524 ANATOMY OF VERTEBRATES. 



with an obtuse apex : the appendix of the right auricle is bifid, 

 one process covering the origin of the pulmonary artery, the other 

 lying upon the right ventricle. The auricular septum seems to 

 be formed by an extension of the left part of the wall of the 

 anterior cava, terminating in an arch to the right of the 

 postcaval orifice, which thus seems to open (as it did in the 

 embryo) into the left auricle. In the younger of two Seals, 

 (Phoca vitulina), which I dissected, 1 the valve that cuts off this 

 original communication between the auricles was incomplete, and 

 left a large ' foramen ovale : ' in the older Seal, not full grown, 

 the ( valvula foraminis ovalis ' was complete as to its extent, and 

 the margins were adherent, save at the upper part where an oblique 

 aperture, admitting a goose-quill, remained. In a young Walrus, 2 

 the entire margin of the valve was adherent, and there was no 

 intercommunication between the right and left sides of the heart. 

 A broad crescentic fold, looking downward, divides the sinus, or 

 fossa, receiving the precaval vein from the larger and deeper one 

 receiving the postcaval one : this fold answers to the upper border 

 of the ' fossa ovalis ' in the human heart ; there is no orifice in the 

 6 fossa ' communicating with the left auricle. There is a small 

 semilunar valve at the coronary orifice, but no Eustachian valve. 

 The appendix of the auricle, in Trichechus, extends in front of the 

 base of the aorta as far as the pulmonary artery, gradually con- 

 tracting to an obtuse point : in Cystophora proboscidea the 

 auricular appendix is short, broad, and bifid ; in both it is occu- 

 pied by a reticular arrangement of carneas columnar. The 

 ventricles are broader in proportion to their length, and the apex 

 is not produced in Trichechus, as in Cystophora proboscidea : . the 

 tendinous cords of the anterior division of the tricuspid valve, and 

 a few of those of the right or external division, are attached to a 

 short and thick fleshy column from the free wall of the ventricle ; 

 this column is connected by a short and thick ( trabecula ' with 

 the septum : most of the other tendinous cords are attached to the 

 septum, and a few to trabecule connecting that fixed wall with 

 the free wall of the ventricle. The pulmonary artery presents no 

 peculiarity ; it is connected by the ligamentous remnant of the 

 6 ductus arteriosus,' which is 10 lines long and 5 lines in diameter, 

 to the under part of the aortic arch, just beyond the origin of 

 the left subclavian ; its cavity is obliterated, but a short, thick, 

 semilunar fold of the lining membrane of the aorta, with its 

 concavity turned toward the end of the arch, indicates the place 

 of the former foetal communicating channel. 



1 clvi". p. 152. 2 cxci". p. 104. 



