366 GENERAL STRUCTURE AND FUNCTIONS 



parts of the body, and to cause them to act in harmony with each 

 other. 



This object may be more fully exemplified as follows : 

 Each organ and tissue in the body has certain properties peculiar 

 to it, which may be called into activity by the operation of a stimu- 

 lus or exciting cause. This capacity, which all the organs possess, 

 of reacting under the influence of a stimulus, is called their excita- 

 bility, or irritability. We have often had occasion to notice this pro- 

 perty of irritability, in experiments related in the foregoing pages. 

 We have seen, for example, that if the heart of a frog, after being 

 removed from the body, be touched with the point of a needle, it 

 immediately contracts, and repeats the movement of an ordinary 

 pulsation. If the leg of a frog be separated from the thigh, its 

 integument removed, and the poles of a galvanic battery brought 

 in contact with the exposed surface of the muscles, a violent con- 

 traction takes place every time the electric circuit is completed. 

 In this instance, the stimulus to the muscles is supplied by the 

 electric discharge, as, in the case of the heart above mentioned, it is 

 supplied by the contact of the steel needle ; and in both, a muscu- 

 lar contraction is the immediate consequence. If we introduce a 

 metallic catheter into the empty stomach of a dog through a gastric 

 fistula, and gently irritate with it the mucous membrane, a secretion 

 of gastric juice at once begins to take place ; and if food be intro- 

 duced the fluid is poured out in still greater abundance. We know 

 also that if the integument be exposed to contact with a heated 

 body, or to friction with an irritating liquid, an excitement of the 

 circulation is at once produced, which again passes away after the 

 removal of the irritating cause. 



In all these instances we find that the organ which is called into 

 activity is excited by the direct application of some stimulus to its 

 own tissues. But^this is noj usually the manner in which the dif- 

 ferent functions are excited during life. The stimulus which calls 

 into action the organs of the living body is usually not direct, but 

 indirect in its operation. Very often, two organs which are situ- 

 ated in distant parts of the body are connected with each other by 

 such a sympathy, that the activity of one is influenced by the 

 condition of the other. The muscles, for example, are almost never 

 called into action by an external stimulus operating directly upon 

 their own fibres, but by one which is applied to some other organ, 

 either adjacent or remote. Thus the peristaltic action of the mus- 

 cular coat of the intestine commences when the food is brought in 



