THE SPECIAL SENSES. 170 



street, or the regular ticking of a clock. All sensations 

 become less vivid with the advance of age, especially hear- 

 ing and vision. 



5. General Sensibility. There is a property pos- 

 sessed by nearly all parts of the human body which we 

 call general sensibility. We have recently seen that the 

 brain is wholly insensitive, and may be cut or pinched 

 without pain. The same is true of the nails, hair, the 

 Scarf-skin or external covering of the body, and a few other 

 structures. In these parts no nerves are found. On the 

 other hand, the sensibility of the true skin, and of mucous 

 membranes, as of the eye and nose, is exquisite, these or- 

 gans having a large supply of sensory nerve-fibres. The 

 bones and tendons have less of these fibres, and are only 

 moderately sensitive. 



6. The sensibility of any part of the body, then, de- 

 pends upon the number of nerves present; and, as a rule, 

 the nervous supply is proportional to the importance of 

 the part, and to its liability to injury. When, therefore, a 

 surgical operation is performed, the most painful part of it 

 is the incision through the skin ; the muscles, cartilage, 

 and bone being comparatively without sensation. Hence, 

 if we could benumb the surface, certain of the lesser oper- 

 ations might be undergone without great inconvenience. 

 This is, in fact, very successfully accomplished by means of 

 the cold produced by throwing a spray of ether, or of some 

 other rapidly evaporating liquid, upon the part to be cut. 



7. Tickling is a modification of general sensibility. At 

 first, it excites a pleasurable sensation, but this soon passes 

 into pain. It. is only present in those parts where the 

 sense of touch is feeble. But all impressions are not 

 received from without: there are, also, certain internal 



5 . General sensibility ? What have we seen as regards the brain ? Of what 

 other structures is the same true ? 



6. The cause of sensibility ? Painful part in a surgical operation ? Benumb- 

 ing the surface ? How done by ether ? 



7. Tickling ? Internal sensations ? The nerves of general sensibility ? 



