254 THE DEVELOPMENT OF THE CHICK 



spinal nerve has four components, viz., somatic motor, somatic 

 sensory, splanchnic motor, and splanchnic sensory, the two latter 

 constituting the so-called sympathetic nervous system. It is 

 obvious, of course, that the splanchnic components must be 

 missing in the caudal nerves. The somatic and splanchnic com- 

 ponents will be considered separately. 



Somatic Components. Each spinal nerve arises from two roots, 

 dorsal and ventral (Fig. 145). The fibers of the former arise from 

 the bipolar neuroblasts of the spinal ganglia; the fibers of the ven- 

 tral root, on the other hand, arise from a group of neuroblasts in 

 the ventral portion of the cord. The roots unite in the interver- 

 tebral foramen to form the spinal nerve. Typically, each spinal 

 nerve divides almost immediately into three branches, viz., a dor- 

 sal branch, a ventral branch, and a splanchnic branch to the sym- 

 pathetic cord; the last is known as the ramus communicans. 



Fig. 145 represents a section passing through the twentieth 

 spinal nerve of an eight-day chick. The dorsal and ventral roots 

 unite just beneath the spinal ganglion; fibers are seen entering 

 the sympathetic ganglion {ramus communicans); the ventral 

 branch passes laterally a short distance where it is cut off; 

 beyond this point it can be traced in other sections in the 

 next posterior intercostal space more than half-way round the 

 body-wall; that is, as far as the myotome has extended in its 

 ventral growth. The dorsal branch arises at the root of the 

 ventral and passes dorsally in contact with the ganglion to 

 branch in the dorsal musculature. This nerve may be regarded 

 as typical of the spinal nerves generally. 



There are thirty-eight spinal nerves in an embryo of eight 

 days. The first two are represented only by small ventral roots. 



The first two spinal ganglia are rudimentary in the embryo and 

 absent in the adult, hence the ganglion illustrated in Fig. 145 is the 

 eighteenth of the functional series (see Fig. 149); it lies between the 

 nineteenth and twentieth vertebrae. 



The fourteenth, fifteenth, and sixteenth are the principal 

 nerves of the brachial plexus, and have unusually large ganglia. 

 The twenty-third to the twenty-ninth are the nerves of the leg 

 plexus, the thirtieth to the thirty-second innervate the region 

 of the cloaca and the remainder are caudal. The special mor- 

 phology of the spinal nerves does not belong in this description. 



