50 STRUCTURE AND CLASSIFICATION OF BIRDS 



the bird's heart and that of the crocodile it appears that this 

 is not the case. If the hearts of the two animals be 

 laid side by side in a corresponding position, it will be 

 seen that the crocodile's heart valve is furnished with a 

 muscle which seems comparable to that lettered a in the 

 bird's heart. And, furthermore, on the septal side of this 

 muscle the fibres which in the bird constitute that half 

 of the valve have a direction which is quite different from 

 that of the fibres in the other and larger half. Finally, 

 while the larger half of the valve is never, so far as is 

 known, fibrous in character, the lesser half occasionally 

 appears to be so wholly or partially. Thus there are some 

 grounds for thinking that the bird's right auriculo-ventricular 

 valve is composed of a complete outer half and of a smaller 

 septal half, presenting, therefore, less difference in this one 

 particular from the monotrematous than from the crocodilian 

 heart. 1 



We can, therefore, derive the bird's heart as regards this 

 valve from a heart like that of the crocodile, in which the 

 septal flap has for the most part disappeared. But in one 

 bird at any rate there appear to be traces of a still further 

 retention of the septal half of the valve. GEGENBAUK, who 

 some years since wrote an exhaustive paper upon the verte- 

 brate heart, 2 made the following remarks about the heart of 

 the condor, which in translation run as follows :- 



' Only in the heart of Sarcorhamphus do I find a peculi- 

 arity which has interest in this connection. From the 

 anterior origin of the muscular valve on the septum ventricu- 

 lorum a fold runs backwards, which is formed by a thicken- 

 ing of the endocardium. The fold runs obliquely backwards 

 and downwards, and crosses in its direction the margin of 

 the muscular va]ve. The course of this fold corresponds to 

 the line of origin of the membranous valvular flap of the 

 crocodile ; I think it reasonable, therefore, to regard it as a 



1 See for a fuller account BEDDARD and MITCHELL, ' On the Alligator's Heart,' 

 P. Z. S. 1895, p. 342. 



- ' Zur vergleichenden Anatomie des Herzens,' Jen. Zeitschr. I860 ; ' Notes 

 on the Anatomy of the Condor,' P. Z. S. 1890, p. 142. 



