I 



260 SPECIAL PHYSIOLOGY. 



oxygenated, or imperfectly decarbonized blood, or blood impaired or 

 impoverished, or poisoned by the natural secretions of the body, or 

 by foreign substances introduced into it, is unfit for the healthy nu- 

 trition of all parts of the nervous system, and more or less interferes 

 with, or may altogether arrest, their functions. The temperature at 

 which the nervous system can act properly, differs in the warm- and 

 cold-blooded animals ; it is presumable that, in all cases, the natural 

 temperature of the blood of the animal, is that best fitted for the func- 

 tional activity of the nervous system. A warm-blooded animal, it has 

 been shown, cannot long survive at a temperature lower than 72, nor 

 yet above 120. The benumbing effects of cold on our sensations, 

 and its ultimate fatal results, are well known, and will be hereafter 

 explained. 



Functions of the Cerebro-spinal Nerves. 



That the nerves are concerned in the functions of sensation and 

 motion, is suggested by the facts, that they are very numerous in all 

 highly sensitive parts, such as the eye, the tongue, and the cutis, or 

 true skin, and are also abundant in the muscles, that they are few in 

 number in the slightly sensitive and non-contractile tissues, such as 

 the ligaments, tendons, and bones, the latter of which have been said 

 to be absolutely insensible in health, and, lastly, that they are entirely 

 absent in the insensible tissues such as cartilage, the cuticle, and the 

 nails. The results of accidental destruction or division of a nerve in 

 the human body, and of its section in experiments upon living animals, 

 afford direct proof of the function of these parts; for when a nerve, 

 the branches of which are distributed to a sensitive part, such as the 

 eye or a portion of the hand, is destroyed by disease, or divided by 

 injury, or in an experiment, the sensibility, general or special, of that 

 part is destroyed; and so also, when a nerve proceeding to certain 

 muscles is cut accidentally or intentionally, those muscles are para- 

 lyzed. If in a frog, the bone and all the soft parts of the thigh, with 

 the exception of the nerves, be cut through, sensibility and power of 

 motion are still manifested, in various ways, in the parts so partially 

 isolated from the body; but, on the other hand, if the nerves them- 

 selves be divided, the other tissues remaining uncut, sensibility and 

 motion are destroyed in the parts previously supplied by the cut 

 nerves. Tight ligature of the nerves produces the same loss of sen- 

 sation and power of movement in the parts below the seat of ligature. 

 There can be no doubt, therefore, that the nerves are concerned in the 

 production of the phenomena of sensation and motion. 



If the upper part or end of a divided spinal nerve, which is still in 

 connection with the cord and brain, be pinched or irritated in any 

 way, a sensation, that of pain, is produced; again, if the lower portion, 

 which is severed from the spinal cord, be pinched or irritated, no sen- 

 sation is felt, but the muscles supplied by the nerve undergo contrac- 

 tion. In these experiments, it is inferred that the upper part of the 

 nerve, being stimulated, conducts the effects of that impression up to 

 the nervous centres, and that the lower part of the nerve conducts 



