NERVOUS SYSTEM. 159 



nervous energy is recruited by repose. There are stated in- 

 tervals, which differ according to the species, in which the 

 powers of volition are allowed to rest, or the individual is 

 said to sleep. This condition, we have seen, is likewise re- 

 quisite, in order to recruit the peculiar irritability of the 

 muscles employed for the purposes of voluntary motion. 



The action which may thus be excited in the nerves sub- 

 servient to voluntary motion, is communicated to the mus- 

 cles. As it exists in the nerves, it is invisible in its opera- 

 tions ; but when communicated to the muscle, its effects 

 are obvious. The different particles of the muscle, enter 

 into a more intimate union ; it contracts, and its tenacity is 

 increased to such a degree, that it is capable of suspending 

 weights which would have torn it asunder when separated 

 from the body. The act of the will by which the action 

 has been generated in the muscle, can be suddenly altered, 

 and the muscle permitted to return to its relaxed state, in 

 the absence of the exciting cause. We are, however, igno- 

 rant of the nature of the change in the condition of the 

 muscle on which the nerve has thus exerted its influence, 

 and we are equally ignorant of the nature of that influence, 

 or its mode of communication. 



c. Involuntary Motion. Besides those nerves which are 

 employed in sensation and volition, there are others which 

 are concerned in the production of involuntary motion. 

 These are distributed to the various organs of digestion, 

 circulation, and respiration, whose continued functions are 

 necessary to existence. These nerves neither communicate 

 to us distinct sensations, nor act immediately under the in- 

 fluence of the will. When the parts, however, with which 

 they are connected, become inflamed, they then communi- 

 cate to us painful sensations ; and when the will is thrown 

 into violent action, corresponding motions are excited in 



