THE NERVOUS SYSTEM. 26 1 



other wire to some particular muscle which under its influence 

 contracts: this is spoken of in physiology as reflexation; in other 

 words an impulse is conveyed to a centre by a sensory nerve and 

 is reflected from that centre through a motor nerve and through 

 the latter produces a muscular contraction. There is further, an 

 automatic action, the difference between which and reflex action, 

 it is somewhat diSicult to define, but perhaps the simplest ex- 

 planation is this, that whereas so-called reflex action is due to some 

 impulse effected from without the body, automatic action is due to 

 impulses effected within the body, and without which life and the 

 various functions of the body could not be maintained, such, for 

 instance, as respiration, the pulsation of the heart, the movements 

 of the alimentary canal, the control of the size of the blood 

 vessels, etc. ; at the same time the principle of conduction is 

 much the same in both, the impulses being received at one place, 

 conveyed therefrom b}' means of nerves to a centre and reconveyed 

 by another nerve to the part to be acted upon; a very familiar 

 illustration of this may be cited in the act of defoecation; the feel- 

 ing of necessity for relief is brought about by the contact of the 

 faeces with the mucous membrane of the lower intestine (the 

 rectum); through the sensory nerves the impulse is conveyed to 

 the nerve centre and from thence to the muscles of the rectum by 

 the motor nerves, which produce contraction of said muscles and 

 expulsion of faeces; this illustration, however, serves a double 

 purpose, first to explain the meaning of automatic action, and 

 second that some automatic actions are capable of restraint by 

 exercise of the will; this is known as the inhibitory action. There 

 are reflex actions other than those that affect the various complex 

 movements referred to, which excite and set up the peculiar func- 

 tion of secretion in the glands, such, for instance, as the secretion of 

 saliva, which is excited by some substance touching the tongue or 

 through the action of the jaws in mastication. In attempting a 

 simple description of the nerve centres, they were referred to as a 

 number of telegraph offices all located under one roof and com- 

 municating with one another, and that these are represented by 

 the brain and spinal cord; some of these offices or nerve centres are 

 located in the brain and some in the spinal cord; at the same time 

 the inter-communication between these centres is maintained by 

 what are called ''paths of conduction " in the spinal cord; this may 



