THE HEART IN SAXJEOPSIDA. 309 



auricles. Generally, a distinct sinus venosus, -with, contractile 

 walls, and communicating by a valvular aperture with the 

 auricle, receives the blood from the vence cavce, and pours 

 it into the right auricle. The pulmonary veins usually 

 open by a common trunk into the left auricle. 



The interauriciilar septum is rarely (in some Chelonia) 

 perforated. Its ventricular edge spreads out on each side 

 into a bi'oad membranous valve, the edge of which, dtu-ing 

 the systole, flaps against a ridge, or fold, developed, on one, 

 or both, sides, from the margin of the auriculo-ventricular 

 aperture, and constituting a rudiment of a second valve. 

 The ventricle contains only one cavity, but that cavity is 

 imperfectly divided into two or three chambers, by septa 

 developed from its muscular walls. 



In the Turtle (Fig. 92), a partly muscular, and partly 

 cartilaginous, septum extends from the front wall of the 

 ventricular cavity towards its right-hand end. It im- 

 perfectly divides the common ventriciilar cavity into a 

 right small, and a left large, moiety. The latter of these 

 receives the blood from the auricles. In consequence of 

 the elongated form of the ventricular cavity, and the pro- 

 jection into it of the large auriculo-ventricular valves, 

 especially of that of the right side, this left and larger 

 moiety of the common ventricle is virtually divided into 

 two, a left and a right, at the time of the auricular systole. 

 The left portion becomes filled with arterial blood from the 

 left auricle, and is distinguished as the cavum arteriosum ; 

 the right receives the venous blood from the right auricle, 

 and is the cavum venosum. 



No arterial trunk arises from the cavv/m arteriosum, but two 

 arterial trunks arise from the light-hand end of the cavum 

 venosum ; these are the two aortic arches. One of these 

 passes to the left and the other to the right side, and they 

 cross one another as they do so, because the origin of the 

 left arch lies more to the right, than does the origin of the 

 left arch. The ostia of both arches are guarded by semi- 

 lunar valves ; and that of the left arch is placed below and 

 to the right of that of the riafht arch. As no arterial trunk 



