56 



MORPHOLOGY OF THE VERTEBRATE BODY 



The Circulatory System. — Any well-developed circulatory sys- 

 tem, like that of the frog, includes a " pump," the heart, by which 

 blood is forced through arteries to all parts of the body. The 

 blood passes from these arteries to the capillaries, which are the 

 smallest of all the blood vessels, and is thence returned to 

 the heart through veins. The heart of a frog (Fig. 32) con- 

 sists of two thin-walled auricles that function as " charging " 

 chambers for the single ventricle which, by the contraction of its 



thick muscular wall, drives the 



c. ft. 



s. a. 



p-c. a. 



au-v. V. 



blood into the arteries and thus 

 around the circuit. The arteries 

 may be traced outward from 

 the ventricle to all parts of 

 the body (Fig. 33). The veins 

 (Fig. 34) parallel the arteries 

 although they do not branch in 

 a manner that exactly follows 

 the divisions of the latter. The 

 connection of arteries and veins 

 by means of capillaries, which 

 is essentially the same in all 

 parts of the body, may be rep- 

 FiG. 32.— Heart of frog with principal resented diagrammatically as 

 outgoing blood vessels, from ventral in Fig, 60, p. 104. Circulation 

 view and with part ^ of ventricle re- ^^ the frog consists in the pas- 

 sage of the blood around such 

 a circuit of heart, arteries, capil- 

 laries, and veins, complicated 

 by the addition of the lesser 

 circuit through the lungs. 

 The chief peculiarity of the frog's circulation consists in the 

 three-chambered condition of the heart, whereby the unoxygen- 

 ated blood from the body and the oxygenated blood from the lungs 

 pass through a single ventricle. In a four-chambered heart they 

 are kept separate in the right and left ventricles respectively 

 and never mixed. The manner in which a partial separation of the 

 blood from body and lungs may be brought about in the frog 

 appears when the structure and manner of action of the heart is 

 examined. The structure of the frog's heart and its principal out- 

 going blood vessel is shown in Figs. 32 and 33. The two auricles 



moved. 



au-».t)., auricular-ventricular valve; c.o., caro- 

 tid artery; l.a., left auricle; p-c.a., pulmo- 

 cutaneous artery; s.a., systemic artery; r.a., 

 right auricle; t.a., truncus arteriosus; «, valve 

 of truncus arteriosus; ve, ventricle. 



