386 NERVOUS ACTION. 



known as the excitability or irritability of the tissue. It 

 is well exemplified in the contraction of muscular tissue 

 on the application of a mechanical or chemical stimulus. 

 Thus, if the point of a knife be applied to the muscle of a 

 recently slaughtered animal, a contraction is induced of a 

 more or less powerful character, according to the previous 

 healthy and vigorous condition of the muscle. Again, if 

 the leg of a frog be separated from the thigh, the skin 

 removed, and the poles of a galvanic battery brought into 

 contact with the surface of the exposed muscles, an ener- 

 getic contraction takes place whenever the electric circuit 

 is completed. In the healthy condition, this excitability is 

 called into play through the nervous system, so that all the 

 animal functions, whether these be of nutrition, sensation, 

 secretion, absorption, or locomotion, &c., are subservient to 

 this important apparatus. 



In this case, however, the stimulus is not usually applied 

 directly to the part the functions of which are to be acti- 

 vated ; it is more commonly originated at a remote part of 

 the system, or in the brain as the consequence of a mental 

 act, and, in either case, is conveyed through the nervous 

 system to the organ it is destined to affect. In this manner 

 it is that the contact of sapid substances with the mucous 

 membrane of the mouth leads to a secretion of saliva, or 

 that the falling of luminous rays on the delicate expansion 

 of the nerve of sight induces a contraction of the pupiL 



It will be further noticed that these reflex actions, as 

 they are called, when the impression is received at one 

 part, and conveyed by the nervous apparatus to an organ 

 more or less remote, co-operate to bring about the healthy 

 exercise of the vital functions, and the maintenance of the 

 frame in a condition of integrity. This is well exemplified 

 in the above-named instances. The secretion of saliva, 



