CIRCULATION AND RESPIRATION IN ANIMALS 163 



First, a closed system of vessels containing BLOOD. Blood is 

 a lifeless liquid PLASMA in which float detached cells, the red 

 and the white blood CORPUSCLES. Second, a series of spaces, 

 channels, and vessels, closely associated with the blood vas- 

 cular system, which is filled with LYMPH. Lymph consists 

 of some of the liquid plasma of the blood, with some white 

 corpuscles, which has passed through the thin walls of the 

 smallest blood vessels and bathes the individual tissue cells. 

 The lymphatic system really acts as an intermediary between 

 the blood and the tissues. It supplies the milieu of the cells, 

 and finally returns the materials again to the blood vascular 

 system. 



The essential elements of the blood vascular system are 

 first, a muscular organ for propulsion of the blood, the HEART, 

 which lies, as has been mentioned, near the mid-ventral line 

 in the anterior part of the coelom; and second, tubes which 

 convey the blood to the heart, the VEINS, and away from the 

 heart, the ARTERIES. The arteries divide and subdivide to 

 form smaller and smaller vessels (ARTERIOLES) which finally 

 merge into exceedingly delicate tubes (CAPILLARIES) that per- 

 meate the tissues of the body. The capillaries, in turn, de- 

 liver the blood to VEINLETS which pass it on through larger 

 and larger veins to the heart. Consequently the blood flows 

 in a circle from heart to heart again, through a closed system 

 of vessels. (Figs. 91, 92.) 



The heart represents that part of the vascular system in 

 which the power of rhythmic contraction has concentrated, 

 and can be regarded as a blood vessel whose walls have 

 become highly modified by an excessive development of the 

 muscular layer. In the lowest Vertebrates and in em- 

 bryonic stages of higher forms the heart consists typi- 

 cally of two chief chambers, an AURICLE and a VENTRICLE, 

 fitted with muscular flaps, or VALVES, which allow the blood 



