THE CIRCULATION OF THE BLOOD. 293 
A 
| 
FE 
Fic. 203.—Diagram to illustrate the relative roportions of the aggregate sectional area of the 
‘different parts of the vascular system (after Yeo). A, aorta; C, capillaries ; V, veins. 
nels or cones with the smaller end toward the heart and the 
widest portions representing the capillaries. 
THE ACTION OF THE MAMMALIAN HEART. 
Very briefly what takes place may be thus stated: The 
right auricle contracting squeezes the blood through the au- 
riculo-ventricular opening into the right ventricle, never quite 
emptying itself probably; immediately after the right ventricle 
contracts, by which its valves are brought into sudden tension 
and apposition, thus preventing reflux into the auricle; while 
the blood within it takes the path of least resistance, and the 
only one open to it into the pulmonary artery, and by its 
branches is conveyed to the capillaries of the lungs, from 
which it is returned freed from much of its carbonic anhy- 
dride and replenished with oxygen, to the left auricle, whence 
it proceeds in a similar manner into the great arterial main, 
the aorta, for general distribution throughout the smaller 
arteries and the capillaries to the most remote as well as the 
nearest parts, from which it is gathered up by the veins and 
returned laden with many impurities, and robbed of a latge 
proportion of its useful matters, to the right side of the heart. 
It will be remembered that corresponding subdivisions of 
each side of the heart act simultaneously, and that any decided 
