clxvi BLOOD-VESSELS. 



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 great venous trunks, named the superior and inferior venae cavse, to 

 the right auricle. In passing from the arteries to the veins the blood 

 changes in colour from 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 ven- 

 tricle, 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 under- 

 goes the requisite change ; and, having now become florid again, it enters the 

 commencing 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 per- 

 forming a circuit, and this motion is what is properly termed the circulation 

 of the blood. Its course from the left ventricle along the aorta, throughout 

 the body, and back by the venae cavse to the right ventricle, is named the 

 greater or systemic circulation, and its passage through the lungs by the 

 pulmonary artery and pulmonary veins from the right to the left side of the 

 heart, is termed the lesser or pulmonary circulation ; but the blood must go 

 through both the greater and the lesser circulations in order to perform a 

 complete circuit, or to return to the point from which it started. As the 

 vessels employed in the circulation through the lungs have been named pul- 

 monary, so the aorta which conveys the blood to the system at large is 

 named the systemic artery, and the venae cavse the systemic veins ; whilst 

 the two sets of capillaries interposed between the arteries and veins, the one 

 in the lungs, the other in the body generally, are respectively termed the 

 pulmonary and the systemic capillaries. 



The blood flows in the arteries from trunk to branches, and from larger 

 to smaller but more numerous tubes ; it is the reverse in the veins, 

 except in the case of the vena portce, a vein which carries blood into the 

 liver. This advehent vein, though constituted like other veins in the first 

 part of its course, divides on entering the liver into numerous branches, after 

 the manner of an artery, sending its blood through these branches and 

 through the capillary vessels of the liver into the efferent hepatic veins to be 

 by them conducted into the inferior vena cava and the heart. 



The different parts of the sanguiferous system above enumerated may be 

 contemplated in another point of view, namely, according to the kind of 

 blood which they contain or convey. Thus the left cavities of the heart, 

 the pulmonary veins, and the aorta or systemic artery, contain red or 

 florid blood fit to circulate through the body ; on the other hand, the right 

 cavities of the heart with the venae cavae, or systemic veins, and pulmo- 

 nary artery, contain dark blood requiring to be transmitted through the 

 lungs for renovation. The former or red-blooded division of the sangui- 

 ferous system, commencing by the capillaries of the lungs, ends in the 

 capillaries of the body at large ; the latter or dark- blooded part commences 

 in the systemic capillaries and terminates in those of the lungs. The heart 



