CIRCULATION 171 



second, tubes which convey the blood to the heart, the veins, and 

 away from the heart, the arteries. The arteries divide and sub- 

 divide to form smaller and smaller arteries which finally merge 

 into exceedingly delicate tubes (capillaries) that permeate the 

 tissues of the body. The capillaries, in turn, deliver the blood to 

 tiny veiNS 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. Indeed, in the 

 meshes of a network of blood-streams all the life of our bodies 

 goes on. About one-twentieth of the weight of the normal human 

 body is blood. (Figs. 120, 124.) 



A. Circulation in the Lower Verterrates 



The heart represents that part of the vascular system in which 

 the power of rhythmic contraction is concentrated, and it can be 

 regarded as a blood vessel whose walls have become highly modified 

 by an excessive development of muscular tissue. In the lowest 

 Vertebrates and in embryonic stages of higher forms the heart 

 consists typically of two chief chambers, an auricle and a ven- 

 tricle, fitted with muscular flaps, or valves, which allow the 

 blood to flow in one direction onlv; that is, from auricle to ventricle. 

 An enlargement, the sinus venosus, connects the veins (venous 

 system) with the auricle, and there is frequently another, called 

 the conus arteriosus, in a similar position at the arterial end. 

 The heart is thus essentially a linear series of chambers. The 

 sinus venosus and auricle function mainly as reservoirs to fill 

 rapidly the especially muscular ventricle. The latter, acting both 

 as a suction and force pump, passes the blood on to the conus 

 arteriosus and from there to the arterial system as a whole. 

 For our purposes, however, we may consider the heart in the lowest 

 Vertebrates (Fishes) as composed of the two chambers, auricle 

 and ventricle. (Fig. 121.) 



The arterial system is the distributing system of vessels which 

 carries the blood to all regions of the body. Soon after its origin 

 at the heart, the circuit in the aquatic forms is temporarily inter- 

 rupted to allow the blood to pass through the gills and exchange 

 carbon dioxide for a supply of oxygen. To facilitate this gaseous 

 interchange, the arteries (afferent rranchial) as they enter 

 the gill membrane break up into smaller and smaller vessels which 

 finally are of microscopic caliber and consisl of but a single layer 



