98 PHYSIOLOGY. 



nothing. That it is not identical with electricity or galvanism, as 

 was once supposed, is now established. The following experiments 

 prove this, if a ligature be placed upon a nerve its power of con- 

 ducting nervous influence is lost, while it still continues to transmit 

 electrical currents. Again, if a section of a nerve be removed, and 

 its place be supplied by an electric conductor, electricity will still pass 

 along the nerve, but no nervous force will be propagated through the 

 conductor to the parts beyond. Lastly, the conducting power of 

 nerve for electricity, according to Matteucci, is -not more than one- 

 fourth that of muscle ; hence, if the nervous force were electric, it 

 would leave the nerve, and follow the muscle in preference. It 

 seems to be a peculiar power developed in the nervous structure 

 under the influence of appropriate stimuli; just as conlractiliiy is 

 developed in a muscle under similar influence. 



The functions of particular nerves may be discovered by examin- 

 ing the anatomical distribution. If a nerve is discovered to lose itself 

 entirely in the substance of muscles it may be inferred to be chiefly 

 or entirely motor, or efferent. If, on the contrary, it can be traced 

 to a membranous expansion, cutaneous, mucous, or otherwise, there 

 is equal reason to believe it an affere7it, or sensory nerve. If a nerve 

 is entirely distributed upon a surface adapted to receive impressions 

 of a special kind, it may be inferred that it is incapable of receiving 

 or transmitting any others. Such a nerve is said to be one of special 

 sensibility, to distinguish it from those that transmit impression of a 

 general character, and which are called nerves o{ general sensibility. 



In considering the functions of the various parts of the nervous 

 system, it is best to begin with the spinal marroiv, which, with its 

 cranial prolongation, the medulla oblongata, may be regarded as the 

 essential part of the nervous system of vertebrata. 



or THE MEDULLA SPINALIS. 



The spinal cord is to be considered in a two-fold light. First. As 

 a conveyor of nervous agency to and fro?n the brain. Second. As 

 an originator of nervous influence. All the cerebral nerves are sub- 

 ject to the influence of the brain, and all the spinal nerves are subject 

 to the same influence through the medium of the spinal cord. As 

 soon as the transmission of this influence is interrupted by division,^ 

 or wounds inflicted upon the cord, impressions on sensitive nerves 

 cease to be propagated to the sensorium, and the brain loses the 

 power of voluntarily exciting the action of the motor nerves which 

 are given off below the injured point. All the nerves above this 

 point, however, still transmit impressions and are subject to the influ- 

 ence of the brain.- 



The white fibrous matter of the cord is to be regarded as the 

 conimon trunk of all the nerves of the body. It must not be sup- 



