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 nmscular 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. Owen ' and by Dr. Martin - 

 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. Jn 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 jiarietes of 

 the thorax, separating the thoraco-abdominal space into a respiratory 

 and a cardio-abdoniinal 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 sav 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 obhque 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 Pvffin 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. 



