NERVOUS SYSTEM.— GENERAL CONSIDERATIONS. 213 



lower (peripheral) end stimulated, a decided change in the ac- 

 tion of the heart follows, which may be in the direction of 

 weakening or slowing, or positive arrest of its action. 



Assuming, for the present, that the cells (center) of the me- 

 dulla have the power to bring about the same result, it is seen 

 that such nervous influence is preventive or inhibitory of the 

 normal cardiac beat, so that the vagus is termed an inhibitory 

 nerve. Such inhibition plays a very important part in the 

 economy of the higher animals, as will become more and more 

 evident as we proceed. The nature of the influfences that pro- 

 duce such remarkable results will be discussed when we treat 

 of the heart. 



An illustration will probably serve in the mean time to make 

 the meaning of what has been presented in this chapter more 

 clear and readily grasped. 



In the management of railroads a very great variety of com- 

 plicated results are brought about, owing to system and orderly 

 arrangement, by which the wishes of the chief manager are 

 carried out. 



Telegraphing is of necessity extensively employed. Sup- 

 pose a message to be conveyed from one ofB.ce to another, this 

 may (1) simply pass through an intermediate office, without 

 special cognizance from the operator in charge ; (3) the operator 

 may receive and transmit it unaltered ; (3) he may be required 

 to send a message that shall vary from the one he receives in a 

 greater or less degree ; or (4) he may arrest the command alto- 

 gether, owing to the facts which he alone knows and upon 

 which he is empowered always to act according to his best dis- 

 cretion. 



In the first instance, we have an analogy with the passage 

 of a nervous impulse through central fibers, or, at all events, 

 unaffected by cells ; in the second, the resemblance is to cells 

 acting as conductors merely ; in the third, to the usual behavior 

 of the cells in reflex action ; and, in the fourth, we have an in- 

 stance of inhibition. The latter may also be rendered clear by 

 the case of a horse and its rider. The horse is controlled by the 

 rider, who may be compared to the center, through the reins 

 answering to the nerves, though it is not possible for either rider 

 or reins to originate the movements of the animal, except as 

 they may be stimuli, which latter are only effective when there 

 are suitable conditions — when, in fact, the subject is irritable in 

 the physiological sense. 



