330 



PHYSIOLOGY 



this development by the evolution of animals, such as the worms 

 and crustaceans, which are segmented and capable of locomotion. 

 The fact that these animals are segmented determines the collection 



of the central cells into a chain of ganglia, 

 one ganglion or pair of ganglia being provided 

 for each segment. In the act of locomotion 

 it is of advantage to the animal that those 

 sense-organs or sensory cells which are pro- 

 jicient, i.e. which are stimulated by changes 

 in the environment originating at a distance 

 from the animal, should be collected together 

 near that part which goes first, namely, the 

 head end. Thus the projected sensations of 

 sight, those which are excited by chemical 

 changes in the surrounding medium and repre- 

 sent the sense of smell, and those which are 

 specially aroused by vibrations in the surround- 

 ing medium and correspond to those which 

 we call the sense of sound, are in the majority 

 of these animals subserved by organs situated 

 near the head end. 



The wisdom of a man is measured by his 

 foresight. The chances of an animal in the 

 struggle for existence are determined by the 

 degree to which the reactions of the animal 

 to its immediate environment are held in check 

 in response to stimuli arising from approaching 

 events. An animal without power to see, 

 smell, or hear its enemy will receive no 

 impulse to fly until it is already within its 

 enemy's jaws. It must therefore be of ad- 

 vantage to a segmented animal that the 

 activities of the whole chain of segmented 

 ganglia should be subservient to those central 

 nerve-cells which are in direct connection 

 with the projicient sense-organs at the head. 

 The influence exerted by the head ganglia will 

 be in the first place inhibitory of the direct 

 reaction excited in each segment by stimulation 

 of its surface, and, for this influence to be propagated, long tracks must 

 be laid down, joining up ganglion to ganglion and propagating impulses 

 from the head ganglia to the most distant part of the chain. As a 

 type of such a system we may refer to the crayfish. In this animal 

 the ' central nervous system consists of a chain of thirteen ganglia, 



FIG. 138. View of central 

 nervous system of cray- 

 fish. (After YUNG and 

 VOGT. ) 



a, cerebral ganglion. 



b, commissure. 



e, subcesophagealganglion. 



g, first abdominal ganglion. 



I, oesophagus. 



m, optic nerve. 



p, antennary nerve. 



s, stomato-gastric nerve. 



