164 BLOOD-YESSELS. 



the circulation of the blood. Its course from the left ventricle along the 

 aorta, throughout the body, and back by the venre cavte to the right 

 ventricle, is named the grecder or sijstemk 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 pulmonnr)j 

 circulation ; but the blood must go through both the greater and the 

 lesser circulation 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 pulmonary, so the aorta 

 which conveys the blood to the system at large is named the systemic 

 artery, and the vence cavee 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 pul- 

 monary 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 venas cavse, or 

 systemic veins, and pulmonary artery, contain dark blood requiring to 

 be transmitted through the lungs for renovation. The former or red- 

 blooded division of the sanguiferous system, commencing by the capil- 

 laries 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 occupies an intermediate position be- 

 tween the origin and termination of each, and the capillaries connect the 

 dark and the red sets of vessels together at their extremities, and serve 

 as the channels through which the blood passes from the one part of the 

 sanguiferous system to the other, and in which it undergoes its alternate 

 changes of colour, since it becomes dark as it traverses the systemic 

 capillaries and red again in passing through those of the lungs. 



AKTERIES. 



These vessels were originally supposed to contain air. This error, 

 which had long prevailed in the schools of medicine, was refuted by 

 Galen, who showed that the vessels called arteries, though for the 

 most part found empty after death, really contain blood in the living 

 body. 



Mode of Distribution. — The arteries usually occupy protected 

 situations ; thus, after coming out of the great visceral cavities of the 

 body, they run along the limbs on the aspect of flexion, and not upon 

 that of extension where they would be more exposed to accidental injury. 



