SCHl'LTK, SKI WHALK. 441 



Above its fibrous layer blends with the adventitia of the precava, aorta and pulmonary artery. 

 Below it is very broadly attached to the ventral slope of the diaphragm. The pericardium attains 

 its greatest breadth at about its middle and thence contracts somewhat towards the diaphragm. 

 The resulting groove at their junction is filled by a large subpleural fat pad, which is continuous 

 across the median line, but attains its greatest size at the sides. While this compensates in 

 large part for the contraction of the pericardium, it does not do it so completely but that the 

 lung presents a ridge as the boundary between its phrenic and mediastinal surfaces. 



Not only is the pericardiac-phrenic adhesion very extensive, but a further evidence of 

 crowding in this region is offered by the serous layer of the pericardium, in which the oblique 

 sinus has been obliterated. The reflection on the dorsal wall after passing from the right side 

 of the precava to that of the postcava sweeps ventrad to the latter, and there passes directly 

 to the lower left pulmonary vein, which is just to the left and rostrad of the postcava as it enters 

 the atrium. The line of reflection then ascends to the upper left pulmonary vein, there turning 

 obliquely to the right to reach the precava. In the area thus exposed are situated the pulmon- 

 ary veins of which there are three on the left, a small intermediate one entering the auricle inde- 

 pendently. To the right of the pulmonary veins is the large obliquely directed right pulmonary 

 artery, which grooves the dorsum of the right atrium between the caval veins. The pulmonary 

 artery and aorta are enclosed as usual in a common tube of serous pericardium. The circular 

 sinus, on account of their large size and the rostro-caudal compression of the heart, is of small 

 dimensions. 



Heart. The heart is markedly contracted in the diameter corresponding to the longi- 

 tudinal axis of the foetus and markedly broadened from side to side. Its greatest breadth is 

 31 mm.; from base to apex it measures 29 mm.; its base has a rostro-caudal height of 19 mm. 

 That this shape of the heart stands in relation to the shortening of the ventral wall of the thorax 

 and is one of the effects of adaptation of the body-form to aquatic life, has been demonstrated 

 by Miiller on the basis of unusually abundant comparative material. He has also found further 

 consequences of these factors in the extensive adhesion of the pericardium to the diaphragm 

 and the suppression of the lobation of the lungs. The long axis of the heart itself tends further 

 to approach more or less a dorso-ventral direction. In this foetus it not only deviates slightly 

 to the left the faint notch at the distal end of the interventricular furrow was situated about 

 3 mm. to the left of the midline but it has in addition a distinct inclination caudad, correspond- 

 ing to the slope of the ventral plane of the diaphragm. The heart itself has undergone but 

 the slightest degree of rotation upon its axis, and the areas occupied by the ventricles on its 

 phrenic and ventral surfaces are subequal. This may be expressed in terms of the right ventricle, 

 the ventral surface of which at its middle has a breadth of 10 mm., the phrenic 9 mm. Simi- 

 larly the phrenic surface of the left ventricle slightly exceeds the ventral. The right and left 

 borders of the heart are equally acute and almost symmetrical in position. By reason of the 

 great size of the pulmonary artery and the aorta, the interauricular notch is very wide and 

 deep, and the atrial septum reduced to little more than the narrow limbus of the fossa ovalis 

 and its valve. The postcava enters the right atrium far to the left in reference to the precava. 

 The latter is marked at its entrance ventrad by a deep veno-atrial angle, which forms the start- 

 ing point of the sulcus limitans. This can be followed only about two thirds of the way to 

 the postcava. Between the cavae the wall of the atrium is inflected by a deep oblique groove 

 which lodges the right pulmonary artery, occasioning an ental prominence approximately in the 



