ACCOMPLISHED IN THE ENCEPHALON. Ill 



The sensations are numerous, but they may all be comprised in two 

 divisions, the external and the internal. Vision and audition afford 

 us examples of the former, in which the impression made upon the 

 organ is external to the part impressed. Hunger and thirst are in- 

 stances of the latter, the cause being internal; necessary ; and depend- 

 ing upon influences seated in the economy itself. Let us endeavour to 

 discover in what they resemble each other. 



In the first place, every sensation, whatever may be its nature, ex- 

 ternal, or internal, requires the intervention of the encephalon. The 

 distant organ as the eye or ear may receive the impression, but it 

 is not until this impression has been communicated to the encephalon, 

 that sensation is effected. The proofs of this are easy and satisfactory. 

 If we cut the nerve proceeding to any sensible part, put a ligature 

 around it, or compress it in any manner; it matters not that the 

 object, which ordinarily excites a sensible impression, is applied to the 

 part, no sensation is experienced. Again, if the brain, the organ of 

 perception, be prevented in any way from acting, it matters not that 

 the part impressed, and the nerve communicating with it, are in a con- 

 dition necessary for the due performance of the function, sensation is 

 not effected. We see this in numerous instances. In pressure on the 

 brain, occasioned by fracture of the skull ; or in apoplexy, a disease 

 generally dependent upon pressure, we find all sensation, all mental 

 manifestation, lost ; and they are not regained until the compressing 

 cause has been removed. The same thing occurs if the brain be stu- 

 pefied by opium ; and, to a less degree, in sleep, or when the brain is 

 engaged in intellectual meditations. Who has not found, that in a 

 state of reverie or brown study, he has succeeded in threading his way 

 through a crowded street, carefully avoiding every obstacle, yet so little 

 impressed by the objects around as not to retain the slightest recollec- 

 tion of them ! On the other hand, how vivid are the sensations when 

 attention is directed to them ! Again, we have numerous cases in which 

 the brain itself engenders the sensation, as in dreams, and in insanity. 

 In the former, we see, hear, speak, use every one of our senses appa- 

 rently ; yet there has been no impression from without. Although we 

 may behold in our dreams the figure of a friend long since dead, there 

 can obviously be no impression made on the retina from without. 1 



The whole history of spectral illusions, morbid hallucinations, and 

 maniacal phantasies, is to be accounted for in this manner. Whether, 

 in such cases, the brain reacts upon the nerves of sense, and produces 

 an impression upon them from within, similar to what they experience 

 from without during the production of a sensation, will form a subject 

 for future inquiry. Pathology also affords several instances where the 

 brain engenders the sensation, most of which are precursory signs of 

 cerebral derangement. The appearance of spots flying before the 

 eyes, of spangles, depravations of vision, hearing, &c., and a sense of 

 numbness in the extremities, are referable to this cause ; as well as the 

 singular fact well known to the operative surgeon, that pain is often 



1 Adelon, Art. Encephale (Physiologic), in Diet, de Med., vii 514, Paris, 1823, and Phy- 

 siol. de 1'Homme, torn. i. p. 239, 2de edit., Paris, 18'29. 



