CHAPTER XII 



THE PERIPHERAL NERVOUS SYSTEM 



The nerves, ganglia and sense organs constitute the peripheral nervous 

 system. The peripheral nerves consist of bundles of medullated and non-medul- 

 lated nerve fibers and aggregations of nerve cells, the ganglia. The fibers are of 

 two types: Afferent fibers which carry sensory impulses to the central nervous 

 system, and efferent fibers, which carry effective impulses away from the nervous 

 centers. The peripheral efferent fibers of both brain and spinal cord take their 

 origin from neuroblasts of the basal plate. Typically they emerge ventro-later- 

 ally from the neural tube. Those arising from the spinal cord take origin in the 

 mantle layer, converge and form the ventral roots of the spinal nerves. The 

 efferent fibers of the brain take origin from more definite nuclei and constitute 

 the motor or effector portions of the cerebral nerves. The peripheral afferent fibers 

 take origin from nerve cells which he outside the neural tube. Those sensory 

 nerve cells related to the spinal 

 cord and to the brain stem caudal 

 to the otic vesicle are derived from 

 the ganglion crest, the origin of 

 which has been described (Chapter 

 X, p. 314). 



X-Xigang. crest, 

 xlfibres. 



bridge. 



Fig. 339. — Reconstruction of an embryo of 4 mm. 

 showing the development of the cerebrospinal nerves. 

 Ci., 2., etc., cervical spinal nerves (Streeter). 



A. SPINAL NERVES 



The spinal nerves are segment- 

 ally arranged and each consists of 

 dorsal and ventral roots, spinal 



ganglion and nerve trunks. In embryos of 4 mm. the ventral roots are already 

 developing as outgrowths of neuroblasts in the mantle layer of the spinal cord 

 (Fig. 339) . The spinal ganglia are represented as enlargements along the ganglion 

 crest and are connected by a bridge of cells. 



In 7 mm. embryos (four weeks old) the cells of the spinal ganglia begin to 

 develop centrally directed processes which enter the marginal zone of the cord as 

 the dorsal root fibers (Fig. 340). These fibers course in, and eventually form the 



351 



