340 STRUCTURE OF THE SUEEP. 



called the aorta, whence it enters smaller arteries, to be dis- 

 tributed throughout the whole system. The remote divis- 

 ions of the arteries are called the capillary vessels, and in 

 ihem the blood, after having accomplished its purposes and 

 conveyed nourishment to all parts, becomes black and im- 

 pure, and in this state enters the capillary veins, which, con- 

 joining and increasing in size and diminishing in number, 

 convey the blood again to the right auricle of the heart. 

 Just before it enters the heart it receives a supply of chyle, 

 which, as we have before observed, is extracted from the 

 food, absorbed by certain small vessels called lacteals, and 

 conveyed by a specific channel to the heart. Such, then, is 

 the circle, or rather the double circle, which the blood takes, 

 and by which so many important purposes are beautifully 

 and correctly accomplished. 



The circulation of the blood is accomplished by the joint 

 action of the heart and arteries, but principally by that of 

 the former. The contraction of the ventricles and of the 

 auricles immediately succeed each other : as the one ex- 

 pands to receive the blood, the other contracts to force it 

 forward, thus producing the unequal double action of the 

 heart that we feel. These actions, however, of the different 

 cavities could not be correctly performed unless some provis- 

 ion were made for preventing the blood, when the ventri- 

 cles contract, from retrograding into the auricles. This, 

 however, is effected by means of a valve, situated between 

 these cavities, which is formed by a duplication of the inner 

 membrane of the heart, thickened by fibrous substance. 

 The floating edges of this valve in the right ventricle pre- 

 sent three points and in the left two ; whence the former is 

 called the tricuspid, and the latter the mitral. The edges 

 of each valve are joined by numerous short tendons to the 

 fleshy columns of the heart ; and whilst the blood is flow- 

 ing into the ventricles the fleshy columns are passive ; but 

 when the ventricles act these columns also contract and 

 draw the edges of the valve together, and thus close the 

 cavity in that direction and prevent the blood re-entering 

 the auricle. 



There are also valves that guard the entrance of the 

 aorta and pulmonary arteries, but they are of a different de- 

 scription, being of less strength, because they are not called 

 upon to oppose the powerful action of the ventricles. Ac- 

 cordingly we find that they consist of three folds of mora- 



