CHAPTER IV. 



GENERAL PHYSIOLOGY. 



A LIVING organism is wont to be guided in its actions by im- 

 pressions received from external sources. A simple impression, 

 such as a sharp sound, a flash of light, a prick on the skin, is 

 called a stimulus. If we apply such a stimulus as a light pin- 

 prick to the leg of a frog, the animal will spring away. Thus a 

 very simple stimulus gives rise to a very complex action, one in- 

 volving a great number of exceedingly delicate muscular adjust- 

 ments. It is characteristic of the higher animals that very 

 complex activities may be the outcome of very simple stimuli. 



This is sometimes explained by saying that the stimulus 

 affects the mind or consciousness of the animal, and then the 

 mind or consciousness causes the activities which follow. But 

 we know so little about the mind of frogs, that it will be well 

 rather to confess our ignorance of what goes on within the 

 organism than to accept this as an explanation. 



If we kill a frog, rendered insensible by chloroform, by 

 rapidly severing the muscles between the skull and vertebral 

 column, so as to lay bare the central nervous cord, and then 

 extirpating the brain by thrusting a large pin or stout wire 

 within the skull, we shall find that it still responds to certain 

 stimuli. If, for example, its side be touched with acetic acid, 

 the hind-limb of that side will be drawn up and the foot passed 

 over the spot so as to remove the source of irritation. Although 

 the frog, as an organism, is quite dead, the tissues of its body 

 are still living. We have no reason to suppose that the brain- 

 less frog possesses anything like consciousness, and yet complex 

 activities follow a simple stimulus. Such a response is called a 



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