148 The Study of Animal Life PART u 



very difficult of determination, because of the immense 

 quantity of blood that .passes through any tissue in a short 

 time. This concludes our sketch of the interchange of 

 matter within the body. 



The Activities of the Nervous System. We have now 

 to consider the arrangement of the nervous system first 

 merely as the means by which all the varied activities of 

 the tissues of the separate parts of the body are co-ordi- 

 nated and wrought into an harmonious series of actions, 

 and then as the associate of consciousness and of mental 

 processes. 



Just as protoplasm may be called the physical basis of 

 life, so is nervous tissue par excellence the physical basis of 

 consciousness and of mind. Throughout the whole animal 

 kingdom it has a superficial similarity of structure, and 

 consists of the same three parts. 



(1) First there are cells adapted to receive notice of 



change in the outer world. Changes in the sur- 

 rounding medium and affecting such cells are 

 called stimuli. These cells sensitive to stimuli 

 form the chief part of the sense organs the 

 eyes, ears, nose, tongue. Also in the skin 

 there are cells sensitive to alterations of touch 

 and temperature. The effect of stimuli upon 

 such cells is probably to set them into a state of 

 molecular agitation, which may or may not result 

 in chemical changes. 



(2) There are connecting fibres or nerves, which, 



being connected with the sensory cells, take up 

 the vibrations or possibly the chemical changes 

 of the sensory cells and transmit them to the 

 " centre." 



(3) There are " central " cells, in which the nerves 



end, and which are set in molecular agitation by 

 the vibrations of the nerves. This molecular 

 agitation is often, when the central cells are in 

 the brain, accompanied by consciousness. Ap- 

 parently also agitations may arise " spontane- 

 ously" within these central cells and stimulate 



