Part 209

The =ventral= (or inferior) =root= (Radix ventralis) contains fewer fibers than the dorsal root, except in the case of the first cervical nerve. It arises from the ventral surface of the spinal cord (Fig. 495) by means of numerous small bundles of fibers which do not form a linear series but emerge from the cord over an area three to five millimeters in width (ventral root zone). The fibers are processes of the large cells of the ventral gray columns of the spinal cord. There is no ganglion on the ventral root.

In the cervical, thoracic, and anterior lumbar regions the bundles of both roots pass through separate openings in linear series in the dura mater before uniting into a root proper. Further back the bundles of each root unite within the dura. In the anterior part of the cervical region and in the thoracic part of the cord there are intervals of varying length between adjacent roots, but in some places the fibers of adjacent roots overlap and an exchange of fibers may be observed. Many of the roots are directed almost straight outward or incline slightly backward, but the posterior lumbar, sacral, and coccygeal roots and nerves run backward to reach the foramina through which they emerge. The distance thus to be traversed increases from before backward, so that these nerves form a tapering sheaf around the conus medullaris and filum terminale in the last lumbar vertebra and the sacrum which is known as the cauda equina.

FIG. 519.—VERTEBRAL CANAL OPENED BY SAWING OFF THE ARCHES. (After Schmaltz. Atlas d. Anat. a. Pferdes.) ]

The =size= of the spinal nerves varies greatly. The largest are connected with the cervical and lumbar enlargements.

In or immediately after its emergence from the intervertebral foramen each spinal nerve gives off a small =meningeal= or =recurrent branch= (Ramus meningeus). This is joined by a bundle of fibers from the ramus communicans and enters the vertebral canal, in which it is distributed. Each nerve then divides into two primary branches, =dorsal= and =ventral= (Ramus dorsalis, ventralis). The =dorsal branches= are smaller than the ventral, except in the cervical region. They are distributed chiefly to the muscles and skin of the dorsal part of the body. The =ventral branches= supply in general the muscles and skin of the ventral parts of the body, including the limbs. Each nerve or its ventral branch is connected with an adjacent ganglion of the sympathetic system by at least one small short branch known as a =ramus communicans=. Many nerves have two and some have three such rami. A nerve may be connected with two ganglia, and a ganglion may be connected with two nerves.

The dorsal root is sensory or afferent, i. e., it conveys impulses to the central system. Its fibers are axones of the cells of the spinal ganglion. The ventral root is motor or efferent, and conveys impulses toward the periphery. Its fibers are axones of the large cells in the ventral gray columns of the spinal cord. The common trunk or nerve formed by the union of the two roots contains both kinds of fibers, as do also their primary divisions. In addition to these fibers, which are distributed to the skeletal muscles and the skin, the spinal nerves contain fibers derived from the sympathetic system through the rami communicantes; these go to the glands and unstriped muscle and are designated secretory and vasomotor fibers.

THE CERVICAL NERVES

The =cervical nerves= (Nervi cervicales) (Figs. 431, 433, 518, 523) number eight pairs. Of these the first emerges through the intervertebral foramen of the atlas, the second through that of the axis, and the eighth between the last cervical and the first thoracic vertebræ.

The =dorsal branches= are distributed to the dorso-lateral muscles and skin of the neck. They divide usually into inner and outer branches. The internal branches (Rami mediales) run in general across the multifidus and the lamellar part of the ligamentum nuchæ to the skin of the upper border of the neck; they supply the deep lateral muscles and the skin. The external branches (Rami laterales) are chiefly muscular in their distribution. The dorsal branches of the third to the sixth nerves are connected by anastomotic branches to form the =dorsal cervical plexus=.

