VASCULAR SYSTEM. 163 



BliOOD-VESSELS. 



The blood, from which the solid textures immediately derive material 

 for their nom-ishmeut, is conveyed through the body by branched tubes 

 named blood-vessels. It is driven along these channels by the action 

 of the heart, "which is a hollow muscular organ placed in the centre of 

 the sanguiferous system. One set of vessels, named arteries, conduct 

 the blood out from the heart and distribute it to the different regions 

 of the body, whilst other vessels named veins bring it back to the heart 

 again. From the extreme branches of the arteries the blood gets into 

 the commencing branches of the veins or revehent vessels, by passing 

 through a set of very fine tubes which connect the two, and which, 

 thougii not abruptly or very definitely marked off from either, are 

 generally spoken of as an intermediate set of vessels, and by reason of 

 their smallness are called the capillary {i.e., hair-like) vessels, or, simply, 

 the capillaries. 



The conical hollow muscular heart is divided internally into four 

 cavities, two placed at its base, and named auricles, and two occupying 

 the body and apex, named ventricles. The auricles are destined to re- 

 ceive the returning blood from the great veins, which accordingly open 

 into them, and to pass it on into the ventricles ; whilst it is the office 

 of the latter to propel the blood through the body. The ventricles 

 have therefore much thicker and stronger sides than the auricles, and 

 the gi'eat arterial trunks lead off from them. Each auricle opens into 

 the ventricle of the same side, but the right auricle and ventricle are 

 entirely shut off from those of the left side by an impervious partition 

 placed lengthwise in the heart. 



The blood is sent out by the left ventricle into the main artery of the 

 body, named the aorta, and passes through the numerous subordinate 

 arteries, which are branches of that great trunk, to the different parts 

 of the system ; then, traversing the capillaries, it enters the veins, and 

 is returned by two gi-eat venous trunks, named the superior and inferior 

 ven^e cavje, to the right auricle. In passing from the arteries to the 

 veins the blood changes in colour fi'oni red to dark, and is otherwise 

 altered in quality ; in this condition it is unfit to be again immediately 

 circulated through the body. On returning, therefore, to the right side 

 of the heart, the blood, now dark and venous, must re-acquire the florid 

 hue and other though less obvious qualities of arterial blood before it is 

 permitted to resume its course. For this purpose, being discharged by 

 the right auricle into the right ventricle, it is driven, by the contraction 

 of that ventricle, along the pulmonary artery and its branches to the 

 lungs, where, passing through the capillary vessels of these organs, it 

 is exposed to the influence of the air, and undergoes the requisite 

 change ; and, having now become florid again, it enters the commenc- 

 ing branches of the pulmonary veins, which, ending by four trunks in 

 the left auricle, convey it into that cavity, whence it is immediately dis- 

 charged into the left ventricle to be sent again along the aorta and 

 through the system as before. 



The blood may thus be considered as setting out from any given 

 point of the sanguiferous system and returning to the same place again 

 after performing a circuit, and this motion is what is properly termed 



M 2 



