. The development of the chick; an introduction to embryology . rii)heral nervous system comprises the nerves whichspan between peripheral oi-gans and the central nervous are fifty pairs in a chick embryo of eigiit days, of whichtwelve innervate the head, and thirty-eight the trunk, distin-guished respectively as cranial and spinal nerves. It is con-venient for purposes of description to consider cranial and spinalnerves separately, and to take uj) the sj^nal nerves first becausethey are much more uniform in their mode of developmentthan the cranial nerves, and also exhibit a more
. The development of the chick; an introduction to embryology . rii)heral nervous system comprises the nerves whichspan between peripheral oi-gans and the central nervous are fifty pairs in a chick embryo of eigiit days, of whichtwelve innervate the head, and thirty-eight the trunk, distin-guished respectively as cranial and spinal nerves. It is con-venient for purposes of description to consider cranial and spinalnerves separately, and to take uj) the sj^nal nerves first becausethey are much more uniform in their mode of developmentthan the cranial nerves, and also exhibit a more primitive ortypical condition, on the basis of which the development of thecranial iumvcs must be, in part, at least, explained. The Spinal Nerves. Each spinal nerve may l)e divided intoa somatic portion rclatcMl jirimarily to thesomatopleure and axis ofthe embryo, and a s])lanchnic poilion related primarily to thesi)lanchnopleure and its derivatives. In each of these again amotor and scMisorv component mav ])e distinguished. Thus each THE NERVOUS SYSTEM 253. Fig. 151. — Six transverse sections fhroush the brain of an 8-day chick inthe planes represented in Fig. , Cerebellum. F. M., Foramen of Mom-o. (in. V., Ganghon of thetrigeminus. Isth., Isthmus. It. d., Diverticuluni of the iter. lat. V.,Lateral ventricle. Otlier abbreviations as before (Fig. 147). 254 THE DEVELOPMENT OF THE CHICK spinal nerve has four conijjonents, viz., somatic motor, somaticsensory, splanchnic motor, ami splanchnic sensory, the two latterconstituting the so-called sympathetic nervous system. It isobvious, of course, that the splanchnic components must bemissing 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 fromthe bipolar neuroblasts of the spinal ganglia; the fibers of the ven-tral root, on the other hand, arise from a group o
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