408 



COMPARATIVE ANATOMY 



this 



A n.s 



I. 



A n 



..Vf.il 



c.d 



Anura shows indications of becoming sunk into the right atrium 

 is now usually no longer recognisable as a distinct chamber ex- 

 ternally, though it still persists, with the two typical valves 

 (cf. Figs. 311-313). It becomes partially divided into two portions 

 by a septum, on the left of which the left precaval opens : 

 subdivision of the sinus, hardly indicated in Chelonians, is 



well marked in Crocodiles, and is 

 complete in Birds and Mammals. 

 The pulmonary veins unite into a 

 single trunk before entering the left 

 atrium. 



The atrial septum is solid and 

 unperforated ; and, as its margin 

 reaches much further posteriorly 

 towards the ventricle than in Am- 

 phibians, it not only completely 

 separates the two atria, but also 

 divides the atrio-ventricular aperture 

 into two. The two primarily dorsal 

 and ventral atrio-ventricular valves, 

 moreover, become fused together and 

 then again subdivided in the plane 

 of the atrial septum, so that a right 

 and left valve are now seen. 1 



The principal advance in structure 

 as compared with the amphibian 

 heart is, however, seen in the appear- 

 ance of a muscular ventricular septum, 

 which may be incomplete (Hatteria, 

 Lizards, Snakes, Chelonians), or com- 

 plete (Crocodiles). The higher Liz- 

 ards (c.y. Yaranidjp, Fig. 310, B) come 

 nearest the Crocodiles in this respect, but in no Reptiles except 

 the latter is there more than a periodic physiological separation 

 between the tw r o halves of the ventricle. A complete septum 

 ventriculorum thus appears for the first time in Crocodiles (Fig. 

 314), in which consequently the right ventricle contains unmixed 

 venous blood, and the left ventricle unmixed oxygenated blood, 

 although, as will be seen presently, an admixture takes place in the 

 systemic arteries. In Crocodiles the right atrio-ventricular aper- 

 ture is guarded by a large muscular flap on the right (outer) 

 side of the aperture. 



The conus arteriosus now becomes practically absorbed into 

 the ventricular portion of the heart, and each aortic root may be 



1 In Ascalabota (< .;/. T<tr<_ utol</ iimnrit<uii<-<i} and C'helonia the proximal part of 

 the aortico-pulmonary septum encloses a plate of hyaline cartilage (cartilage of 

 ]'>ojanus), and cartilages may occur in the neighbourhood of the semilunar valves 

 of the aorta and pulmonary artery in other Reptiles (e.g. Crocodiles). 



I. V.... 



Fii;. 311. HEART OF Cydodu* 

 boddatrtei. From the dorsal 

 side. (After Rose. ) 



An, A /i.*, innominate arteries ; 

 Ao.rtlxt, dorsal aorta ; D.C.<I, 

 D.C.x, precaval veins; /../, 

 pulmonary vein ; P. 3, P.d, 

 pulmonary arteries ; tip. >', spa- 

 tium intersepto-valvulare (cf. 

 Fig. 316); V.C.d, posterior 

 cardinal; V.j.d, jugular, and 

 V.x.d, subclavian vein of the 

 right side; V.c. i, postcaval vein. 



