460 GENERAL BIOLOGY 



7) The development of coordination centers (groups of 

 integrated accessory circuits) among the added neurones of 

 the principal centers. 



8) The development of control centers in connection 

 with ganglia at the front end of the body, establishing a 

 brain. 



9) The development of the upper brain as the chief con- 

 trol center of vertebrates. 



Relations between parts and functions in the nervous system 

 of vertebrates. 



We have already given very brief consideration to 

 the parts in the nervous system of the salamander, (general 

 structure, pages 188192; development, pages 195- 

 201). Essentially the same features of both structure and 

 development characterize all the higher vertebrates. The 

 same parts are present and in the same relations, and they 

 come into existence by the same developmental processes. 

 The relatively greater development of the higher centers of 

 the brain has also been suggested (see figure 128 on page 

 201). Figure 260 shows roughly the location of the princi- 

 pal parts of the nervous system in the human body. Brain 

 and spinal cord occupy the dorsal axis, and cranial and 

 spinal nerves connect these multiple centers with every part 

 of the surface of the body. The sympathetic ganglia and 

 nerves of the organs of the coelom are not shown. 



As in the salamander so in other vertebrates each 

 spinal nerve has two roots. The ganglion-bearing posterior 

 root is a bundle of afferent or sensory fibers that bring in 

 the effects of stimuli from the surface of the body, and 

 the anterior root is a bundle of efferent (mostly motor 

 fibers,) that carry impulses to muscle or gland (or, through 

 the ventral commissures, to the ganglia of the sympa- 

 thetic system). The bodies of the afferent neurones, as t we 



