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Show 304 MR. F. E. BEDDARD ON THE [Nov. 3, the vein has a much shorter course within the pericardium, and leaves that cavity at its posterior corner on each side, instead of passing forward and running for some space within the pericardium at an angle with its former course. Hyrtl figures veins in the place of the gubernaculum which he describes as ramifying over the heart; but he describes no coronary artery. On the contrary, I find a very distinct coronary artery (see p. 308), but have seen no coronary veins. In the interior of the ventricle I could find no traces of a subdivision into two cavities. The demarcation between the pylangium and the synangium is quite visible externally, and plainly to be seen in the accompanying drawing (text-fig. 29, p. 303). The entire truncus arteriosus measures 43 m m. as against 34 mm., which is the extreme length of the ventricle. Of this 43 mm., 28 m m . belong to the pylangium. The pylangium is rather plainly divisible into three divisions, which are subequal. The first division, that nearest to the ventricle, seems on a dissection to belong partly to the ventricle and partly to the truncus arteriosus. Externally, however, as the drawing shows, it appears to be clearly a part of the truncus ; its smooth walls and sharp demarcation from the ventricle mark it out as a portion of the truncus. The synangium does not abut upon the anterior wall of the pericardium as in Menobranchus *. It divides while within the pericardium into right and left halves, which separately perforate the pericardium. The diameter of the synangium is very much greater than that of the pylangium, about twice as great, and it has an almost square outline. The auricle is attached to the ventricle for the greater part of the anterior left-hand border. It is naturally, on acccount of its thin walls, of somewhat irregular outline. It is a little difficult to compare the sizes of the auricle and the ventricle on account of their different shape ; but it appeared to m e that the auricle is slightly, but not very distinctly, the larger. Whether the auricle does during life I do not know, but it can lie entirely on the left hand of the ventricle owing to the large pericardium. The sinus venosus is spacious and opens into the auricle not far from the opening of the latter into the ventricle. It receives below and approximately in the middle line the vena cava inferior. On each side opens into the sinus venosus a transverse trunk which is, I presume, partly ductus Cuvieri and partly superior cardinal. Each transverse trunk runs along the whole of the transverse diameter of the pericardium and in contact with its posterior wall. The auricle is very distinctly divided into two cavities (text-fig. 30). The larger, which 1 presume to be the equivalent of the right auricle, lies nearest to the ventricle into which it opens. The sinus venosus opens into this, and its orifice is shielded on the side remote from the ventricle by a flap, which so far completely partitions the auricle, since it extends from roof to floor, and has * Huxlev, "On the Structme of the Skull and of the Heart of Menobranchus lateralis," P. Z. S. 1874, p. 186. |