532 COMPARATIVE ANATOMY 



Evolution of the Spinal Nerves. In man thirty-one pairs of nerves are 

 connected with the spinal cord. Of these, eight are cervical, twelve 

 thoracic, five lumbar, five sacral, and one coccygeal. Unlike cranial 

 nerves, each spinal nerve is connected with the central nervous system 

 by two roots, one dorsal, sensory and ganglionated; the other ventral, 

 motor and non-ganglionated. The fibers of the dorsal root are outgrowths 

 of the nerve cells which form the ganglion; the nerve cells of the ventral 

 root lie in the ventral horn of the spinal cord. The two roots unite near 

 the ganglion to form a mixed nerve which has its exit from the vertebral 

 canal by way of intervertebral foramina. As a spinal nerve passes 

 towards the periphery, it divides into dorsal and ventral branches, and a 

 visceral branch, the ramus communicans, which connects with a sympa- 

 thetic ganglion. (Fig. 439) 



The similarity of vertebrate spinal nerves, in their metameric repeti- 

 tion, to the nerves of amphioxus has been one of the reasons for assuming 

 amphioxus to be the prototype of the vertebrates. Except the first two 

 pairs, which are in some respects peculiar, all the segmental nerves of 

 amphioxus are similar, each having a dorsal mixed root and a ventral 

 motor root, which do not unite. Another peculiarity of amphioxus is the 

 absence of a ganglion on the dorsal root. The cell bodies from which 

 sensory neurites arise lie either in the dorsal wall of the cord or scattered 

 along the course of the nerve. It is, however, asserted that some of the 

 cells of the dorsal nerves are neuroepithelial cells located in the skin. 

 The general somatic afferent fibers of the dorsal nerves pass directly 

 to the skin by way of the myocommata. The general visceral efferent 

 fibers leave the dorsal nerve by way of intestinal branches which supply 

 the muscles of the intestine. 



The ventral nerves emerge from the cord by numerous fibrillar roots, 

 and connect directly with the myotomes opposite. The dorsal nerves are, 

 therefore, intermetameric, and the ventral metameric. 



In myxine the dorsal and ventral nerves unite peripherally as they 

 do in all higher vertebrates; and only in petromyzon among cyclostomes 

 are all dorsal and ventral nerves separate. But in the head region of all 

 vertebrates, the somatic motor nerves, oculomotor, trochlear, abducens, 

 and hypoglossus do not unite with sensory nerves. In this regard, as in 

 the mixed character of such nerves as V, VII, IX and X, the cranial nerves 

 of higher vertebrates retain a primitive characteristic which is lost by 

 the spinal nerves. 



Plexuses. Although the spinal nerves of all chordates show a 

 segmental one-to-one correspondence with myotomes and vertebrae, this 

 regular metamerism is modified in the region of the paired appendages by 

 the union of a number of ventral rami into an interlaced network or 

 plexus. Four such plexuses are recognized in vertebrates, cervical, 



