For the embryology and comparative anatomy of the cranial nerves. see NERVOUS SYSTEM_ The spinal nerves arise from each side of the spinal cord and are distributed to the trunk and limbs, though some of the upper ones supply the lower parts of the head and face. There are generally thirty-one pairs, eight cervical (abbreviated C.), twelve thoracic (Th.)—formerly called dorsal,—five lumbar (L.), five sacral (S.) and one coccygeal (Coc.). As the thoracic nerves are as far as the edge of the sternum, when it ends in an anterior cutaneous branch to the front of the chest. The dorsal primary division divides into an external (lateral) and internal (mesial) branch through which the skin and muscles of the back are supplied.
It will be seen from the foregoing that the thoracic nerves are almost completely segmental in their distribution, in other words, each supplies a slice of the body, but in the other regions this segmental character is masked by the development of the branchial skeleton and the limbs. In the cervical region the first cervical or suboccipital nerve comes out between the occiput and atlas and does not always have a posterior root. When it has not, it obviously can supply no skin. Its anterior primary division joins those of the second, third and fourth cervical nerves to form the cervical plexus, from which the skin of the side of the neck and lower part of the head and face are supplied by means of the small occipital, great auricular, superficial cervical, supra sternal, supraclavicular and supraacromial nerves (fig. 7), as well as those muscles of the neck which are not supplied by the cra nial nerves. The phrenic nerve, which comes chiefly from the fourth cervical, runs down, through the thorax, to supply the greater part of the diaphragm. The explanation of this long course (see DIAPHRAGM) is that the diaphragm is formed in the neck region of the embryo. The posterior primary division of the sec ond cervical nerve is large, and its inner (mesial) branch (great occipital) supplies most of the back of the scalp (fig. 7). The fifth, sixth, seventh and eighth anterior primary divisions of the cervical nerves as well as a large part of that of the first thoracic are prolonged into the arm, and in the lower part of the neck and armpit communicate with one another to form the brachial plexus. It will be seen from fig. 8 that each component nerve except the first thoracic divides into an anterior (ventral) and a posterior (dorsal) division which are best spoken of as secondary divisions in order to prevent confusion with the anterior and posterior the simplest and most generalized in their arrangement, a typical one of these, say the fourth or fifth, will be first described.
The nerve is attached to the spinal cord by two roots, of which the ventral is purely efferent or motor and the dorsal purely afferent or sensory. On the dorsal root is a fusiform ganglion which lies in the foramen between the vertebrae through which the nerve passes. The two roots then join together to form a mixed nerve (fig. 5), but very soon divide once more into anterior (ventral) and posterior (dorsal) primary divisions. These, how ever, each contain sensory and motor fibres. Just bef ore it divides in this way the mixed nerve gives and receives its rami communi cantes with the sympathetic (see NERVOUS SYSTEM).
The anterior primary division runs round the trunk, between the ribs, forming an intercostal nerve and giving off a lateral cutaneous branch, when the side of the body is reached, which divides into anterior and posterior secondary branches. The rest of the division runs forward, supplying the intercostal muscles, primary divisions which all the spinal nerves undergo. In the diagram the anterior secondary divisions are white, while the posterior are shaded. Later on in the plexus three cords are formed of which the posterior is altogether made up of the posterior secondary divisions, while the anterior secondary divi sions of the fifth, sixth and seventh cervical nerves form the external cord, and those of the eighth cervical and first thoracic the inner. As a general rule the nerves which rise from the ven tral secondary divisions of the limb plexuses run only to that surface of the limb which was ventral in the embryo, while the dorsal secondary divisions are confined to the original dorsal area, but, in order to apply this to the human adult, it must be realized that the limbs are at one time flattened buds coming off at right angles from the side of the body and having dorsal and ventral surfaces, one (preaxial) border toward the head of the embryo, and one (postaxial) toward the tail.