalis ganglion. Indirect fusion of the recurrent maxillary division of this nerve takes place near the Gasserian ganglion through the sympathetic nerves, the temporo-lacrimalis, the facial nerves, the large cervical ganglion, and indirectly with the glosso-pharyngeal and the vagus. This fusion has been called the superior recurrent branch of the trigeminus, or trifacialis.

There is a direct fusion of the superior recurrent maxillary division of the fifth just before entering the upper jaw. This fusion is with the spheno-palatine ganglion and the sympathetic carotidis cephalica, and also with the large cervical ganglion.

=The Abducens= (Fig. 62, No. C, 13).—This nerve originates in the somatic and motoric column along with the other nerves of the eye muscles. Its nucleus lies in the circle of the pars commissuralis of the cerebellum. The nerve then passes ventrally, as does the motoris oculi and leaves the brain along the median line. This is a comparatively large nerve and passes somewhat laterally and ventrally from the foramen opticum through a canal in the sphenoid. It then enters the orbital cavity. Some muscular twigs are given off to the quadratus and to the pyramidalis muscle. This nerve also innervates the external rectus muscle of the eye. The abducens anastomoses with the ramus ciliaris and the ophthalmic nerve and gives fine nerve twigs to the ramus ciliaris externus of the ciliary ganglion.

=The Facialis and the Acusticus.=—The facial nerve originates with the auditory (Fig. 62, No. C, 14 and 15) in a very vaguely known manner, from the cerebellum. It divides into three parts.

The first of these probably comes from the complex ganglion with the posterior roots of the auditory. This part belongs to the somatic sensory group of nerves. From this same group originates the auditory, which spreads out into the cochlea and takes the impressions of sound. This nerve is short and thick, and, at the point where it loses its medullary covering on entering the cochlea, there is developed a ganglion. This ganglion is similar to the spinal ganglia.

The second part is provided with one root which originates mesially and ventrally, from the deeper ganglion cells. Some of the fibers from this root constitute the vestibular branches. They accompany the auditory and supply the anterior part of the ear labyrinth and the semicircular canals. The larger part of the fibers of this trunk make up the intermediate part of the facial. The geniculate ganglion is formed at their fusion. The sympathetic spheno-palatine nerve emerges from this ganglion, coming out of the aqueduct of Fallopius.

The third part, called the portio dura, is the main facialis. It is located opposite the auditoria intermedia. Its roots may be traced to the complex ganglion, from which they take a direction ventrally from the median portion.

The facialis, after emerging from the aqueduct of Fallopius, takes a curved course and partly fuses with the sympathetic temporo-lacrimalis on the upper posterior wall of the ear drum. It is here accompanied by the carotid and the cephalic arteries. It leaves the ear drum through an opening in the quadrate bone, giving off a large branch to the digastricus muscle and a small one to the stapedius of the columella auris. The facialis trunk is quite large. It then passes downward along the quadrate bone, where it receives a large branch from the glosso-pharyngeus. This nerve also gives branches to the mylo-hyoideus and the stylo-hyoideus muscle, crosses laterally the glosso-pharyngeus, and finally fuses with branches of the subcutaneous and with the first, the second, the third, and the fourth cervical nerves. After this fusion the nerves innervate the skin of the anterior of the neck and the constrictor colli muscle.

The facialis anastomoses as follows: with a fine branch of the sympathetic temporo-lacrimalis, indirectly with the ramus trigeminus, with the large cervical nerve ganglion and with the spheno-palatine ganglion and nerve.

=The Vagus Group.=—The ninth, the tenth, and the eleventh cranial nerves are by some anatomists called the vagus group, first so called by Willis. They are made up of both sensory and motor nerves.

