PRESSURE SENSE. 287 



we descend the animal scale we find many of the lower 

 animals lacking some of the senses that we possess. But 

 here we must be careful in our statements. Because an 

 earthworm, for instance, lacks eyes, we may not say that it 

 lacks sight. That is, while there is pretty good evidence that 

 it cannot see objects distinctly as we can, it is not therefore 

 necessarily unable to distinguish between light arid darkness. 

 Many of the lower animals lacking eyes show that they 

 discern light. In many of the simpler forms of animal life 

 there is 110 evidence 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 

 exterior 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, to hear. Still 

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

 pretty sure that it feels. 



Further, the organs of special sense, and the nervous sys- 

 tem generally, may be said to originate in the skin, and to be 

 modifications of the sense of touch. It has long been known 

 that the brain and spinal cord originate as an infolding of 

 the skin, which afterward becomes completely shut off from 

 the outer surface. " In general, it may be said that a study 

 of the facts of development shows us that nerve-cells appear 

 at first upon the surface of the body, but that during the 

 growth of the organism the cells become shut off from the 

 surface; and in order to maintain their connection with 

 the periphery, long processes called nerves pass from the 

 cells thus deeply imbedded to the surface." 



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

 strictly a Pressure Sense. If we test the skin to find what 



