FUNCTIONS OF THE SPINAL NERVES. 261 



those effects downwards to the muscle. Hence the special office which 

 the nerves perform in the phenomena of sensation arid motion, is said 

 to be that of acting as conductors. Certain structural arrangements 

 in the nerves and the nerve fibres, have been supposed to favor and 

 render more precise this conducting power, by insulating the channels 

 of conduction ; thus, the axis fibre being supposed to be the conduct- 

 ing substance, the medullary sheath and the tubular envelope of the 

 white fibres, are said to act as insulators. In the nerves, the sheath 

 or neurilemma may serve as an additional insulating as well as a pro- 

 tecting investment. No matter how close to the spinal cord the nerves 

 are cut, similar results to those just mentioned ensue, the portion of 

 the nerve detached from the cord never being capable of producing 

 any sensory phenomenon, nor yet being able, if left unirritated, to 

 determine any regular movement in the muscles with which it is still 

 connected. Hence we arrive at the negative conclusion, that the 

 nerves of themselves are not either seats of sensation, or natural 

 centres of origin of motorial stimulus. It has already been stated that 

 the fibres of a spinal nerve, which convey the sensory impressions up- 

 wards, are called the afferent nerve fibres ; whilst those which conduct 

 the effects of motorial stimuli downwards, are named efferent fibres. 



In the trunks of all spinal nerves, these two kinds of fibres are in- 

 termixed; but at the roots of the nerves, it has been discovered, as 

 the result of experiment, that these two kinds of fibres are separated; 

 and that, whilst the afferent or sensory fibres pass entirely through 

 the posterior ganglion-bearing root up to the cord, the efferent or 

 motor fibres pass from the cord exclusively through the anterior roots. 

 This remarkable natural separation or analysis of the two sets of 

 fibres, was discovered by our countryman, Sir Charles Bell, and forms 

 the basis of his, and of still later, discoveries in the functions of the 

 nervous system.* The doctrine of Sir Charles Bell is demonstrated 

 by the following experiments. If the anterior root only of a spinal 

 nerve be cut across, sensation in the part below remains unaltered, but 

 the muscles are paralyzed, or the power of movement is lost. If the 

 posterior sensory root only be divided, sensation is lost in the parts 

 below, but voluntary power remains. If both roots are divided, sen- 

 sation and motion are both destroyed. Again, if the lower or distal 

 portion of the divided posterior root be irritated, no signs of pain or 

 other sensation, and no motions take place in the parts below; but if 



* Experiments made on the various parts of the nervous system in living animals, 

 have led to the formation of the most important inferences as to the respective uses 

 of these parts, whether composed of white or gray nervous matter. Sections of 

 various kinds, and stimulation before or after such sections, have, indeed, been the 

 chief methods employed in the investigation of the intricate problems of nervous 

 action. Very numerous instances will have to be mentioned in the following pages. 

 "Whilst we may designate as cruel and profitless, the mere repetition of well-known 

 experiments upon living animals, with the exception of a few of fundamental im- 

 portance, we must seriously maintain the right of the physiologist to employ, and 

 the propriety of employing, the lower animals in well-considered experiments 

 fqr the elucidation of those laws of life, which our intelligence prompts us to ex- 

 plore, and on a knowledge of which the alleviation of human suffering so largely 

 depends. 



