110 SENSATIONS. 



others, it has been shown, that an animal may live days, and even weeks, 

 after the hemispheres have been removed; nay, that in certain animals, 

 as reptiles, no change is produced in their habitudes by such abstraction. 

 They move about as if unhurt. Injuries of the surface of the cere- 

 bellum exhibit, that it also is not sensible; but deeper wounds, and 

 especially such as interest the peduncles, have singular results, to be 

 explained hereafter. The spinal cord is not exactly circumstanced in 

 this manner. Its sensibility is exquisite on the posterior surface ; much 

 less on the anterior, and almost null at the centre. Considerable sen- 

 sibility is also found within, and at the sides of, the fourth ventricle; 

 but this diminishes as we proceed towards the anterior part of the 

 medulla oblongata, and is very feeble in the tubercula quadrigemina 

 of the mammalia. 



It has been shown, that the spinal nerves, by means of their poste- 

 rior roots, convey general sensibility to the parts to which they are dis- 

 tributed. But there are other nerves, which, like the brain, are them- 

 selves entirely devoid of general sensibility. This has given occasion 

 to a distinction of nerves into those of general and of special sensi- 

 bility. Of nerves, which must be considered insensible or devoid of 

 general sensibility, we may instance the optic, olfactory, and auditory. 

 Each of these has, however, a special sensibility ; and although it may 

 exhibit no pain when irritated, it is capable of being impressed by 

 appropriate stimuli by light, in the case of the optic nerve ; by 

 odours, in that of the olfactory ; and by sound, in that of the auditory. 

 Yet we shall find, that every nerve of special sensibility seems to re- 

 quire the influence of a nerve of general sensibility ; the fifth pair. 



Many nerves appear devoid of sensibility, as the third, fourth, and 

 sixth pairs; the portio dura of the seventh; the ninth pair of encephalic 

 nerves ; and, as has been shown, all the anterior roots of the spinal 

 nerves. 



The parts of the encephalon, concerned in muscular motion, will fall 

 under consideration hereafter. 



2. PHYSIOLOGY OF SENSIBILITY. 



Sensibility we have defined to be the function by which an animal 

 experiences feeling, or has the perception of an impression. It in- 

 cludes two great sets of phenomena ; the sensations, properly so called, 

 and the intellectual and moral manifestations. These we shall investi- 

 gate in succession. 



a. Sensations. 



A sensation is the perception of an impression made on a living 

 tissue ; or, in the language of Gall, it is the perception of an irrita- 

 tion. By the sensations we receive a knowledge of what is passing 

 within or without the body ; and, in this way, our notions or ideas of 

 them are obtained. When these ideas are reflected upon, and compared 

 with each other, we exert thought and judgment; and they can be re- 

 called with more or less vividness and accuracy by the exercise of 

 memory. 



