STRUCTURE OF THE VERTEBRATES 



231 



narrows and forces the unaerated blood of the pulmonary cham- 

 ber into the pulmonary artery, and the aerated blood of the left 

 chamber into the two loops (radices) of the ventral aorta. In the 

 alligator the septum is complete, making a four-chambered heart, 

 but there is an opening between the right and left ventral aortae 

 which permits a slight admixture of bloods. 



Pulmonary vei 



Auricles. 



Chordae tendinae 



■Aorta 



Pulmonary artery 

 Semilunar valves 



Left ventricle 

 Papillary muscles 



Cava-]— s=i 



Tricuspid valve 

 Right ventricle 



Fig. 133. Structure of the Mammal Heart. A portion is removed to show 



the internal structure. The ventricular septum curves toward the right, 



separating the semi-lunar valves of the aorta and pulmonary artery. 



The mammal heart is structurally and functionally completely 

 four-chambered. The sinus venosus is left as a vestigial band of 

 tissue within the right auricle. The septum is complete, dividing 

 the heart into left auricle and ventricle, and right auricle and 

 ventricle. The auriculo-ventricular opening is guarded by two 

 sets of valves, each valve being a flap of tissue attached to the 

 inner periphery of the opening, and the loose edge held by 

 strands of connective tissue (the chordae tendinae) which are 

 attached to the wall of the ventricles. The left (mitral) valve 

 has two cusps while the right {tricuspid) has three. The ventral 

 aorta consists of a single loop (radix) and is completely sep- 

 arated from the pulmonary. Therefore the blood is never mixed, 

 except in pathological conditions. The heart of the birds is es- 

 sentiallv like that of the mammals. 



