HEART OF SIRENIA. 521 



the abdominal muscles to reach the diaphragm, which has a like 

 low position anteriorly, to which it adheres broadly : and the 

 precavals unite and terminate in the auricle by one orifice: 

 the thoracic part of the postcaval is very short. The musculi 

 pectinati are well developed in the right auricle, and the 

 appendix is distinct, but undivided. The fossa ovalis is feebly 

 marked in the Cachalot, is deeper in some Delphinidas, but in all 

 Cetacea it is closed : there are neither Eustachian nor coronary 

 valves. In the Cachalots and Whales the ventricular mass is 

 subdepressed and semicircular, the apex being rounded or rather 

 flattened, and sometimes indented : for the right ventricle is co- 

 extensive with and sometimes terminates, as in the Mammalian 

 embryo, distinctly from the left. In Phoccena and most Del- 

 phinidce, the apex of the ventricle is simple and better marked. 

 The movable wall of the right ventricle has about half the 

 thickness of that of the left, showing the exercise of greater force 

 in propelling the blood through the lung, than in land Mammals. 

 The tendons of the tricuspid valve go to three short and thick 

 columns in most Cetacea ; but the rest of the inner surface is 

 broken by strong trabecular bands. Hunter notes the soft 

 yielding substance of the semilunar valves in the Hyperoodon he 

 dissected, suggesting that they were naturally less strong than 

 in land Mammals. 1 The left auricle is less than the right, with 

 many well-defined muscular columns on the inner surface, and 

 a distinct appendix ; but is less 

 fleshy than the right auricle. 

 In the left ventricle both tra- 

 becular and mammillary forms 

 of muscular processes of the 

 inner surface are numerous. 



The most striking feature 

 in the anatomy of Whales is 

 the vast size of their several 

 organs : the heart may be more 

 than a yard in transverse dia- 

 meter, and not much less in 

 length. 



D. Heart of Sirenia. — The 

 outward division of the ventri- 



Heart of the Dugong. 



cles indicated in some Cetacea 



is carried to an extent very characteristic of the present group : 

 but in Rhytina and Manatus the cleft is not quite so deep as in 

 the heart of Halicore, fig. 404. 



1 ccxxvi. ii. p. 111. 



