THE ASCENT OF MAN. 459 



cient to cause jit limes retention ;um1 consecjntMil iiisi)issatiou of tlio 

 bile, leading to the lorniation of j:jalI-.stones. 



The quadruped's liver hangs suspended from the spine, but as the 

 erect attitude is assumed it depends more and more from the diai)hragm. 

 The diaphragm in its turn develops adhesions with the fibrous covering 

 of the heart, which is continuous with the deep fascia of the neck, so 

 that iu effect the liver hangs suspended from the top of the thorax and 

 base of the skull. This restricts in some degree the action of the dia- 

 phragm and confines the lungs. This must have an effect upon the 

 aeration of the blood, and consequently upon the ability to sustain pro- 

 longed and rapid muscular exertion. An extra lobe of the right lung 

 that in animals intervenes, either constantly or during inspiration, be- 

 tween the heart and the diaphragm, is occasionally found in a vestigial 

 state in man. 



The vascular system abounds in evidences that it was primarily 

 adapted to the quadrupedal position. By constant selection for enor- 

 mous periods of time, the vessels have become located in the best pro- 

 tected situations. It is scarcely possible to injure a vessel of any size 

 in an animal without deeply penetrating the body or passing quite 

 through a limb. In man, on the contrary, several great trunks are 

 comparatively exposed, notably the great vessels of the thigh, those of 

 the forearm, and of the ventral wall. 



The influence that gravity has upon the circulation is well known. 

 The horizontal position of the great venous trunks favors the easy flow 

 of blood to the heart without too greatly accelerating it. Man, in whom 

 these trunks are vertical, suffers thereby from two mechanical defects, — 

 the difficulty of raising blood through the ascending vena cava, whence 

 come congestion of the liver, cardiac dropsy, and a number of other 

 disorders, and the too rapid delivery through the descending cava, 

 whence the tendency to syncoi)e or fainting if for any cause the action 

 of the heart is lessened. Clevenger's admirable discovery that the 

 valves of the veins are arranged for a quadrui)edal jjosition should also 

 be mentioned here. Evidently intended to resist the action of gravity, 

 they should, to be effective, be found in the large vertical trunks. But 

 in the most important of these they are wanting. Hence are caused 

 many disorders arising from hydrostatic pressure, such as varicose veins, 

 varicocele, hemorrhoids, and the like. Yet the values occur in several 

 horizontal trunks, where they are, as far as we know, of no use what- 

 ever. Place man on all fours however, and it is seen that the entire 

 system of valves is arranged with reference to the action of gravity in 

 that position. The great vessels along the spine and the portal system 

 being then api)roximately horizontal do not recpiire valves, while all 

 the vertical trunks of considerable size, even the intercostal and jugular 

 veins, are provided with them. A confirmation of this view is found in 

 the fact that the valves are variable in chara<;ter and tend to disappear 

 in the veins where they are no longer needed. 



