198 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



valves which is found in the bird from that which 

 obtains in reptiles, no such comparison is possible in 

 the case of the mammalian valves ; in most of the 

 Mammalia we find that the valves are membranous 

 and not fleshy, and that there are three (tricuspid) 

 valves in the right auriculo-ventricular orifice, and 



two in the left (mitral 

 valves) ; as in the left 

 side of the heart of the 

 bird, there are chorda? 

 tendiiiese and musculi 

 papillares, but the num- 

 ber of these is not con- 

 fined to three. In the 

 rabbit the valve of the 

 right side is continuous, 

 and not produced into 

 three cusps. In. the 

 Ornithorhynchus, the 

 valve of the right side 

 is, as in the Sauropsida, 

 muscular membranous 

 tissue being only com- 

 paratively feebly de- 

 veloped in it. 



As in the other parts of the heart, so in the 

 ventricle, we find an instructive series of gradations. 

 Single and undivided in all Fishes except the Dipnoi, 

 it is always much stronger, and has much thicker 

 walls, which are largely composed of muscular tissue, 

 than has the auricular region ; for while the office of 

 the contraction of the auricle is merely to drive the 

 blood into the adjoining chamber, the ventricle has 

 the chief part to play in forcing it through the body. 

 In the Teleostei and some Ganoids the wall of the 

 ventricle is arranged in two layers, between which is 

 a lymphatic space. 



Fig. 85. Left auriculo-veutricular 

 valve of the Swan, showing the 

 chordae tendinese and the musculi 

 papillares (pp). (After Wieder- 

 sheim.) 



