522 ANATOMY OF VERTEBRATES. 



In half-grown specimens of Dugong 1 I found the foramen 

 ovale completely closed, and the ductus arteriosus reduced to a 

 thick ligamentous chord, permeable only for a short distance by 

 an eye-probe from the aorta, where a crescentic slit still repre- 

 sented the original communication. In the smoothness and 

 evenness of their exterior, and their general form, the auricles of 

 the Dugong, ib. «, d, resemble those of the Turtle ( Chelone, vol. i. 

 fig. 335): the appendix can hardly be said to exist in either. 

 The right auricle, a, is but little larger than the left, e : the 

 musculi pectinati are well developed, especially in the left : they 

 are irregularly branched, and with many of the small round 

 fasciculi attached only by their two extremities to the auricular 

 parietes. There is but one precaval and one postcaval orifice in 

 the right auricle, with a smaller coronary inlet. The pulmonary 

 veins terminate in the left auricle by a common trunk one inch in 

 length. The free wall of the right ventricle scarcely exceeds 

 at any part a line in thickness, and is in many places even less. 

 The tricuspid valve is attached to three fleshy columns by chordae 

 tendineae given off from the sides and not the extremities of those 

 columns, both of which extremities are implanted, as trabeculae, in 

 the walls of the ventricles. There are several other columnae 

 carneae passing freely from one part of the ventricle to another, 

 like the musculi pectinati of the auricles, and which have no con- 

 nection with the tricuspid valve. The mitral valve is adjusted to 

 its office by attachments to two short and transversely extended 

 mammillary columnae. The thickness of the parietes of the left 

 ventricle varies from half an inch to an inch. The valves at the 

 origins of the great arteries, c,f, present the usual structure. 



E. Heart of Ungulata. — In all hoofed beasts the ventricles are 

 conical ; the apex being longer and sharper in Ruminants than in 

 most other Mammals. The auricles are relatively smaller to the 

 ventricles than in the preceding groups. The three parts of the 

 tricuspid valves are distinct from their confluent bases, and are 

 pointed at the apex : the basal union of the two parts of the 

 mitral valve is of a greater extent, forming there an annular 

 valve about the left auriculo- ventricular opening. The smooth 

 inner surface of the ventricles is but little interrupted by fleshy 

 columns. The Horse resembles the Ruminant in the general 

 shape and structure of the heart : but in the Tapir 2 it is shorter 

 and broader, as it is in the Rhinoceros 3 and Elephant. The 

 right auricle in the Rhinoceros, as in most Ungulates, has but 

 one precaval orifice, and shows no valve at the termination of 



1 cxvn". p. 35. 2 clii". 3 y". p. 46. 



