TRANSPORTATION SYSTEM 



123 



able pressure, and they are firm enough to stand open even when empty 

 of blood. The veins are not called upon to endure such pressures as are 

 the arteries; their walls are comparatively thin and collapsible. More- 

 over, in the veins there are at invervals valves, consisting of membranous 

 flaps directed forward (in the direction of flow), which close and stop 

 the blood if it starts at any time to flow backward (Fig. 96). 

 The capillaries are of various sizes, the smallest ones 

 being just large enough to allow the blood cells to pass 

 along single file. They have very thin walls, only one cell 

 thick. Being thin, they are collapsible, and at times of rest, 

 when the circulation is slow, many of them are closed. 



Blood is kept coursing through these vessels by the 

 motive power of the heart. Any muscular activity is apt 

 to exert pressure on near-by veins, and this in conjunction 

 with the valves in the veins helps to keep the blood mov- 

 ing; but the heart action is the main source of power. 



Chambers of the Heart and Course of Circulation. — 

 The hearts of various vertebrates have two, three, or four 

 chambers, and the course of the circulation is in part 

 related to this feature of heart structure. A diagram of 

 the circulator}^ system in the dogfish, an animal with a 

 two-chambered heart, is shown in Fig. 97. This diagram indicates that 

 the blood of animals with gills and a two-chambered heart passes from the 

 ventricle of the heart through the gills and then forward to the head or 

 backward through the dorsal aorta to the organs of the body, where it 

 passes through capillaries and returns to the auricle of the heart by means 

 of the veins. 



TO HFAn DORSAL AORTA 



, H°^"° ^ I ) } I 



Fig. 96.— 

 Vein slit open 

 to show 

 valves. 

 Course of 

 blood is 

 upward. 



GILLS- 



'llllllll 



VEIN 



VENTRAL AORTA 



BODY AND 

 ORGANS 



Fig. 97. — Simplified diagram of the circulatory system of the dogfish. 



Except for the fact that the blood in the arteries is distributed to 

 different organs, from each of which it returns independently to the veins, 

 the blood of a fish covers only one circuit. It passes through two sets of 

 capillaries, one in the gills and another in the head or some body organ or 

 tissue, and goes to the heart only once in each circuit. This course is a 

 consequence of the two-chambered construction of the heart. 



In animals with lungs and a heart of more than two chambers the 

 circulatory system is more complicated. The heart of amphibians and 



