BRANCH CHORDATA; CLASS PISCES: THE FISHES 267 



the posterior end of the body-cavity note a small red 

 glandular mass, the spleen. 



At the anterior end of the body in front of the liver 

 and between the sets of gills note the small pcricardial 

 cavity within which is contained the heart. The peri- 

 cardial cavity is separated from the peritoneal cavity by 

 a thick muscular wall against which the liver abuts. The 

 heart consists of four parts. The posterior part is a thin- 

 walled reservoir, the sinus venosus, into which blood 

 enters through the jugular vein from the head and through 

 the cardinal vein from the kidney. From the sinus 

 venosus it passes forward into a large chamber, the 

 aiiride. Next it flows into the ventricle, where, by the 

 contraction of the walls, rhythmical pulsations force it into 

 the conns arteriosus, thence into the ventral aorta, and 

 lastly into the gills, where it is purified. After passing 

 through the capillaries in the fine gill-filaments it is again 

 collected, now pure, by paired arteries from each pair of 

 gills, which arteries unite to form the dorsal aorta ex- 

 tending backward just below the backbone to the end of 

 the tail. From the dorsal aorta a pair of arteries, the 

 subclavian, are given off to the pectoral fins. At this 

 point two other arteries branch off ventrally, the first being 

 the cardiac artery, which distributes blood to the stomach 

 and pyloric caeca. The second divides into several long 

 mesenteric arteries supplying blood to all parts of the in- 

 testine and spleen. In the caudal region blood is taken 

 up through the caudal vein and carried forward to the 

 kidneys. These strain out the impurities arising from 

 waste of tissues, after which the blood is carried back to 

 the sinus venosus through the cardinal vein. From the 

 intestine it is gathered into the large portal vein as in the 

 toad. The portal vein carries blood to the liver, where 

 nutriment may be stored up, and from thence it flows back 



