326 



CLASiS REPTILIA. 



entirely to the kidney. In crocodiles and chelonians the caudal veins 

 bifurcate in front and are continued as the two anterior abdominal veins, 

 which receive the iliac veins. In the crocodile the rena.l-portal veins start 

 from a transverse vessel which connects the two branches of the caudal. 



There is an imperfect septum in tlie sinus venosus. The interauricular 

 septum] is always complete. The ventricular cavity is largely broken 

 up by muscular trabeculae, the central clear space being not very large. 

 The interauricular septum reaches right down to and divides the aurieulo- 

 ventricular opening into a right and left division (Fig. 180). To the 

 posterior free edge of this septum are attached two valves (v, v'), one 

 guarding the right aviriculo-ventricular aperture and the other the left ; 

 these are the only auriculo-ventricular valves, except in crocoddes (see 

 below). The dorsal and ventral ends of these valves are attached to the 

 dorsal and ventral wall of the ventricle by muscular bands, which consti- 

 tute together with the valves an incipient septum dividing the ventricle 



MA 



Fig. 180.— Tlie heart of a turtle {Chelune midas). A, a diagram explanatory of the 

 arrangement of the cavities and vessels. B, a drawing from nature, the ventral face 

 of the ventricle being laid open (after Huxley), a muscular flap projecting from the 

 ventral wall and forming an incomplete septum dividing the cavum venosum C.v ; Cp 

 part of the cavum venosum from which the pulmonary artery rises ; L.A left, R.A 

 right auricle ; L.Ao left, R.Ao right aorta ; P. A pulmonary artery ; s arrow showing 

 course of blood in left aorta, t in right, z in pulmonary artery and y behind the 

 incomplete septum ; v, v' the right and left auriculo-ventricular valves ; w, x arrows 

 in auriculo-ventricular openings. 



into a right and left portion. Of these the right, wliich is the larger and 

 receives the opening of the right auricle, is called the cavum venosum (C.r), 

 while the smaller left receives the left auricle and is called the cavum 

 arteriosum. The cavum arteriosum gives off no vessels ; whereas the 

 cavum venosum gives off three — a dorsally arising pulmonary artery, and 

 a right and left systemic aorta. The left systemic aorta arises to the right 

 of the other and crosses to the left side (Fig. 180), passing ventrally to it. 

 The cavujn venosimi is imperfectly divided into two by a muscular 

 projection of its ventral wall (a). From the right side of this projection, 

 froni the cavuiii pulmonale as it may be called, arises the ptilmonary 

 artery, from the left the two systemic arches. When the ventricle con- 

 tracts the free edge of the imperfect septum so constituted meets the dorsal 

 M'all of the ventricle and the heart becomes functionally divided for the 



