MULTICELLULAR ANIMALS 221 



become recognized by the central system. Other ectodermal cells 

 from the neural crest also migrate elsewhere and become associated 

 with mesodermal elements to form endocrine glands; for example, 

 the adrenal glands are in part derived from such cells. 



Spinal Nerves (Figs. 141, 146). But the greater number of the 

 cells that are in the embryonic neural crest on both sides of the 

 newly organized neural tube remain in their original position with 

 reference to the tube and form a series of ganglia on either side and 

 slightly lateral to the dorsal region of the spinal cord. These ganglia 

 correspond to the primitive segments and so in the adult human 

 are vestiges of primitive segmentation. Nerve fibres, that is, the 

 dendrites, grow out from the nerve cells in these ganglia and inner- 

 vate the surface and other regions of the vertebrate body. All the 

 endings of these fibres are associated with organs for the reception 

 of stimuli; hence the spinal ganglia are sensory in function. Other 

 fibres, axones, grow centrally from the ganglia and penetrate the 

 dorsal region of the spinal cord to establish connections with the 

 nerve tracts within. Thus a continuous communication is set up 

 between the sensory nerve endings on the surface of the body and 

 the spinal cord and brain (Fig. 141). 



The motor fibres growing out from the cells of the ventral region 

 of the spinal cord on either side become associated with the sensory 

 fibres growing out from the dorsal ganglia to form nerve trunks. 

 The ventral fibres all carry outward bound impulses and as they 

 branch oflF from the nerve trunks they pass to muscles. Thus a 

 spinal nerve is composed of sensory and motor fibres, associated 

 with the communicating fibres from the sympathetic system. 



Reflex Arc. Because of the numerous communications existing 

 between sensory and motor fibres within the wall of the spinal cord 

 an impulse may be received and traverse a sensory fibre, enter the 

 cord, be transferred to a motor fibre, and finally activate a set of 

 muscles without passing up the cord through the complex association 



