STRUCTURE AND PHYSIOLOGY OF ANIMALS 



or contraction of the blood-vessels of the part ; and a similar 

 initiatory or controlling influence is exerted over the activities of 

 all the organs. 



In certain groups of animals all the impressions from the 

 external world are received 

 through the integument of 

 the general surface, and 

 this is the case in all 

 animals with the general 

 impressions of touch and 

 of heat and cold. The 

 sensitiveness of the integu- 

 ment to such general im- 

 pressions may be increased 

 by the presence in it of a 

 variety of tactile papillae 

 or corpuscles having nerve- 

 fibres terminating in them. 

 In most animals, however, 

 there are certain organs, 

 the organs of special 

 sense, adapted to receiving 

 impressions of special kinds 

 eyes for the reception of 

 the impressions produced 

 by light, ears for the recep- 

 tion of those produced by 

 the waves of sound, ol- 

 factory organs or organs of 

 smell, and gustatory organs 

 or organs of taste. The 

 most rudimentary form of 

 eye is little more than a 

 dot of pigment which 

 absorbs some of the rays 

 of bright light these pro- 

 ducing a nerve-disturbance 

 in certain neighbouring 

 nerve-cells. To this may 

 be added clear, highly- 

 refracting bodies which in- 

 tensify the effect. In the 

 higher types of eye there 

 are the same characteristic 



parts the clear, highly-refracting substance, the pigment, and 

 the nerve-cells ; but each has undergone a development resulting 

 in the construction of an organ adapted to the reception of light- 



sc 



FIG. 26. Nervous system of the Frog. (From 

 Howes's Atlas.) 



