48 STRUCTURE AND CLASSIFIGATION OF BIRDS 



Professor Huxley, we have already referred to the existence 

 in birds of what may be considered their homologue, and we 

 have only to add that in a hornbill the left-hand portion of the 

 horizontal septum was muscular — that, at any rate, a strong 

 band of muscle bound the gizzard to the left oblique septum. 



Circulatory System 



The bird's heart is ^ery uniform in structure ; there are 

 very few and but slight differences in any part of the heart 

 between the most and the least specialised forms. It is, 

 however, in certain particulars equally distinctive in structure, 

 and differs in a number of well-marked points from the heart 

 of either reptile or mammal. As might be expected, the 

 reptile which shows the nearest approximation in the anatomy 

 of its heart to the bird is the crocodile, while the Monotre- 

 mata are the mammals which on the other side occupy a 

 corresponding position. 



As with the mammalia the heart is completely separated 

 into four chambers ; in the bird the heart has perhaps more 

 of an elongated form than in the mammal, the apex (which, 

 as in the mammal, is formed by the left ventricle alone) being 

 rather more pointed than in the heart of any mammal. In 

 a transverse section through the ventricular walls a notable 

 difference in the relative dimensions of the right and left 

 ventricles for the two types is apparent. It will be noted 

 that in the bird the cavity of the right ventricle is, as it were, 

 partially wrapped round that of the left, and is in consequence 

 of a decidedly crescentic form. The cavity of the right 

 ventricle of the mammal's heart is more oval in form, and is 

 not vsrapped round that of the left. In this particular the 

 Monotremata stand midway between the bird and the higher 

 mammals. 



The interest of the structure of the bird's heart, however, 

 largely, for reasons of comparative anatomy, centres in that 

 of the valve which guards the orifice from the auricle. The 

 interior of that ventricle has fairly smooth walls, a sculp- 

 turing so conspicuous in the mamgaalian ventricle being 



