CHAPTER XIII 

 THE PERIPHERAL NERVOUS SYSTEM 



THE nerves, ganglia, and sense organs constitute the peripheral 

 nervous system. The peripheral nerves consist of bundles of myelinated 

 and unmyelinated 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 motor im- 

 pulses 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-laterally from the neural tube. 

 Those arising from the spinal cord take origin in the mantle layer, con- 

 verge, 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 components 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 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. 302). 



X-XI gang, crest 

 XI fibers 



^=^ 



F IG - 357- Reconstruction of an embryo of 4 mm., showing the development of the cerebro- 

 spinal nerves (Streeter). X 17. Cr-6, Cervical spinal nerves. 



A. THE SPINAL NERVES 



The spinal nerves are segmentally 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 neuro- 

 blasts in the mantle layer of the spinal cord (Fig. 357). The spinal ganglia 

 are represented as enlargements along the ganglion 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 



23. 353 



