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Show 704 MR. F. G. PARSONS ON T H E [June 16 middle layer consists of three double-headed flexores breves, innermost serving for the two small inner toes. The deep layer contains two abductor muscles, one to the fourth, the other to the fifth toe. The Vascular System. The Heart in Petrogale agrees in most respects with Owen's description of the organ in the Great Kangaroo1; I propose, however, to describe it a little more in detail. On opening the right ventricle the ventral wall is seen to be covered by columns: carn^ae of the second and third kinds; these are so broad and numerous that the wall presents quite a smooth appearance, the intervals between the columnae appearing as small rounded pits. There are no musculi papillares or chordae tendineae attached to this wall, nor is there any moderator baud. The right auriculo-ventricular valve has four cusps, one of which is much larger than the rest and lies against the septal wall, to which its free edge is connected by a large number of short chordae tendineae without any musculi papillares. There are only two (Owen says three) m^culi papillares in the right ventricle; they are attached to the septal wall, the larger about halfway down, the smaller dorsally and above. Each of these sends chordae tendineae to two cusps, and in this way the three anterior segments of the right auriculo-ventricular valve are stayed. In the left ventricle the auriculo-ventricular or mitral valve has only two cusps, one of which is close to the aortic opening, the other away from it. There are numerous chordae tendineae which spring from several large musculi papillares arranged in two groups. The aortic and pulmonary valves have the normal human arrangement, the corpora Arantii being well marked. The right auricle is elongated transversely, at its right extremity is the opening of the posterior vena cava; in tbe dorsal wall close to this and a little to the left is the opening of the left anterior vena cava. The right anterior vena cava opens on the upper part of the dorsal wall about the junction of the left third with the right two-thirds. At the left extremity of the auricle are the openings of two appendages, one of which projects ventral to the aorta, the other dorsal. Owen describes the right auricular appendage as being notched, but in Petrogale tbe notching is so deep that there are two distinct appendages. There are many and well-marked musculi pectinati in the right auricle, which are best developed on the ventral wall. The right auricle, as Owen points out, is remarkable for the absence of all traces of foetal structures; there are no signs of the Eustachian or Thebesian valves, nor is there any indication of the fossa or annul us ovalis. The left auricle has musculi pectinati only in the appendage, which is large and forms a great part of the cavity. Owen describes it as being notched, but this I failed to make out. The two pulmonary veins open close together into the dorsal wall. 1 'Anatomy of Vertebrates,' vol. iii. p. 517. |
