352 ELEiMENTARY PHYSIOLOGY less. 



Each corpuscle consists of an elongated central core of 

 glassy-looking material in which the axis of the nerve is 

 embedded and terminates. The core is surrounded by 

 some 30 to 40 capsules made of connective tissue, and 

 placed one outside the other like the layers of an ordinary 

 onion. 



(i) The Sensation of Pressure. — Mere contact of 

 a single object with the skin exerts a pressure on it 

 which results in a stimulation by means of which we 

 become aware that something is touching us. The power 

 of discriminating pressure and its differences we may call 

 the sense of pressure. The sensitiveness of the various 

 regions of the skin in responding to pressure varies, and 

 the difference may be measui-ed for each {)art of the skin 

 by determining either what the least weight is which can 

 be just felt when allowed to rest on that part, or else by 

 determining the least difference in weight which can be 

 distinguished between two weights laid in succession on 

 the same spot. Experimenting in this way it may be 

 shown that the sense of pressure is most acute on the 

 skin of the forehead and of the back of the hand. The 

 sense is less acute in the skin of the finger tips. Careful 

 investigation seems to show, with but little doubt, that 

 some points on the skin of any part are peculiarly sensi- 

 tive to pressure. Hence we may perhaps speak of 

 " pressure spots " in the same way that we do of " heat 

 spots" and "cold spots." 



(ii) The Sensations of Heat and Cold. — The feel- 

 ing of warmth, or cold, is the result of an excitation 

 of sensory nerves distributed to the skin, which are 

 possibly distinct from those which give rise to the sense 

 of touch. And it would appear that the heat must be 

 transmitted through the epidermic or epithelial layer, to 

 give rise to this sensation ; for, just as touching a naked 

 nerve, or the trunk of a nerve, gives rise only to pain, so 

 heating or cooling an exposed nerve, or the trunk of a 



