60 



MORPHOLOGY OF THE VERTEBRATE BODY 



tract passes through the liver, and the connections, as shown, 

 through the kidneys and the posterior extremities. In such a 

 system there is no such problem of the separation of oxygenated 

 from unoxygenated blood as appears in the frog. 



Fig. 35. — Schematic representation of circulation in a vertebrate having 

 a two-chambered heart as in fishes. 



o, auricle; d.t., digestive tract; kd, kidney; I, liver; s.v., sinus venosus; v, ventricle. 



The circulatory systems of birds and mammals (Fig. 36) 

 have four-chambered hearts, consisting of two auricles and 

 two ventricles, and two entirely distinct circuits, one through 

 the body and another through the lungs. The blood is forced by 



Fig. 36. — Schematic representation of circulation in a vertebrate having 

 a four-chambered heart as in mammals. 



d.t., digestive tract; f.b., fore body; h.b., hind body; kd, kidney; I, liver; la., left 

 auricle; Is, lungs; l.v., left ventricle; r.a., right auricle; r.v., right ventricle. 



the left ventricle into the general arterial system and so to the 

 capillaries in all parts of the body. Returning by the veins, it 

 goes to the right auricle and thence to the right ventricle, which 

 forces it through the lesser circuit of the lungs, whence it is returned 



