THE FROG, A TYPICAL VERTEBRATE 



249 



shown that the entire blood supply passes from the heart through- 

 out the body and back again to the heart in from twenty to thirty 

 seconds. This means that 

 the six quarts of blood in 

 the body of a man pass 

 three or four thousand 

 times per day throughout 

 the tissues of the body. 



The heart (Fig. 143) of 

 the frog is a conical organ 

 consisting of a muscular 

 ventricle and two thin- 

 walled auricles. In man 

 there are two ventricles 

 instead of one. Tubes, 

 the arteries, carry the 

 blood from the heart to 

 the tissues, and others, 

 the -veins, carry it back to 

 the heart. The muscular 

 ventricle by its contrac- 

 tions forces blood into the 

 arteries. The walls of the 

 arteries are elastic and 

 by their pressure force 

 the blood along until it 

 reaches the finest of all 

 the blood tubes, the cap- 

 illaries. The capillaries 

 unite the ends of the ar- 

 teries with the ends of the 

 veins and the blood from 

 the former passes through 

 them into the veins and 

 thence back to the heart. 



sc 



FIG. 143. Diagram of the arterial system of 

 the frog, ventral view. 



ao", aortic arch; au', right auricle; au", 

 left auricle ; br, brachial artery ; c.c, carotid ; 

 c.gl, carotid gland; c.il, common iliac; coe, 

 coeliaco-mesenteric ; coe', cceliac ; cu, cuta- 

 neous ; d.ao, dorsal aorta ; fin, femoral ; g, 

 gastric ; h, hcemorrhoidal ; hp, hepatic ; hy, 

 epigastrico-vesical ; k, kidney; 1, lingual; 

 Ig", left lung; m, anterior mesenteric; m.i, 

 posterior mesenteric ; oc, occipital ; pc', pan- 

 creatic ; p.cu, pulmocutaneous ; pul, pulmo- 

 nary ; re, renal ; sc, sciatic ; sp, splenic ; tr.a, 

 truncus arteriosus; ts, testis; v, vertebral 

 (After Howes.) 