=The Glosso-pharyngeus.=—The roots of the glosso-pharyngeus (Fig. 62, No. C, 16; Fig. 76, No. 13) emerge, along with those of the vagus, from the medulla oblongata, and enter, as a short trunk, the foramen jugulare et caroticum. Between these two nerves there is usually found a thin portion of bone. The trunk of the glosso-pharyngeus forms a ganglion in the foramen jugulare et caroticum where it receives connecting branches from the near-by ganglion radicis and vagus nerve. The glosso-pharyngeus passes out of the foramen jugulare et caroticum above the large superior sympathetic nerve ganglion, with which it communicates. The glosso-pharyngeus passes diagonally over the ramus temporo-lacrimalis of the sympathetic system, and then receives a very strong branch from the vagus (Fig. 76, No. 12). It sends a short branch to the recurrent lacrimalis of the sympathetic system. At this point there is formed a reddish-yellow ganglion, the petrosal ganglion, which is similar to the petrosal ganglion of mammals. Frequently it is found close below the large superior cervical nerve ganglion. The petrosal ganglion is frequently connected by special fibers with the cervical nerve ganglion, the large superior cervical nerve ganglion and the ganglion radicis vagi. The glosso-pharyngeus is divided into the following branches.

First, the recurrent pharyngeus which gives branches to the upper part of the throat and which is tortuous in its course. It receives a branch from the superior cervical nerve ganglion and gives off branches to the salivary glands and papillæ of the posterior tongue region.

Second, the recurrent lingualis, which passes with the lingual artery over the hyoid bone to the base of the tongue and to the papillæ. Another branch passes below the tongue bone and supplies the tongue and the pharynx. The glosso-pharyngeal is a mixed nerve.

FIG. 76.—Photograph of spinal cord of a hen. 1, Cerebrum. 2, The olfactory nerve terminating at 3, the turbinated bones. 4, The superior maxillary division of the fifth nerve dividing. 5, The eye with the upper part laid open. 6, The cervical segment. 7, The lumbo-sacral segment. 8, The lungs. 9, The kidneys lying above the peritoneum. 10, The terminal filament of the spinal cord. 11, The hypoglossal nerve. 12, The vagus nerve. 13, The glosso-pharyngeal nerve. 14, The branch from the vagus. ]

=The Vagus.=—The vagus, or pneumogastricus (Fig. 18, No. 5; Fig. 62, No. C, 17; Fig. 76, No. 12), a mixed nerve, forms a ganglion in the foramen through which it passes. The foramen lies between the os petrosum and os occipitale, close to the foramen jugulare, above and to the inside of it.

The ganglionic root of the vagus communicates with the superior cervical nerve ganglion. The vagus, after coming out of the foramen, takes up branches of the spinal accessory, passes to the superior nerve ganglion, crosses the carotid artery, and then, accompanied by the internal jugular, it connects by a branch with the hypoglossal. It then communicates with the petrosal ganglion, and receives a long branch from the superior cervical ganglion. It then extends along the neck with the jugular vein and often fuses with the sympathetic ganglionic plexuses. As it passes, it is interwoven with the glosso-pharyngeus, but each nerve element retains its own individuality. Extending down the neck, on entering the thoracic cavity, it lies between the plexus brachialis and the carotid artery. Then it passes below the subclavian artery, and between the bronchial tubes, the aorta pulmonalis, and the subclavian vein. Then ventrally it rests upon the glands, and the right and the left vagi fuse or unite. From here they radiate down to the stomach in fan-shape, and, continuing, they fuse with the sympathetic system.

The other branches of the vagus are:

The first branch is the recurrent laryngeus, which supplies the lower end of the bronchial tubes, and the esophagus, then enters above the bronchus near the origin of the subclavian artery as the recurrent cardiacus.

The second are the recurrent pulmonale which pass into the lungs, each fusing with its fellow from the opposite side, and giving off branches inferior to the vena cava including a branch to the heart.

The third are the recurrent hepatici which pass through the diaphragm and are distributed to the liver.

