EESPIEATION. 473 



are seen to contract, but not coincidently with the expira- 

 tory process. This muscle is spasmodically contracted in 

 asthma. 



Eespiration takes place in a regular manner, or with a 

 rythm. Inspiration occupies a much shorter time than ex- 

 piration, by which it is immediately succeeded. Between 

 each expiratory and inspiratory act is a short interval, quite 

 perceptible, when the respiration is undisturbed, though ob- 

 viated by exciting the animal so as to accelerate breathing. 



When respiration is undisturbed, it commonly maintains 

 a constant standard in each species of animals. Most fre- 

 quently it holds to the pulse a relation of about one to four, 

 but it is relatively faster in some animals. In the horse, 

 it varies from 9 to 12 in the minute, in the ox from 15 to 

 20, in the sheep from 13 to 17, and in the dog from 15 to 20. 

 It is always most rapid in young animals, for which the 

 higher figures may accordingly be taken. Breathing is ac- 

 celerated by anything that impedes the dilatation of the lung, 

 or excites the animal. The former cause is well exempli- 

 fied in an overloaded stomach, especially in the cow, while the 

 latter is readily shown by subjecting to exercise. Colin 

 found that a horse respiring at the rate of 10 per minute 

 had the respirations increased to 28 when walked for several 

 hundred yards. In a few minutes' it had returned to the 

 ordinary standard. After five minutes trot, they had in- 

 creased to 52 per minute, but became perfectly quiet again 

 three minutes later. He was then galloped for five minutes, 

 after which the respirations numbered 65 for the first minute, 

 and 60 the second. 



In birds the lungs are confined to the posterior part of the 

 chest, to which they are bound down by areolar tissue. 

 They are much less complex in form than in mammalia, but 

 aeration of the blood is also conducted in a number of air 



