DANGERS OF IMPERFECT REFLEX ACTION. 435 



vapour, and keeps them closed so long, that tlie carbonic acid 

 accumulating in the blood begins to act on the vagus and stop 

 the heart. But this only occurs after the vapour has been held 

 before the nose for some time : while the retlex stoppage which 

 I have just mentioned takes place at once, almost simultaneously 

 with the closure of the nostrils. Tliis reflex stoppage has been 

 shown by Hering and Kratschmer to be due to the irritation 

 being conveyed along the nasal branches of the fifth nerve 

 to the medulla, whence it is reflected along the vagus to the 

 heart, and stops it. 



Yet, notwithstanding the stoppage of the heart, the rabbit 

 does not die ; nor is it, indeed, any the worse. Why is this ? 

 Usually, when the heart is stopped, as, for example, when a 

 ligature is put round the aorta, the blood all runs out of tho 

 arteries into the veins ; and then, as I liave said, it is useless for 

 nutrition. But there is a nervous arrangement wliich prevents 

 this wdien the heart stops in consequence of an irritation 

 applied to the fifth nerve. This nerve not only contains 

 branches which are connected with the vagus and stop the 

 heart or retard it, but it also has branches which go to the 

 cerebral hemispheres, and there excite an action which passes 

 down the vaso-motor nerves, causing the auricles to contract, 

 and preventing the blood from running out of the arteries into 

 the veins, except very slowly indeed ; so that, as soon as the 

 irritation stops, the circulation is ready to go on normally. 

 But it is only when the cerebral hemispheres are in good work- 

 ing order that this occurs. When they are removed, or when 

 their function is destroyed by chloroform, morphia, or chloral, 

 irritation of a sensory nerve, such as the fifth, no longer has the 

 same effect ; and it then always, according to Cyon, lessens the 

 pressure of blood in the arteries. As it is the pressure of blood 

 within the arteries which keeps up the flow within them, just 

 as it is the pressure of water within the pipes supplying a town 

 which keeps up the supply to the houses, we can readily see 

 that the diminished pressure which occurs on the irritation of a 

 sensory nerve, after the cerebral hemispheres have been rendered 

 useless by a small quantity of chloroform, is a most serious thing 

 for the animal. . But here it is a little chloroform which is a 



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