170 Rev. P. Keith on the Susceptibilities of Living Structures, 



the impression made to or from a common centre of feeling and 

 of consciousness. It is a case of excitability or of irritability 

 peculiar to animal structures, and is essentially perceptible. — 

 Perceptible impressions are either external or internal, and 

 the movements which they occasion are either voluntary or 

 involuntary, — voluntary if they originate in the brain, invo- 

 luntary if they originate in any other organ.— External im- 

 pressions are either general or particular. Thus there is a 

 general susceptibility that extends over the whole surface of the 

 animal fabric, which physiologists denominate tact, by which 

 we are apprised of the presence and influence of such bodies 

 or accidents as approach and affect us, — as heat, cold, mois- 

 ture, pressure, — producing pleasure, or producing pain. But 

 there are also peculiar susceptibihties which are confined to 

 peculiar organs. Thus the eye is adapted to receive, exclu- 

 sively, the impression of light; the ear the impression of sound, 

 the nostril the impression of smell, the tongue the impres- 

 sion of taste, and the hand or the finger the impression of 

 touch. — Internal impressions may originate in, or be trans- 

 mitted from, any or all of the internal organs, if not in their 

 natural, at least in their morbid state. Thus the empty sto- 

 mach perceives and transmits the sensation of hunger ; the 

 exhausted lungs that of the want of a due supply of air ; 

 the distended bladder that of the necessity of being emptied ; 

 and the excited sexual organs that of the urgency of the ve- 

 nereal desire. — The muscular mechanism of the organs is 

 the means by which the impression is received, and the nerves 

 of sensation are the instruments by which the received im- 

 pression is transmitted to the brain. This is a truth that 

 cannot be controverted. For if the nerve of sensation leading 

 from the organ to the brain is injured, there is no more sen- 

 sation ; — no seeing without an optic nerve; no hearing without 

 an auditory nerve; and no touch nor tact without the integrity 

 of the nerves that terminate in the skin. 



A nerve as seen in its course in the human body is a dense 

 white cord covered with three membranes corresponding with, 

 and traceable to, the membranes of the brain. The cords 

 consist of a number of distinct filaments, each composed of a 

 fine membrane inclosing a soft and pulpy matter, and each 

 endowed with its own specific capability. It is either a fila- 

 ment of sensation only, or of motion only, — and that through- 

 out the whole of its course. The filament of sensation can- 

 not act as a filament of motion, nor the filament of motion as 

 a filament of sensation*. 



• Bell on the Nerves, p. 18. 



If 



