Chap. V.] 



HEART OF VERTEBRATES. 



197 



portion of the heart by membranous valves, just as 



the auricle is shut off from the venous sinus by 



similar structures ; these are in fishes ordinarily, 



though by no means always, two in number; and their 



function is clearly to close the way back into the 



venous system, and thereby to aid in forcing the 



blood forwards, on the contraction 



of the walls of the cavity. In the T ft 



Amphibia (Fig. 84) the two valves 



are fibrous, and are connected by 



fibres with the wall of the ventricle, 



so that when this part of the organ 



contracts it draws down the valves ; 



when the auricular chamber (atrium) 



becomes divided, each opening into 



the ventricle is provided with 



valves. 



In the turtle the auriculo- ven- 

 tricular valves are formed by the 

 development of the ventricular edge 

 of the auricular septum into two 

 folds, which, on the contraction of 

 the ventricle, meet a ridge on the 

 correspondingly opposite side of the 

 ventricle ; in the crocodile these 

 folds are distinct valves. In the Bird 

 we find a difference between the valves of the right 

 and left side ; on the right two folds of muscular 

 tissue, close together at their auricular end, diverge 

 from one another, and extend far down into the right 

 ventricle ; on the left the muscular is largely replaced 

 by fibrous tissue which gives off fine tendons (chordae 

 teiidinese) to projecting muscular processes of the 

 wall of the ventricle (musculi papillares) (Fig. 

 85) ; these tendinous chords are grouped into 

 three masses, and there are three muscular elevations. 



Though it is possible to derive the arrangement of 



Fig. 84. Heart of the 

 edible Frog (Rana 

 esculenta), to sho* 

 the auricle and ve 

 tricle opened from 

 the left side. 



s, Septum atriorum ; la, 

 left auricle ; ra, right 

 auricle ; w, auriculo- 

 ventricula.- valves ; o, 

 orifice of arterial cone 

 or trunk : v. ventricle. 

 (After Ecker.) 



