THE SPECIAL SENSES 253 



5. General Sensibility. There is a property possessed 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 cover- 

 ing 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 organs 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, depends 

 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 operations might be undergone without 

 great inconvenience. This is, in fact, very successfully accom- 

 plished 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 sensations, as they are 

 called, which depend upon the condition of the internal organs, 

 such as appetite, hunger, thirst, dizziness when looking down 

 from some lofty position, drowsiness, fatigue, and other feel- 

 ings of comfort or discomfort. General sensibility, whether of 

 the internal or external organs of the body, chiefly depends 



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 ? Benumbing the 

 surface ? How done by ether ? 



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



