642 THE POPULAR SCIENCE MONTHLY. 



in ano from which he had been suffering, the irritation produced by it 

 appearing' to have been the real cause of the incontinence. 



We will now pass from reflex changes in the respiratory and intes- 

 tinal tracts to reflex changes in the blood-vessels and heart. It is well 

 known that, usually, irritation of a sensory nerve causes dilatation of 

 the vessels in the part supplied by that nerve, and contraction of the 

 vessels in the other parts of the body. This may take place without 

 any alteration whatever in the beats of the heart itself ; but if the 

 irritation be very strong, or applied to certain nerves, the heart also 

 may be acted upon. There seem to be certain nerves which act more 

 readily upon the heart than others, and more especially is this power 

 possessed by the fifth nerve, the roots of which are very closely asso- 

 ciated with those of the vagus. On stimulation of the branches of the 

 fifth nerve passing to the nose in animals — as, for example, by holding 

 ammonia, strong acetic acid, or chloroform before the nose of a rabbit — 

 the beats of the heart may be suddenly and completely arrested. To 

 a similar arrest of the cardiac pulsations by irritation of the dental 

 branches of the fifth, I attribute the numerous deaths which have 

 occurred through the extraction of teeth under chloroform. It is 

 probable that the extraction of teeth would, under all circumstances, 

 be an exceedingly dangerous operation, were it not that in the waking 

 condition irritation of the dental nerves sets in motion two pieces of 

 mechanism, one of W'hich, to a great extent, counteracts the effects of 

 the other. As I have already mentioned in a former paper, I was once 

 asked how it was that the application of ammonia or acetic acid to the 

 nose of a fainting person was proved by experience to be beneficial, 

 when theoretically it ought to be injurious by arresting the already 

 enfeebled heart. The answer to this is, that ammonia or acetic acid, 

 held before the nose of a fainting person, by irritating the branches of 

 the fifth nerve, does not act upon the heart alone — it causes contraction 

 of all the vessels of the body, and thus keeps the arterial system full, 

 and the blood-pressure high, despite the diminished current poured into 

 it by the flagging heart. So much is this the case, that I have found, 

 in an animal in which the heart was weakened and the vessels dilated 

 by shock, that the application of ammonia or acetic acid to the nose 

 raised the pressure by one fourth of the whole. In the ordinary 

 waking condition, the sudden stimulus of extracting the tooth has its 

 effect upon the heart completely counteracted by the coincident con- 

 traction of the arterioles throughout the body which it also causes. In 

 chloroform narcosis, however, these two reflexes are not influenced 

 equally by the drug, and the reflex upon the heart may remain after 

 the reflex action upon the vessels has been abolished, so that the heart 

 may stop, and death will then ensue ; for, the capillaries being no 

 longer contracted, the blood at once drains out of the arteries into the 

 veins, and circulation ceases. Another very important reflex upon the 

 heart is that which is effected through the solar plexus and the sym- 



