434 ONE OF THE CAUSES OF DEATH UNDER CHLOEOFORM. 



Stoppage of the heart's action, then, beino^ of such importance 

 as a cause of death, we must now inquire how it is produced. 

 Tlie heart is kept pulsating rhythmically by the motor ganglia 

 which it contains within iti-elf, and will continue to pulsate for 

 some time after its complete removal from the body. But 

 though it thus shows its power to contract independently of the 

 central nervous system, it is, nevertheless, influenced to a great 

 extent by the nerve-centres within the cranium. It would 

 never do to have the heart acting without reference to the 

 wants of the system, and pumping blood vigorously into the 

 arteries when the pressure within them was already too great, 

 or acting slowly and feebly when the limbs were engaged in 

 severe work, and wanted an abundant supply of blood to enable 

 them to perform it. There are, therefore, nerves, some accelerat- 

 ing and others retarding the heart, which pass to it from the 

 medulla oblongata, and, acting as the spur and reins of a rider 

 do upon, his horse, regulate its beats in accordance with the 

 wants of the system. The retarding fibres are contained in the 

 vagus nerve; and, when this nerve is irritated strongly, the 

 heart will either stop immediately in diastole, or will beat very 

 slowly and more feebly. Nor is it only by direct irritation of 

 the vagus that this result can be attained. Just as irritation of 

 a sensory nerve sets motor nerves in action, and produces 

 various muscular movements by reflex action through the 

 spinal cord, so may irritation of a sensory nerve set the vagus 

 in action and produce stoppage of the heart, by acting reliexly 

 through the medulla oblongata. A good many sensory nerves 

 can do this ; but there is one which possesses the power in an 

 especial degree. The roots of the fifth nerve are anatomically 

 closely connected with those of the vagus ; reflex stoppage of 

 the heart is produced more readily by irritation of the fifth than 

 of any other norve. In many rabbits, the heart can be instan- 

 taneously stopped by irritating the nasal branches of this nerve 

 by a pungent vapour, such as ammonia, held before the nose. 

 In every rabbit, or almost every rabbit, indeed, we can stop the 

 heart by a pungent vapour applied to the nose ; but we do not 

 always do it iix the reflex manner I have just described. The 

 animal always closes its nostrils to prevent the entrance of the 