The =ventral branches= are smaller than the dorsal ones—an exception to the general rule. They increase in size from first to last. They supply in general the muscles and skin over the lateral and ventral aspect of the vertebræ, but the last three enter into the formation of the brachial plexus, and the three preceding the last give off the roots of the phrenic nerve. An irregular =ventral cervical plexus= is formed by anastomoses established between the ventral branches. The following special features may be noted:

The =first cervical nerve= emerges through the intervertebral (antero-internal) foramen of the atlas. Its =dorsal branch= (N. occipitalis) passes upward and outward between the obliqui capitis and the recti capitis dorsales and supplies branches to these muscles, the scutularis and posterior auricular muscles, and the skin of the poll. The =ventral branch= descends through the alar (antero-external) foramen of the atlas, crosses over the ventral straight muscles and the carotid artery under cover of the parotid gland, and divides into two branches. The anterior branch enters the omo-hyoideus muscle. The posterior branch passes downward and backward under cover of that muscle, unites with a branch of the ventral division of the second cervical nerve, and continues its course on the ventro-lateral surface of the trachea to enter the sterno-thyro-hyoideus behind the intermediate tendon. In the recessus atlantis the ventral branch is connected by one or more twigs with the superior cervical ganglion of the sympathetic, and a little lower with the hypoglossal nerve. It also sends branches to the ventral straight muscles of the head and the thyro-hyoideus. Below the atlas the ventral branch is crossed superficially by the spinal accessory nerve, the occipital artery, and the inferior cerebral vein.

The =second cervical nerve= is larger than the first. It emerges from the vertebral canal through the intervertebral foramen of the anterior part of the arch of the axis. Its =dorsal branch= ascends between the complexus and the ligamentum nuchæ and ramifies in the skin of the poll. The =ventral branch= gives off =muscular branches= to the rectus capitis anterior major, and =anastomotic branches= to the spinal accessory and the ventral divisions of the first and third cervical nerves; one of these crosses over the carotid artery and concurs in the formation of the nerve to the sterno-thyro-hyoideus mentioned above. The ventral branch then becomes superficial by passing between the two parts of the mastoido-humeralis, and divides into posterior auricular and cutaneous branches. The =posterior auricular nerve= passes upward and forward on the parotid gland parallel with the posterior border of the parotido-auricularis to ramify on the convex face of the external ear. The =cutaneous nerve= of the neck (N. cutaneus colli) crosses the mastoido-humeralis muscle and turns backward along the course of the jugular vein. On the lower part of the parotid gland it is connected by a twig with the cervical cutaneous branch of the facial nerve. It gives off twigs to the subcutaneous muscles and the skin of the parotid and laryngeal regions and a long branch which passes forward in the submaxillary space.

The =third cervical nerve= leaves the vertebral canal through the foramen between the second and third cervical vertebræ. Its =dorsal branch= emerges between two bundles of the intertransversalis muscle accompanied by a branch of the vertebral artery, turns upward on the multifidus, and divides into several branches which radiate on the deep face of the complexus. It gives branches to these muscles and to the skin, and a twig which joins the corresponding branch of the fourth nerve. The =ventral branch= emerges through the intertransversalis below the bundle above which the dorsal branch appears. It gives branches to the trachelo-mastoideus, rectus capitis anterior major, longus colli, splenius, and mastoido-humeralis. It also gives off a large cutaneous nerve which passes out between the two parts of the mastoido-humeralis and divides into several divergent branches.

The =fourth= and =fifth cervical nerves= are distributed in general like the third. Their =dorsal branches= are united by anastomotic twigs with each other and with those of the third and sixth nerves to form the dorsal cervical plexus. The =ventral branch= of the fifth nerve often contributes a small twig to the phrenic nerve.

The =sixth cervical nerve= has a smaller =dorsal branch= than the fifth. Its =ventral branch= is larger and goes in part to the brachial plexus; it supplies twigs to the intertransversales, the longus colli, the mastoido-humeralis, and the cervical parts of the serratus and rhomboideus, furnishes a root of the phrenic nerve, and gives off several considerable subcutaneous branches. One of the latter ramifies on the thick part of the cervical panniculus, to which it gives branches; another and larger branch (N. supraclavicularis) sends twigs to the skin over the shoulder joint, and descends to the skin over the superficial pectoral muscles (Fig. 466).

