THE MUSEUM 



NATUEAL HISTORY. 



ZOOLOGY. 



CLASS II.-BIRDS. 



Although the numerous mid varied tribes of Birils 

 undoubtedly come next in order after tbe Mammalia, 

 we cannot point to any member of the latter class, 

 which, in its general characters, really makes an ap- 

 proach to the birds. Some of the lowest mammals 

 certainly present some resemblance to the oviparous 

 Vertebrata in a physiological point of view; but the 

 pecuHarities exhibited by these rather indicate a rela- 

 tionship to tlie class of reptiles, and thus, in our classi- 

 fications, the Birds form, as it were, a supplementary 

 class, interpolated between the two similarly-quadruped 

 groups of Mammalia and reptiles. 



This view is remarkably in accordance with geolo- 

 gical facts, as far as we can judge from the present state 

 of our knowledge ; the preponderating reptilian Fauna 

 of the secondary period gives place, in the gradual 

 evolution of organic nature, to the similarly prepon- 

 derating mammalian Fauna of the tertiary epoch, 

 whilst the traces of birds, such as they are, occur 

 simultaneously with these from a very early period of 

 time.* 



Birds, Iflie mammals, are warm-blooded, air-breathing, 

 vertebrate animals, and, Uke them, possess a heart com- 

 posed of four distinct cavities and voluminous minutely 

 cellular lungs. The latter organs, however, present 

 some differences from the corresponding parts in the 

 Mammalia ; they are not lobed, and, instead of being 

 freely suspended in the cavity of the chest, tliey are 

 attached to the inner surface of the dorsal part of this 

 cavity. Moreover, tire surface of the lung, instead of 

 forming a closed sac, as in the Mammalia, is perforated 

 by several large apertures, passing down through its 

 substance to the main branches of the air-tubes, and, 

 on the other hand, commimicating externally with an 

 extensive system of air-sacs, which penetrate to nearly 

 every part of the body of the bird, and even occupy the 

 internal cavities of those hollow bones, which, in the 

 Mammalia, and indeed in the young bird, are filled 

 with marrow. By this arrangement the air taken into 



* The foot-prints of birds occur in the new red sandstone, 

 the earliest of the secondary formations, both in Europe and 

 America. They are accompanied by similar traces of gigantic 

 Batrachiau reptiles. 



the lungs may, to a considerable extent, penetrate, as 

 it were, into the very substance of the bird's body, 

 a circumstance of no small importance in reducing 

 its specific gravity, and rendering it capable of being 

 readily supported in the air by tlie action of the wings. 

 So ready is the commimication between these air-sacs 

 and the lungs, that birds have even been known to 

 breathe through a fractured wing-bone, when the ordi- 

 nary air-passages have been closed by compression.* 



Another anatomical character by which birds are 

 distinguished from mammals, consists in tlie absence 

 of the diaphragm or muscular partition, which, in the 

 latter, separates the cavity of tlie chest from that of 

 the abdomen, and which, by its movements, assists 

 greatly in respiration ; tlie alternate enlargement and 

 diminution of the cavity of tlie chest by which this 

 function is executed La birds, is effected by the alter- 

 nate elevation and depression of the broad flat sternum. 



As tlie power of flight is tlie prmeipal general charac- 

 teristic of the whole class of birds, we naturally expect 

 to find tlie structure of the skeleton specially modified 

 for the accomphshment of this object; and so com- 

 pletely are these expectations fuMUled, that it is impos- 

 sible ever to mistake the skeleton of a bird for tliat of 

 any other form of animal (see Plate 3C). Nevertheless 

 the parts described as occurring in the skeleton of a 

 mammal, may invariably be traced chstinctly. The dis- 

 tinctions of head, neck, and trunk are always clearly 

 visible ; but the tail is very short, and tlie proportions 

 of the different regions of tlie body are usually quite 

 different from those which prevafl among the Mam- 

 malia. 



The skuU is generally of smaU size, and its cavity is 

 much smaller in proportion than in the Mammaha. 

 This indicates a smaller brain, and less general intelli- 



* Although it is by no means certain that the air per\-ading 

 the body of the bird by means of these air-sacs, is in any way 

 subservient to respiration, it is impossible not to recognize in 

 this provision tor the passage of air amongst the tissues of the 

 body an analogy with that arrangement of the respiratory 

 apparatus in insects, which, in like manner, serves to render 

 the bodies of those animals sufficiently light to enable them to 

 exercise the power of flight. In this, as in some other respects, 

 the analogy between birds and insects is unmistakable. 



