The Organs of Respiration 107 



cavity by a sort of "diaphragm" formed of thickened peri- 

 toneum. In certain eels the heart is remote from the head. 



The heart consists of four parts, the sinus venosus, into which 

 the veins enter, the auricle or atrium, the ventricle, and the 

 arterial bulb at the base of the great artery which carries the 

 blood to the gills. Of these parts the ventricle is deepest in 

 color and with thickest walls. The arterial bulb varies greatly 

 in stnicture, being in the sliarks, rays. Ganoids, and Dipnoans 

 muscular and provided with a large number of internal valves, 

 and contracting rhythmically like the ventricle. In the higher 

 fishes these structures are lost, the walls of the arterial bulb are 

 not contractile, and the interior is without valves, except the 

 pair that separate it from the ventricle. 



In the lancelet there is no proper heart, the function of the 

 heart being taken by a contractile blood-vessel situated on the 

 ventral side of the alimentary canal. In the Dipnoans, which 

 are allied to the ancestors of the higher vertebrates, there is the 

 beginning of a division of the ventricle, and sometimes of the 

 auricle, into parts by a median septum. In the higher verte- 

 brates this septum becomes more and more specialized, sepa- 

 rating auricle and ventricle into right and left cavities. The 

 blood in the fish is not returned to the heart after purification, 

 but is sent directly over the body. 



The Flow of Blood. — The blood in fishes is thin and pale red 

 (colorless in the lancelet) and with elliptical blood-corpuscles. 

 It enters the sinus venosus from the head through the jugular 

 vein, from the kidney and body walls through the cardinal vein, 

 and from the liver through the hepatic veins. Hence it passes 

 to the auricle and ventricle, and from the ventricle through the 

 arterial bulb, or conus arteriosus to the ventral aorta. Thence 

 it flows to the gills, where it is purified. After passing through 

 the capillaries of the gill-filaments it is collected in paired 

 arteries from each pair of gills. These vessels unite to form the 

 dorsal aorta, which extends the length of the body just below 

 the back-bone. From the dorsal aorta the subclavian arteries 

 branch off toward the pectoral fins. From a point farther back 

 arise the mesenteric arteries carrying blood to the stomach, in- 

 testine, liver, and spleen. In the tail the caudal vein carries 

 blood to the kidneys. These secrete impurities arising from 



