38 ELExMENTARY THYSIOLOGY less. ;i 



desired direction. Hence it is that the valves are most 

 numerous in those veins which are most subject to 

 muscular pressure, such as those of the arms and legs. 



The only arteries which possess valves are the primary 

 trunks — the aorta and pulmonary artery — which spring 

 from the heart, but these valves, since they really belong to 

 the heart, will be best considerocl with that organ. 



3. The General Arrangement of Blood-vessels 

 in the Body. — It will now be desirable ta take a general 

 view f)f the arrangement of all these different vessels, and 

 of their relations to the great central organ of the vascular 

 system — the heart (Fig. 11). 



All the veins of every part of the body, except the 

 lungs, the heart itself, and certain viscera of the abdomen, 

 join together into larger veins, which, sooner or later, 

 open into one of two great trunks (Fig. 11, I'.C.S. V.C.I.) 

 termed the superior and the inferior vena cava, 

 which debouch into the upper or broad end of the right 

 half of the heart. 



All the arteries of every part of the body, except the 

 lungs, are more or less remote branches of one great 

 trunk — ^the aorta (Fig. 11, Ao.), which springs from the 

 lower division of the left half of the heart. 



The arteries of the lungs are branches of a great trunk, 

 the pulmonary artery (Fig. 11, F.A ), springing from 

 the lower division of the right side of the heart. The 

 veins of the lungs, on the contrary, open by four ti'unks 

 int:; the upper part of the left side of the heart (Fig. 11, 

 P. V. ), by tlie pulmonary veins. 



Thus the venous trunks open into the upper division of 

 each half of the heart : those of the body in general into 

 that of the right half, those of the lungs into that of the 

 left half ; while the arterial trunks spring from the lower 

 moieties of each half of the heart : that for the body in 

 general from the left side, and that for the lungs from the 

 right side. 



Hence it follows that the great artery of the body, and 



