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 
astand. This condition, which is termed Asphysia, 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 Mammats, and in Brrps, as 
in May. 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 
