SENSE ORGANS. 



199 



vations,* that different nerve fibers take cognizance of 

 contact, pain, heat, cold, and muscular resistance (Fig. 

 68, page 95). In physics cold is a negative term, a mere 

 absence of heat; but in physiology, as we all know, it 

 is a very positive sensation. 



Now, it is impossible to take up all the sensations in 

 detail. We take only that which is the most universal 

 and fundamental, viz., contact (pressure being only a 

 stronger contact). This, together with the muscular 

 sense of resistance, gives us externality, or the existence 

 of the external world. 



Again, it is necessary to distinguish between general 

 sensibility and special sense of touch. The former, to- 

 gether wifh the so-called muscular sense, gives the exist- 

 ence of the external world ; the latter may be regarded 

 as the same, specially organized to give definite knowl- 

 edge of some of its properties, such as shape, hardness, 

 roughness, etc. These two kinds of sense of contact are 

 by no means developed in the same degree. The con- 

 junctiva of the eye is exquisitely sensitive to contact, 

 but it does not appreciate the properties of the touching 

 body as does the finger tips or the tongue tip. The same 

 difference, it will be remembered, we found in the retina. 

 Mere sensitiveness to light is keenest a little way from 

 the central spot, but this spot alone is specially organized 

 to give us accurate knowledge of shape, color, etc. 



General Organ of Touch. — The general organ 

 for perception of contact is the skin and portions of the 

 mucous membranes near the outlets of the passages, 

 especially the mouth. This is for all contact. Besides, 

 there are certain portions of the skin specially organized 

 as organs of touch, such as the hands and the parts about 

 the mouth. 



* Sci., vii, 151, 1886, and 459, 1886. 



