82 MANUAL OF CATTLE-FEEDIKG. 



derived from the burning of waste products in the capil- 

 laries, and for the same reason is poor in oxygen ; while 

 the air in the vesicles of the hmgs, on the contrary, is rich 

 in oxygen and contains but little carbonic acid. Under 

 these circumstances each gas moves from the place where 

 it is most abundant to the place where there is a deficiency 

 of it. The carbonic acid of the blood diffuses through the 

 membrane of the blood-vessels into the air of the vesicles till 

 the latter is as rich in that gas as the former, while the oxy- 

 gen, at the same time, passes from the vesicles to the blood. 



The carbonic acid is largely contained in the plasma of 

 the blood, and simply diffuses into the air in the lung 

 vesicles and is expelled in expiration, but for the taking 

 •up of oxygen there is a special provision in the coloring 

 matters of the corjDuscles. The venous blood contains 

 both haemoglobin and oxyhsemoglobin. When the blood 

 passes through the hmgs the hsemoglobin unites with the 

 oxygen which diffuses into it, and when the aeration is 

 properly performed is all converted into oxyhsemoglobin, 

 which gives the arterial blood its bright-red color. The 

 corpuscles thus act as vehicles for conveying oxygen from 

 the lungs to the remotest regions of the body. 



In the capillaries this oxygen is given up again in part, 

 and hemoglobin formed once more, giving to the venous 

 blood its darh-red color. 



if by any means respiration is stopped, the air in the 

 lung vesicles speedily becomes so charged with carbonic 

 aeid and exliausted of oxygen that the exchange of gases 

 with the blood can no longer go on ; the carbonic acid is re- 

 tained in the latter, the waste products of the tissues are not 

 burned, and the animal's blood is poisoned — it is suffocated. 



If its supply of air, however plentiful, contains more 

 tlxan a certain amount of carbonic acid, the removal of this 