=The Accessorius Spinalis.=—This is a very small motor nerve (Fig. 62, No. C, 18). It comes out between the dorsal and the ventral root of the third cervical nerve. It lies close to the neck and extends anteriorly, receiving roots from the first and the second cervical nerve. It passes through the occipital foramen into the brain cavity and then enters the ganglion radicis vagi. It passes out of the cranial cavity through the foramen jugulare. It then partly fuses with the vagus and partly, as a fine branch, with the subcutaneous colli.

=The Hypoglossus.=—The hypoglossus is a motor nerve (Fig. 62, No. C, 19). It originates anterior to the eleventh pair of cranial nerves, and from the same ganglion as the abducens and the motor oculi. It leaves the medulla oblongata from its ventral surface, and passes with a posterior and an anterior branch out of the cranial cavity through two separate foramina in the os occipitale laterale. The posterior branch, much smaller than the anterior, passes between the cranial bones and the cervical sympathetic nerve; it then passes at right angles through the rectus capitis anticus, and, while closely following the carotid artery, it fuses with the anterior branch. The larger, anterior branch, much stronger than the posterior, sends a short strong branch to the complexus muscle. The rest of this branch, which probably has sympathetic elements, is disposed as is a spinal nerve. It crosses the sympathetic nerve, and at this point forms a typical cervical sympathetic ganglion. It also forms a short loop with the recurrent ventralis of the first cervical plexus. From this loop, after it has given off several branches to the muscles of the neck, it gives off one or two strong branches, which fuse with each other and with the posterior thin branch, thus forming the trunk of the hypoglossus. It receives many elements from the first cervical nerves.

The hypoglossus sometimes communicates with the vagus, crosses over the latter, and divides itself into two main branches, as follows:

First, the recurrent laryngo-lingualis, which passes between the cornua of the os hyoideum and the larynx to the anterior part, and furnishes the principal tongue muscles. It extends along the inferior surface of the tongue and fuses with the one of the other side and extends to the free tip of the tongue. This nerve probably receives sensory elements from the second root and from the confluent of the first cervical nerve. This form of anastomosis is well marked in birds with thick tongues, as ducks.

Second, the recurrent laryngeus furnishes the muscles of the superior larynx and of the tongue skeleton. It extends downward and also furnishes the muscles of the trachea. It follows the course of the jugular vein and at the entrance of the chest supplies the furcula. The nerve passes into the thorax, downward along the side of the bronchial tubes, and supplies innervation to all the muscles of the inferior larynx.

THE SPINAL CORD

=Structure of the Cord.=—The spinal cord is called the myelon or medulla spinalis. It is a comparatively large, white, irregularly cylindrical cord, flattened from above downward. It extends from the foramen magnum at the base of the medulla to the caudal portion of the spinal canal where it terminates in a fine filament. In order to allow considerable motion of the spinal column without danger of injury to the spinal cord, the cord is loosely suspended in the canal. The meninges, or coverings, are continued from the brain. In addition to these there are to be found arteries, veins, and nerves entering and others making their exit from the canal and from the cord. At the cervico-dorsal juncture, where the brachial plexus is given off, the cord is enlarged. There is another enlargement at the point where the lumbo-sacral plexus is given off. At this point, superiorly, there is a longitudinal cavity called the sinus rhomboidalis. This sinus contains a gelatinous substance. There is a pair of nerves given off at each intervertebral foramen. Thus there are as many pairs of spinal nerves as there are vertebral segments. The inferior root is the motor root and carries the impulses from the cord to the periphery. The superior root is the sensory branch and carries the impulses from the periphery to the cord, and thence the impulse is carried to the centers in the brain. The sensory and the motor roots are of about equal size. The inferior have more numerous filaments. The ganglion on the superior or sensory root is relatively large. In the sacral region the sensory and the motor branches pass through their own bony canal.

White rami communicantes are given off at each intervertebral foramen to the sympathetic system and gray rami are received from the sympathetic ganglia. Thus there is established a direct communication between the sympathetic and the spinal system. There is a ganglion on the superior root just outside the spinal cord.