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[393] IN 18711 I gave a brief description of a structure which I had observed.in the dog, in the following terms:
"In the myology of the dog, the insertion of the tendon of the external oblique muscle of the abdomen presents some interesting peculiarities. The outer and posterior fibres of this muscle end in a fascia, which is partly continued over the thigh as fascia lata, and partly forms an arch (Poupart's ligament) over the femoral vessels; by its inner end it is inserted into the outer side of a triangular fibro-cartilage, the broad base of which is attached to the anterior margin of the pubis, between its spine and the symphysis, while its apex lies in the abdominal parietes. The internal tendon of the external oblique unites with the tendon of the internal oblique to form the inner pillar of the abdominal ring, and is inserted into the inner side of the triangular fibro-cartilage. The pectineus is attached to the ventral face of the cartilage; the outer part of the tendon of the rectus into its dorsal face; but the chief part of that tendon is inserted into the pubis behind it. This fibro-cartilage appears to represent the marsupial bone, or cartilage, of the Monotremes and Marsupials."
The only reference to this statement which I have met with is by Professor Macalister, in his "Introduction to the Systematic Zoology and the Morphology of Vertebrate Animals" (1878), p. 265 :
"Professor Huxley describes a fibro-cartilaginous 'marsupial' above the pubis, from whose anterior surface the pectineus arises. I have failed to satisfy myself of its existence as a constant structure in many dogs, in the common and Bengal foxes, in the. dingo, jackal, Canispallipes, and wolf."
The wording of this passage does not make it quite clear whether [394] the writer has not found the structure in any case, but does not mean to deny that it may occur occasionally in the various Canido he mentions; or whether he has found it occasionally, but not constantly, in all or some of them.
Under these circumstances, it may be desirable to publish the fact that, having recently dissected, for purposes of comparison, a male and female fox and a male and female dog, I have not had the slightest difficulty in demonstrating the existence of the structure which I described in 1871, in all four. And the only phrase which appears to require modification in that description is the use of the term fibro-cartilage. I do not remember whether, formerly, I submitted the structure to microscopic examination or not; but in the specimens lately examined, notwithstanding the firmness and density of the triangular plate, it contains no true cartilage cells, but is entirely composed of fibrous bands which lie parallel with one another in the middle of the plate, while, at the thickened edges, they become closely interwoven.
A comparison of this triangular fibrous plate in the fox, with the "marsupial" bones of Phalangista vulpina, shows that the fibrous plate in the former animal exactly answers to the basal part of the "marsupial" bone in the latter. It may properly, therefore, be termed the epipubis ligament, and must be regarded as a structure of the same order as the rudimentary clavicle and the rudimentary hallux of the Canidæ; that is to say, as the remains of an organ which was fully developed in the ancestral forms of that group.
It is interesting to remark, in connexion with this interpretation of the facts, that, in the existing Thylacinus, which presents so many curious points of resemblance to the dogs, the epipubis is not ossified. As, however, the Canido have certainly existed since the Eocene epoch, there is no likelihood of the existence of any direct genetic connexion between the dogs and the Thylacines. The existing carnivorous Marsupialia have evidently all proceeded from ancestral forms, characterised by the possession of a thumb-like hallux, a peculiarity which is presented neither by the dogs, when they possess a hallux, nor by any other Carnivora with pentadactyle hind feet. Moreover, the early birth of the young and the development of a marsupium in the female, are evidences of the departure of the existing Marsupialia from the direct line by which the Mammalia have advanced from the ornithodelphous type. That the ancestors of all mammals possessed bony or cartilaginous epipubes is, I think, highly probable, but it does not follow that they had the marsupial method of bearing and nourishing their young.
1 "Manual of the Anatomy of Vertebrated Animals," p. 417.
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