4 1 o ELEMENTAR Y ANA TO MY. [LESS 



or bulbils arteriosus, which may be rhythmically contractile, 

 as in Elasmobranchs, or not so, as in Teleosteans. Its en- 

 trance has valves variously conditioned in different animals. 



The heart may consist of but two cavities, one auricle and 

 one ventricle, as in Fishes. This reduction is, however, ac- 

 companied by a dilatation of the root of the aorta into a 

 bulbus arteriosus, and of the termination of the venous chan- 

 nels at the heart into a sinus venosiis. 



Finally, the heart may consist of but a simple tube, as in 

 the Lancelet, so often referred to. 



FIG. 356. HEART OF CRYPTOBRANCHUS OPENED ON ITS DORSAL ASPECT. 



(After Hyrtl.) 



n, ventricle ; b, auricle ; c, vena cava ; d, right innominate vein ; e, coronary veins 

 from the heart opening into the right innominate vein ;_/", left innominate vein ; 

 g, five fleshy, semi-lunar valves at the root of the bulbus aortse ; /z, three fleshy 

 seiui-hmar valves placed at the entrance of the right subdivision of the bulb ; 

 Ji ' i a prominence below the left subdivision of the bulb ; z, membranous semi- 

 lunar valve at the root of the left subdivision of the bulb. 



The envelope of the heart, the pericardium, may have its 

 cavity continuous with that of the peritoneum, as in the 

 Myxinoid and Elasmobranch Fishes. 



The right ventricle may, as in all Mammals and Birds, 

 give rise to the pulmonary artery only, or, as in Crocodiles, to 

 an aortic arch also. 



The valve separating that ventricle from the right auricle 

 may, as in Birds, be muscular in structure, instead of mem- 

 branous as in man. 



In forms such as Frogs and most Reptiles (in which pul- 

 monary arteries co-exist with a single or an imperfectly 

 divided ventricle), special arrangements of valves cause the 

 venous blood to be propelled into the pulmonary arteries, and 

 mainly arterial blood into the aortic arches. 



