RESPIRATION 



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pressure restored. The remarks on this subject on p. 108 

 should be re-read. 



The Number of Respirations varies with the class of animal ; 

 as a rule, the larger the animal the slower the respirations : 



Horse - - - - - -8 to 10 per minute. 



Ox- 12 „ 15 „ 



Sheep and goat - - - - 12 ,, 20 „ „ 



Dog 15 „ 20 „ 



Pig 10 „ 15 „ „ 



Rumination increases the frequency of, and causes irregu- 

 larity in, the respiration, and muscular exertion in all animals 

 at once causes it to rise. In experiments on respiration this is 

 most marked ; walking a horse will nearly treble the number 

 of respirations, but the breathing begins to fall immediately 

 the horse stops, though it does not reach the normal for a few 

 minutes. 



The ratio of heart-beats to respiration has been placed at 

 1 : 4 or 1 : 5. 



The Effect of Respiration on the Circulation. — We have 

 previously alluded to the influence of respiration on the circula- 

 tion, and have pointed out that the negative pressure in the 

 thorax assists the circulation by favouring the return of blood 

 by the veins. This aspiratory action of the thorax mainly 

 affects the anterior vena cava, but the contents of the posterior 

 vena cava, we have seen, are also influenced during inspiration 

 by the contraction of the diaphragm and the compression to 

 which the vessels are exposed. These two important features 

 have the effect of increasing at each inspiration the flow and 

 velocity of the blood to the heart. Aspiration of the thorax 

 favours the filling of the right heart, while the distension of the 

 lungs in inspiration is generally believed to dilate the vessels, 

 and so favour the flow of blood through these organs. 



In dealing with the question of blood-pressure (p. 71), it 

 was pointed out that the curve obtained shows certain undula- 

 tions which are generally regarded as due to the influences of 

 respiration, for it is found, if the blood-pressure and respiration 

 curves be simultaneously obtained and superposed, that the 

 undulations in the latter correspond very closely with the undula- 

 tions in the blood-pressure. On comparing the curves, it is 

 found that the blood-pressure rises with inspiration and falls 

 with expiration ; further, it is observed that the rise does not 

 take place immediately inspiration begins, nor does the fall 

 occur immediately expiration starts, but shortly after in both 

 cases, as may be seen in Fig. 29. These results have never 

 been adequately accounted for ; but until the question was 

 reopened by Lewis in recent years, the following was the 



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