280 PHYSIOLOGY. 



As we descend the animal scale we find many of the lower 

 animals lacking some of the senses that we possess. In 

 many of the simpler forms of animal life there is no evi- 

 dence of a sense of hearing, and it is extremely likely that 

 if they have taste and smell, these senses are in a very 

 rudimentary state of development. But in all these forms 

 it is believed that " feeling " exists. Contact of their exte- 

 rior with foreign objects is so often immediately followed 

 by action that little doubt remains about their having the 

 sense of touch. Even ameba may have, in a rudimentary 

 state, the power to distinguish light, to taste, and to hear. 

 Still we have little or no evidence on these points, while 

 we are pretty sure that it feels. 



The Pressure Sense. The sense of touch, proper, is 

 strictly a pressure sense. If we test the skin to find what 

 regions are able to detect the least pressure, it is found 

 that the forehead is most sensitive, and nearly equally so 

 are the temples, back of the hand, and forearm. 



Ability to detect Differences of Pressure. The 

 ability to detect differences of pressure is tested by finding 

 what is the least addition to a weight required to make it 

 seem heavier. For instance, if a weight of 1 1 grains is 

 just perceptibly heavier than one of 10 grains, it does not 

 follow that i grain added to a weight of 100 grains will 

 give any palpable increase. To 100 grains must be added 

 10 grains before additional pressure is felt ; that is, what- 

 ever the weight, there must be the same ratio of increase 

 to increase the sensation. This is part of the law, already 

 stated, of the relation of stimulus and sensation. The law 

 is true only in a general way and will not apply in extreme 

 cases. It is stated that the forehead, the lips, and temples 

 appreciate an increase of one fortieth to one thirtieth of the 



