254 



PHYSIOLOGY OF THE HEART 



[CH. XXI. 



doses over long periods of time ; the term inhibition is not applicable 

 in this case, and the effects of the poisonous action of chloroform on 

 the heart itself can be avoided by keeping the proportion of chloro- 

 form in the inspired air at 2 per cent, or less. But in other cases 

 which are seen both in animals and human beings who may be 

 peculiarly susceptible to the influence of chloroform, heart stoppage 

 occurs during the onset of anaesthesia long before the percentage of 

 chloroform in the blood has reached a value which is toxic to the 

 heart. Some have considered that death during the induction of 

 chloroform anaesthesia is due to the vapour irritating the vagal 

 terminations in the lung, and so leading to reflex inhibition of the 

 heart. Embley's experiments, however, lead to the conclusion that 

 the chloroform acts on the vagus centre in the medulla oblongata. 

 In animals, cutting the vagi immediately sets the heart going again. 

 In man this operation cannot be performed, and it is therefore a wise 

 precaution, whenever it is necessary to administer chloroform, to 

 give beforehand a small dose of atropine under the skin so as to 

 temporarily paralyse the vagus endings in the heart. 



Gaseous Exchanges in the Heart. The using up of oxygen by the 

 living heart was well illustrated by an old experiment of Yeo's. He 

 passed a weak solution of oxyhseinoglobin through an excised beat- 

 ing frog's heart, and found that after it had passed through the heart, 

 the solution became less oxygenated and venous in colour. 



This is still better shown by the following numbers, obtained 

 by Barcrof t and Dixon by estimating the gases in the blood entering 

 and leaving the coronary vessels of a cat. It will be seen that the 

 metabolism in the heart tissue is reduced during inhibition; this is 

 followed by increased metabolism during the subsequent period, 

 which corresponds with the increase of visible activity which then 

 occurs, and which is seen in the tracings given in figs. 229 and 230. 



Rhythm, Conduction, etc., in Cardiac Muscle. 



In one time, the rhythm which cardiac muscle exhibits was 

 supposed to be due to the action upon it of the nerves which are 

 present. We now know that the property of rhythmical peristalsis 

 resides in the muscular tissue itself, though normally during life it is 

 controlled and regulated by the nerves that supply it. This conclusion 

 may be expressed by saying that cardiac rhythm is myoyenic, not 



