152 PHYSIOLOGY. 



dark, through intricate passages, with the greatest facility. 

 {The fifth nerve is associated with the organs of the senses 

 of smell, sight, and hearing ; it exercises that of touch, is 

 the immediate instrument of taste, and is affected in some 

 degree by pungent odorous substances, by light, and by 

 sound,! 



21. Spinal nerves.-^The nerves of the spinal cord are all 

 similar in their construction, and in the functions which 

 they perform/ ^The two sets of filaments of which they are 

 composed, though enclosed in the same sheath, yet remain 

 entirely distinct throughout their entire course*? i^Fhey go 

 to every muscular fibre in the system, and spread out over 

 the entire surface of the body) which possesses, accordingly, 

 a more exquisite sensibility than the deep-seated parts. 

 Thus in amputating a limb, the chief pain is in cutting 

 through the external parts ; for the bone may be sawn 

 through, the muscles, tendons, and ligaments cut and la- 

 cerated, and even burnt with a red-hot iron^ and still the 

 patient experiences little or no suffering. 



22. f The object of endowing the skin with such a high 

 degree of sensibility, is, doubtless, to warn us to avoid, not 

 only what is injurious to the skin itself, but also what might 

 endanger internal parts. It thus serves as a protector to 

 the whole body. The extremes of heat and cold, which 

 might prove injurious, produce their painful impressions^ 

 mechanical causes rouse by their sharpness, roughness, or 

 hardness ; acrid and corrosive chemical agents induce un- 

 easy sensation all which serve to admonish us to shun the 

 causes producing such effects, fit is remarkable to notice 

 how every part of the body is endowed with its own kind 

 of sensibility/' J The skin feels changes of temperature ; the 

 muscles experience only a sense of fatigue ; the eye is sen- 

 sible only to light ; the ear to sound ; the nose to odours ; 

 the heart to blood ; the stomach to food^ &c. ; and these 

 sensations are sent along up to the brain by little threads, 

 in close contact with other threads, which bring back the 



