9 8 



HUMAN PHYSIOLOGY 



the hand may be moved in different directions. In the dark 

 the distance of objects is also judged by estimating the extent 

 of movement of the hand and arm necessary to touch them. 

 The sensations derived from the joint and muscle senses are 

 often lost or impaired, and the condition of the mechanism 

 may be tested by ascertaining the smallest difference of 

 weight which can be appreciated. With moderate weights of 

 about one pound, a difference of about 5 per cent, can usually 

 be detected in the normal condition. (Experiment.) 



C. SPECIAL SENSES. 



I. Tactile Sense. 



The Tactile Corpuscles, which consist of a naked branch- 

 ing varicose termination of an axon surrounded by sheaths 



of fibrous tissue, are situated in 

 the papillae of the true skin 

 (Fig. 42). 



The study of the sense of 

 touch may be approached by 

 touching the table and analys- 

 ing the manner in which the 

 conclusion is arrived at that a 

 table is touched. We conclude 

 it is a table because the surface 

 is hard and smooth. This judg- 

 ment may or may not be correct. 

 But even in saying that the 



Fio. 42. Simple form of sensory body W6 touch IS hard and 

 nerve termination. In the tac- ., i r 



tile corpuscle the nerve fibre Smooth, W6 are also forming 



coils round the capsule before judgments from the sensations 

 entering. (DooiEL.) experienced. When we say the 



surface is hard we mean it resists pressure, and when we 

 say it is smooth we mean that the skin of our finger is 

 uniformly touched and not pressed upon at certain points 

 as it would be if the surface were rough. Or it may be 

 that we draw our finger over the table and feel a con- 

 tinuous contact, and not the series of contact which we 

 should experience were the surface rough. The determina- 



