STRUCTURE OF MAN. 35 



valves from returning to the auricle, is thrown forward into 

 the pulmonary artery, which carries it to the lungs. This 

 passage is also guarded by valves. 



At the time of its passage through the right side of the 

 heart, the blood is of a dark bluish red or purple color, ap- 

 proaching almost to black. It is generally called black blood, 

 and is neither fit for circulation in the vessels, nor for the 

 nourishment of the different parts. In this state it is sent to 

 the lungs. These fill up all that part of the cavity of the 

 chest not occupied by the heart, which they surround almost 

 upon all sides. They consist principally of a collection of 

 blood and air vessels, and are constantly supplied with air, 

 which is drawn in through the windpipe, and distributed to 

 every part of them. The blood is circulated, throughout 

 their substance, by the branches of the pulmonary artery, 

 and is, in its course, exposed to the influence of the air. By 

 this means, its color is changed to a bright crimson or ver- 

 milion, and it becomes again fit for the purposes of life. 



It is now brought back to the left side of the heart, by the 

 pulmonary veins, and passes through the left auricle and ven- 

 tricle, in a manner similar to that which has been already 

 described with regard to the right side. The left ventricle, 

 frqm its superior size and strength, gives to the blood a more 

 powerful impulse ; than that which it receives from the right, 

 and this is the more necessary, because it has a wider and 

 more extensive course to traverse. From the left ventricle, 

 it is thrown into the aorta, the great artery which supplies 

 the whole body with blood. This artery ascends from the 

 heart for a short distance, arches over, sends branches to the 

 head and arms, and then descends behind the heart, and dis- 

 tributes them to the other parts of the system. 



The branches thus distributed throughout the body, are 

 subdivided again and again to an almost inconceivable degree 

 of minuteness, and finally terminate in a system of vessels 

 called capillary vessels. These pervade every part, and the 

 blood, after passing through them, enters into another set of 

 vessels, the veins, which gradually collect together and en- 

 large in size, till they terminate, as has been before remarked, 

 in two large trunks at the right auricle of the heart. In the 

 capillary vessels, the blood undergoes a change in its quali- 

 ties, precisely opposite to that which takes place in the lungs. 

 It becomes, from a bright red color, of the same dark red 

 which it was described to possess upon passing through the 

 right side of the heart, in its passage to the lungs. The cause 

 3 



