60 



EVOLUTION AND GENETICS 



B 



ing of two fundamental chambers, but preceded and followed by- 

 two others (Fig. 41A, B). The blood enters this heart from the 

 body through several large veins which join the posterior cham- 

 ber, the sinus venosus. From the sinus venosus it passes into the 

 atrium, thence into the ventricle whose muscular walls contract 

 rhythmically and drive the blood out to the body. As it leaves the 

 ventricle it passes first into the conus arteriosus, which tapers into 

 the truncus arteriosus, a part of the tubular portion of the circu- 

 latory system. In some fishes 

 a muscular bulbus follows the 

 conus. From the truncus sev- 

 eral pairs of afferent branchial 

 arteries are given off (Fig. 43B) 

 — five in some existing fishes — ■ 

 which pass into the branchial 

 arches and Ijreak up into capil- 

 laries wherein the blood, laden 

 with wastes, gives up its car- 

 bon dioxide and receives a 

 fresh supply of oxygen through 

 the delicate tissues of the gills. 

 It is then collected into other 

 Fig. 39. — Section of the jaw of an em- arteries, the efferent bran- 

 bryo kitten A Meckel's cartilage; chials, which unite to form the 

 B, bone trabeculae of the developing , , , i i • i ,i i 



mandible. From a photomicrograph, dorsal aorta behmd the bran- 

 chial region. Through this 

 vessel and its branches the pure blood is carried to all parts of the 

 body and distributed to the tissues by another system of capil- 

 laries (Fig. 40). 



After it has served the tissues the blood is collected again, this 

 time into veins. The veins from the caudal region return their 

 blood to the kidneys, where it is again distributed in capillaries. 

 Such a system is called a portal system. Blood from the alimen- 

 tary tract is conveyed in a similar manner to the liver, and from 

 these organs other veins convey the blood to the heart, the pos- 

 terior cardinals that from the kidneys, and the hepatic vein that 

 from the liver. These portal systems are called the renal and 

 hepatic portal systems, respectively (Fig. 40). Blood from the 

 body is also returned to the heart directly through other veins. 

 In the fish, therefore, we find that the muscular contractions 



