254 THE DEVELOPMENT -OF THE CHICK 
spinal nerve has four components, viz., somatic motor, somatie 
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. ach 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 vertebra. 
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. 
