SCHULTE, SEI WHALE. 443 



tion on the left of the septum similar to that of the usual valve of placentals, from which it would 

 differ in its great length and in its extensively fenestrated margin, which would be attached 

 to the atrial wall by the numerous strands intervening between the fenestrse. The establish- 

 ment of subsequent adhesions would effect the definitive closure of the foramen. The peculi- 

 arity of the valvula here consists primarily in its ballooning into the left auricle, a condition 

 recorded by Rose l only in Terrapene, and it may be noted in passing that at the fundus the sep- 

 tum in this form was very thin in places and the separation of the atria was effected only by 

 endocardium. Though actual communications are not formed in reptiles, the condition is 

 certainly strikingly analogous to the funnel-like valve of the Cetacea with its fenestrated fundus. 

 A second peculiarity, though one shared by birds, monotremes and marsupials (Rose) is in the 

 site and multiplicity of the perforations of the septum primum. This maintains itself in its 

 upper segment where the foramen ovale secundum of placentals is established, which so comes 

 later to be covered on the right by the crescentic downgrowth of the septum secundum. Here 

 the perforations are small and multiple and occupy the middle of the septum primum. In 

 consequence, when the septum secundum develops on the right and comes to be amalgamated 

 with the primum, the latter has no free edge but is attached in its whole circumference per- 

 mitting communication between the atria by way of its fenestrated middle. In birds, mono- 

 tremes and marsupials the perforations are closed by proliferation of the endocardium, and 

 there is plain evidence of the embryonic process in the structure of the valve of the adult (Rose), 

 which is marked by a reticulum of ridges with intervening thinner areas. In the adult Cetacean 

 I have not found in the descriptions of the atrial septum a record of this type of structure. In 

 view of the adhesions ventrally at the base of the valve, I have ventured to suggest the mode of 

 closure outlined above. It is possible to see in the form of the septum primum provision for 

 its valvular function. The funnel, on account of its length, can be applied as a whole to the 

 wall of the left atrium, ventral to the region of the interatrial septum, and being here com- 

 pressed against the wall and its lumen obliterated, may in this way prevent regurgitation from 

 the left atrium to the right. Secondarily this position is favored and in part at least rendered 

 permanent by the adhesions between the base of the funnel and this region of the atrial wall. 

 There is of course a possibility that a circular arrangement of the muscle of the valve may 

 by a sphinter action assist its function. 



In other respects the heart conforms to the usual conditions and presents little that requires 

 comment. The left atrium receives two pulmonary veins from the right lung, three from the 

 left, the small additional vessel opening between the other two. The atrioventricular valves 

 have the usual arrangement. As Turner has pointed out the papillary muscles are numerous, 

 and are not very clearly separated into groups. In the right ventricle there is a large moderator 

 band. The conus arteriosus is capacious in contrast to the rather narrow aortic vestibule. 

 The same disparity obtains between the arch of the aorta and the pulmonary artery, the latter 

 having a wide circular lumen, while that of the former is reduced to a narrow slit. The inter- 

 ventricular foramen is closed. 



Thymus. The two lobes of the thymus rest upon the pericardium and the arch of the 

 aorta immediately caudad of the left brachiocephalic vein. They are subequal, disk-like, the 

 right nearly circular, the left oval with its long axis transverse. Their ventral surfaces are 



1 Rose, C. Beitrage zur vergleichenden Anatomie des Hertzens der Wirbelthiere. Morph. Jahrb., Bd. XVI, 1890. 



