THE BLOOD AND ITS CIRCULATION. 341 



brane, 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 accom- 

 plished by the arteries alone. In the sheep, 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 arte- 

 ries are strong and thick, and consist of three coats ; the 

 outer, the strongest and thickest, gives the vessels the re- 

 markable elasticity which they possess ; the middle coat is 

 the fibrous, which seems to be a modification of muscular 

 power, and enables the arteries to contract on their contents ; 

 the third coat is the serous, vvhich lubricates the interior of 

 the vessel and facilitates the passage of the blood. Thus 

 to these several coats, but particularly to the two former, do 

 the arteries owe the remarkable property they possess of 

 contracting when distended with blood, and almost immedi- 

 ately afterwards expanding to receive a fresh supply, and 

 which, assisted by the action of the heart, constitutes the 

 pulse ; and may be felt in every part of the body where an 

 artery is sufficiently near the surface to be perceptible. 



The arteries, however, do not all possess an equal thick- 

 ness and power ; for instance, the pulmonary artery, though 

 quite as large as the aorta, is neither so thick nor so strong ; 

 and the reason is, that the same power is not required to 

 send the blood over the smaller circuit of the lungs as over 

 the larger one of the whole system ; and, for the same rea- 

 son, the right side of the heart is weaker than the left. 

 The arteries, as they divide and subdivide in their course, 

 become weaker in their coats in proportion to the diminu- 

 tion of their size, till at length they terminate in the minute 

 branches called the capillary vessels, which do not possess 

 any pulsating power, and many of which do not contain red 

 blood. Diminutive, however, as these branches may be, yet 

 it is by them that the most important offices are performed ; 

 by them the different parts of the body are nourished, 

 whether bone, flesh, nerve, or skin ; by them the various 

 29* 



