PRODUCTION OF SENSATIONS 



343 



arranged in separate layers around a narrow central cavity 

 called the core (Fig. 145). Within the core is the termina- 

 tion of a large nerve fiber. These corpuscles are found in 

 the connective tissue beneath 

 the skin, along tendons, around 

 joints, and among the organs 

 of the abdominal cavity. 



The simple forms of sense E 

 organs have a more or less 

 general distribution over the 

 body, and are concerned in 

 the production of at least three 

 special sensations. These are 

 touch, temperature, and the 

 muscular sensation. 



Touch, or feeling, is perhaps A 

 the simplest of the sensations. 

 The sense organs employed 

 are the touch corpuscles, and 

 the external stimulus is some 

 form of pressure or impact. 

 Pressure applied to the skin, 

 by acting on the fiber terminations in the corpuscles, starts 

 the impulses that give rise to the sensation. The touch 

 corpuscles render the fiber terminations so sensitive that 

 the slightest pressure is able to arouse sensations of touch. 

 It is found that a change of pressure, rather than pressure 

 that is constant, is the active stimulus. That all parts of 

 the skin are not equally sensitive to pressure, and that the 

 mind does not interpret equally well the sensations from 

 different parts, are facts easily demonstrated by experiment. 

 (See Practical Work.) 



The Temperature Sensation. Temperature sensations, 



FIG. 145. Pacinian corpuscle, 

 magnified. A. Medullated nerve 

 fiber. B. Axis cylinder terminating 

 in small bulb at C. D. Concentric 

 layers of connective tissue. E. In- 

 ner bulb. 



