240 CIRCULATION IN MAMMALS AND BIRDS. 



obstruction exists. Thus, when the admission of air to the 

 lungs is prevented, the blood will not pass through the 

 pulmonary capillaries, since it cannot undergo the change 

 which ought to be performed there ; and it therefore accumu- 

 lates in the pulmonary artery, the right side of the heart, and 

 venous system ; and if no relief be afforded by the admission 

 of air into the lungs, the whole circulation is thus brought to 

 a stand. This condition, which is termed Asphyxia, occurs 

 in drowning, hanging, and other forms of suffocation ( 338). 



Course of the Blood in the different Classes of Animals. 



281. The Circulation of the Blood takes place on the 

 same general plan in all other MAMMALS, and in BIRDS, as 

 in MAN. In all the animals included in these groups, the 

 heart is composed of two halves quite distinct from each 

 other ; each possessing an auricle or receiving cavity, and a 

 ventricle or propelling cavity. The course of their blood, 

 which goes through a complete double circulation, is shown by 

 the diagram (fig. 129). The vessels and cavities of the heart 

 which contain venous blood are shaded ; whilst those which 

 convey arterial blood are left white : and this distinction is 

 kept-up in the other figures. The direction of the blood is 

 indicated by the arrows. Every drop of blood which has 

 passed through the capillaries of the system, is transmitted 

 to the lungs before it is allowed again to enter the aorta ; 

 and the whole mass of "the blood passes twice through the 

 heart, before any part of it is transmitted a second time to the 

 vessels from which it was before returned. 



282. The two sides of the heart do not possess, when that 

 organ is perfectly formed, any communication with each other, 

 except through the pulmonary vessels ; and thus they might 

 be regarded as two distinct organs, united for the sake of 

 convenience. The right side of the heart, being placed at 

 the origin of the pulmonary artery, and having for its office 

 to propel the blood through the lungs so as to receive the 

 influence of the air, may be called the respiratory heart : 

 whilst the left side, which is placed at the origin of the 

 aorta, and has to propel the blood to the body in general, may 

 be called the systemic heart. The circulation would be per- 

 formed precisely in the same manner, if these two organs 



