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Show 1888.] VISCERAL ANATOMY OF BIRDS. 255 It is possible then, as it appears to me, that the feebly developed muscular layer which extends for a short way over the dorsal attachment of the oblique septum in the Duck and in the Toucan may be the degenerate rudiment of the powerful muscle which extends over so large a portion of the oblique septum in the Penguin and in the Puffin. There is nothing, moreover, in the facts, so far as they have been stated, to disprove the truth of the converse to the above, viz. that the powerfully developed muscular layer of the Penguin and the Puffin is a further development of the feeble musculature of the oblique septum in the Duck. To decide which of these two alternatives is the more probable, it is necessary to go into the question of the nature and homologies of the muscular layer in question. I have at present been unable to discover any bird in which the oblique septum showed characters which would serve to throw any light upon the question. No doubt the structure of the viscera of the extinct Dinosauria would solve the problem at once ; but, failing these, it is clear that the Crocodilia more than any existing group of Reptiles approach birds in their structure. Prof. Huxley has in his paper, so frequently referred to, indicated many striking resemblances between the respiratory organs of Birds and those of Crocodiles. It had already been noted by Sir R. Owen1 and by Dr. Martin2 that the abdominal cavity of Crocodiles is remarkable for the great development of special serous sacs enveloping the various viscera, its cavity being thus greatly subdivided. In this arrangement there is a very close similarity to Birds, as Prof. Huxley pointed out. " A fibrous expansion extends from the vertebral column over the anterior face of the stomach, the liver, and the dorsal and front aspect of the pericardium, to the sternum and the parietes of the thorax, separating the thoraco-abdominal space into a respiratory and a cardio-abdominal cavity, and representing the oblique septum of the bird" (Huxley, loc. cit. p. 568). This supposed homologue of the oblique septum in the Crocodile is not, however, simply made up of a layer of fibrous tissue ; Prof. Huxley goes on to say in the same paper and on the same page :-" A broad, thin muscle arises, on each side, from the anterior margin of the pubis; and its fibres pass forwards, diverging as they go, to be inserted into the ventral face of the posterior part of the pericardium and into the ventral and lateral parts of the fibrous capsule of the stomach, passing between that organ and the adherent posterior face of the liver, and being inserted into the fibrous aponeurosis which covers the anterior surface of the stomach, and represents the oblique septum." If the homologies instituted by Prof. Huxley be allowed, then this muscle is clearly the equivalent of the muscle which I have described in this paper in the Puffin and which M . Filhol has described in the Penguin ; in every case the muscle arises from the pubis and extends as far as the region of the stomach ; in the two birds, 1 P. Z. S. 1831, p. 139. 2 P. Z. S. 1835, p. 129. |
