HEART or REPTILES. 



509 



335 



shape of tlie body, shows its greatest breadth, figs. 304, a, .335. 

 The two auricles, when distended, are of nearly equal size, and 

 of a subquadrate form, fig. 336, M, o. The bivalvcd orifice 

 between the sinus and the auricle, ib. 

 V, o, is a transverse slit. The white 

 arrow, o, shows the course of the blood 

 from the right auricle, past the valve 

 supported by the base of the auricu- 

 lar septum, into the aortic cavity of 

 the ventricle. In fig. 337 a bristle 

 passes through the orifice left by the 

 incomplete septum between the aortic 

 and pulmonic cavities into the latter, 

 which is the largest in Emijs, as in 

 Opliidia and Lacertilia : the bivalved 

 orifice of the pulmonary artery, P, is 

 shown. In fig. 338 that of the left aorta, 

 A, is similarly exposed, and the in- 

 complete septum is cut through : the 



root of the right aorta, a', is beliind 



Heart of T^studij {jrcica 



336 



that of the left. The relative position 

 of the origins of the three arteries from the chelonian ventricle 

 is shown in fig. 336, where P is the pulmonary, a the left aorta, 

 a' the right aorta, the most posterior, or dorsal, of the three 

 arteries. The ventricle is almost wholly occupied by a spongy 

 muscular structure, and the cavities 

 are smaller in Testudo than in ISmi/s. 

 The left auricle, fig. 336, M, receives 

 the arterial blood from the lungs by 

 a single vein, the common trunk of the 

 ])ulmonary veins, ib. I, I: it opens into 

 the back part of the auricle near the 

 septum, and is guarded by a single 

 oblique membranous fold, ib. m. Each 

 auriculo-ventricular orifice is guarded 

 by a fold which extends across it from 

 either side of the base of the inter- 

 auricular septum ; to that of the left 

 auricle a small part of the muscular 

 structure is attached by chordas tcndineje. Opposite to the right 

 valve a semilunar ridge projects, in Testudo indica, which is the 

 rudiment of the second auriculo-ventricular valve in the Cro- 

 codile, and of the fleshy valve of tliat orifice in the right ventricle 



StrucLure of auricles: heart of C7ic/j/s 

 fimbriata 



