STRUCTURE OF THE HUMAN BODY 41 



auricle, from which it enters the left ventricle in order to be 

 pumped through a great artery to the body at large. This 

 artery is known as the aorta (Gk. aeiro, I carry), and it is 

 characteristic for the group of animals to which Man belongs, 

 i.e. the Mammalia, that the curved part or arch with which the 

 aorta begins should turn to the left. In a Bird it would be found 

 curving round to the right. 



If the aorta be traced, its arch will be found to give off large 

 branches to the anterior part of the body, after which it bends into 

 the middle line, and (as the dorsal aorta) runs just ventral to the 

 backbone, supplying the parts it passes, and ending posteriorly by 

 forking into two great vessels for the lower limbs. Any branch 

 artery farther traced will be found to divide, tree-like, into smaller 

 and smaller twigs, which at last pass into a close net-work of 

 excessively minute thin-walled tubes, the capillaries. The same 

 thing will be observed on tracing the pulmonary artery. It may, 

 in fact, be said that almost all parts of the body are traversed by 

 a dense net-work of capillaries, and from these the smallest veins 

 arise, uniting, river-like, into larger and larger trunks till the great 

 veins opening into the heart are constituted. We may therefore 

 say that the heart and blood-vessels form a closed set of tubes, 

 as the capillaries intervene between the smallest arteries and the 

 smallest veins. This may be clearly seen in the web of a frog's foot. 



Capillaries are of enormous importance, because their thin 

 walls permit exchanges between the blood they contain and the 

 body-substance they traverse. 



The veins of the stomach, intestine, and certain other parts 

 unite into a large portal vein, which enters the liver, and there 

 divides up into smaller and smaller branches. The blood in this 

 vein contains most of the digested food which has been absorbed 

 from the gut, and the object of passing it through the liver is 

 to let that organ take up some of these products, and store them 

 for distribution as needed. The impure blood from the liver itself 

 is poured by several blood-vessels into one of the great veins 

 running to the right auricle. The arrangement just described for 

 supplying the liver with impure blood, in addition to the pure 

 blood which it gets from a branch of the aorta, is called the portal 

 circulation. 



The Lymph- System. — The circulatory organs include a 

 lymph-system as well as a blood-system. The lymph is a clear, 



