THE CIRCULATION OF THE BLOOD. 51 



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. Experiments have been instituted, with a view of 

 ascertaining the amount of power possessed by the heart ; and it 

 has been computed by Dr. Hales, that the force exercised by 

 the left ventricle alone is equal to 113 lbs. in the horse, and it 

 is believed that the blood is projected into the aorta, with a ve- 

 locity equal to twenty-one feet per minute. 



The contraction of the ventricles and the auricles immediately 

 succeed each other ; as one expands to receive the blood, the 

 other contracts to force it forward, and thus the unequal double 

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

 the different cavities, could not be correctly performed, unless 

 some provision were made for preventing the blood, when the 

 ventricles 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 presents three points, 

 and in the left two ; whence the former is called the tricuspidy 

 and the latter the mitral. The edges of each valve is joined by 

 numerous short tendons to the fleshy columns of the heart ; and 

 wliilst the blood is flowing 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 

 closes the cavity in that direction, and prevents the blood re- 

 enterinn; the auricle. 



There are also valves that guard the entrance of the aorta 

 and pulmonary arteries ; but they are of a different description, 

 being of less strength, because they are not called upon to oppose 

 the powerful action of the ventricles. Accordingly, we find 

 they consist of three folds of membrane, and are called, from 

 their shape, semilunar. They are so situated, that when the 

 blood passes into the arteries they are thrown against their 

 sides, and when the blood has passed, they are thrown up, so 

 that their edges meet, and thus prevent the blood returning to 

 the heart. 



In fishes, the heart is single, and only serves the office of the 

 pulmonary circulation, that of the system being accomplished by 

 the arteries alone; in the horse, though the heart is the principal 

 power, yet the arteries greatly assist. The Aorta, which receives 

 the blood from the left ventricle, divides into two branches, called 

 the Anterior and Posterior Aorta; the former conveying the 

 blood to the head and neck, and the latter to the lower parts of 

 the body. These arteries are strong and thick, and consist of 



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