ORGANIC EVOLUTION 



191 



a 



graph lines in passing outward from the commercial 

 centres to the remoter districts. 



Within the coelom of vertebrates there are other ganglia, 

 in part arranged in pairs segmentally and connected with 

 the ventral roots of the spinal nerves, as shown in figure 117, 

 and in part variously disposed in the walls of the internal 



organs of the coelom, whose funct- 

 ions they control and coordinate. 

 These are connected with each 

 other by nerve fibres. They to- 

 gether constitute the so-called 

 sympathetic system. The fun- 

 damental nutritive processes of 

 the body, that are performed 

 involuntarily, and that are es- 

 sential to keep life going, are 

 unconsciously controlled through 

 the sympathetic system. Prac- 

 tically all the involuntary mus- 

 cles of the body, those of the skin, 

 as well as those of the viscera, 

 are controlled through nerve 

 fibres that take their origin from 

 the cells of sympathetic ganglia. 

 There is another, more direct 

 line of communication between 

 the organs of the coelom and the 

 brain. One pair of cranial 

 nerves (called the vagi; sing, vagus) which descends 

 through the neck into the coelom, sends branches also to 

 the lungs, the heart, the stomach and part of the intestine. 

 The movements of the involuntary muscles are com- 

 paratively simple and uniform, but those of the volun- 

 tary muscles of the body wall and limbs are infinitely 



Fig. 117. Diagram illustrating 

 the relation of the neural tube 

 to the ganglia a, is a cross- 

 section of the body showing 

 the sympathetic ganglia in 

 the coelom ; b, is a cross- 

 section of the cord and adja- 

 cent ganglia ; showing roughly 

 the location of the groups of 

 nerve cells. 