The =seventh= and =eighth cervical nerves= have small =dorsal branches=, which ascend between the longissimus and multifidus, giving twigs to these muscles, the spinalis and semispinalis, the rhomboideus, and the skin. Their =ventral branches= are very large and concur in the formation of the brachial plexus; that of the seventh nerve contributes the posterior root of the phrenic nerve.

PHRENIC NERVE

The =phrenic nerve= (N. phrenicus) (Figs. 428, 429, 433), the motor nerve to the diaphragm, is formed by the union of two or three roots which cross the superficial face of the scalenus muscle obliquely downward and backward. The chief roots come from the ventral branches of the sixth and seventh cervical nerves. The root derived from the fifth nerve is small and inconstant. The root from the seventh cervical comes by way of the brachial plexus. The course of the nerve is not the same on both sides. On the =right side= the nerve enters the thorax by passing between the brachial artery and the anterior vena cava. It then courses backward and somewhat downward over the right face of the anterior vena cava, crosses the pericardium, and continues along the posterior vena cava to the diaphragm. In the latter part of its course it is inclosed in a special fold of the right pleura and inclines gradually to the ventral face of the vein. On the =left side= the nerve enters the thorax between the left brachial and inferior cervical arteries, and then runs its entire course in the mediastinum. In the anterior mediastinum it lies along the brachiocephalic artery ventral to the left vagus and cardiac nerves, and crosses over the dorso-cervical vein. It then passes over the upper part of the pericardium and runs backward in the posterior mediastinum to reach the tendinous center of the diaphragm considerably to the left of the median plane. Each nerve is usually connected near its origin with the first thoracic ganglion of the sympathetic by a ramus communicans, and each terminates by dividing into several branches which are distributed to the corresponding part of the diaphragm.

THE BRACHIAL PLEXUS

The =brachial plexus= (Plexus brachialis) (Fig. 433) results from anastomoses established between the ventral branches of the last three cervical and first two thoracic nerves. It appears as a thick, wide band between the two parts of the scalenus muscle, and is covered by the anterior deep pectoral and subscapularis muscles. The largest root of the plexus is derived from the first thoracic nerve, while the root supplied by the sixth cervical nerve is quite small. Each of the three chief roots, i. e., those from the last two cervical and the first thoracic nerve, is connected with the sympathetic by a ramus communicans.

The =branches= emanating from the plexus go for the most part to the thoracic limb, but some are distributed on the chest-wall. The names of the branches, and their arrangement so far as they can be conveniently examined before removal of the fore limb, are as follows:

1. The large =suprascapular nerve= (N. suprascapularis) turns outward and forward and disappears between the supraspinatus and subscapularis.

2. The much smaller =subscapular nerves= (Nn. subscapulares), usually two primary trunks, arise close behind the suprascapular, run backward a short distance, and divide into several branches which enter the lower third of the subscapularis.

3. The =anterior thoracic= or =pectoral nerves= (N. pectorales craniales), three or four in number, arise from the anterior part of the plexus and from the loop formed by the musculo-cutaneous and median nerves. One enters the anterior deep pectoral muscle. Another passes out between the divisions of the deep pectoral to supply the superficial pectoral, giving a twig usually to the posterior deep muscle. The latter receives one or two other nerves.

4. The =musculo-cutaneous nerve= (N. musculocutaneus) arises from the anterior part of the plexus and passes over the outer face of the brachial artery, below which it is connected by a large but short branch with the median nerve, thus forming a loop in which the artery lies. One or two branches to the pectoral muscles are given off from the nerve or the loop.

5. The =median nerve= (N. medianus) is usually the largest branch of the brachial plexus. It lies at first above the brachial artery, then passes between the artery and vein to reach the anterior border of the former. It is easily recognized by its large size and the loop which it forms with the musculo-cutaneous nerve.

6. The =ulnar nerve= (N. ulnaris) arises with the median by a short common trunk. It descends behind the brachial artery and is accompanied a short distance by the radial nerve.

7. The =radial nerve= (N. radialis) arises from the posterior part of the plexus and is sometimes the largest branch. It descends behind the ulnar nerve over the origin of the subscapular artery and the lower part of the teres major, and dips into the interstice between that muscle and the long and internal heads of the triceps.