CHAPTER XIII 



THE PERIPHERAL NERVOUS SYSTEM 



The nerves, ganglia, and sense organs constitute the peripheral nervous 

 system. The peripheral nerves consist of bundles of myeKnated and unmyelin- 

 ated 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 

 originate from nerve cells which lie outside the neural tube. Those sensory 

 nerve cells related to the spinal cord 



X-XI gang, crest 

 XI fibers 



and to the brain stem caudal to the 

 otic vesicle are derived from the gang- 

 lion crest, the origin of which has 

 been described (Chapter X, p. 304). 



A. SPINAL NERVES 



The spinal nerves are segment- 

 aUy arranged and each consists of 

 dorsal and ventral roots, spinal gang- 

 lion, 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. 357). The spinal gangha are represented as enlargements along the gang- 

 lion crest and are connected by cellular bridges. 



In 7 mm. embryos (five 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. 358). These fibers course in the dorsal funiculi and 



23 353 



Fig. 357. — Reconstruction of an embryo of 4 

 mm., showing the development of the cerebrospinal 

 nerves (Streeter). X 17. Ci., a., etc., cervical 

 spinal nerves. 



