GENERAL CHARACTERS OF BIRDS. 59 



and at the same time possessed of considerable resistance. The degree of 

 muscular power required for support and propulsion in the air, involves the 

 necessity of a very high amount of respiration ( 275), for which it has been 

 seen that an express provision exists in Insects; and as the general activity 

 of the vital processes depends greatly upon the high temperature, which this 

 energetic respiration keeps up, a provision is required for keeping in this heat, 

 and not allowing it to be carried away by the atmosphere, through which the 

 Bird is rapidly flying. 



35. The first and third of these objects, the lightening of the body, and 

 the extension of the respiratory surface, are beautifully fulfilled in a mode, 

 which will be found to correspond with the plan adopted for the same purpose 

 in Insects. The air which enters the body, is not restricted to a single pair 

 of air-sacs or lungs placed near the throat; but is transmitted from the true 

 lungs, to a series of large air-cells, disposed in the abdomen and in various 

 other parts of the body. Even the interior of the bones is made subservient 

 to the same purpose; being hollow, and lined with a delicate membrane, over 

 which the blood-vessels are minutely distributed. In this manner, the respi- 

 ratory surface is greatly extended; whilst, by the large quantity of air intro- 

 duced into the mass, its specific gravity is diminished. The subservience of 

 the cavities in the bones to the respiratory function, is curiously shown by 

 the fact, which has been ascertained both accidentally and by a designed ex- 

 periment, that, if the trachea of a Bird be tied, and an aperture be made in 

 one of the long bones, it will respire through this. 



36. The other two objects, the extension of the surface, and the retention 

 of the heat within the body, are also accomplished in combination, by a most 

 beautiful and refined contrivance, the covering of feathers. Like hair or 

 scales, feathers are to be regarded as appendages to thecutis; the stem is 

 formed from it by an apparatus, which may be likened to a hair-bulb on a 

 very large scale ; but there are some additional parts for the production of the 

 laminae, which form the vane of the feather, and which are joined to the stem 

 during its development. These laminae, when perfectly formed, are connected 

 by minute barbs at their edges, which hook into one another, and thus give 

 the necessary means of resistance to the air. The substance of which feathers 

 consists, is a very bad conductor of heat ; and when they are lying one over 

 the other, small quantities of air are included, which still further obstruct its 

 transmission by their non-conducting power. Thus the two chief objects are 

 fulfilled ; power of resistance and slow-conducting properties being obtained, 

 in combination with lightness and elasticity. At the two extremes of the 

 class, however, we meet with remarkable modifications in the typical structure 

 of feathers. In the Penguin, those which cover the surface of the wings 

 have a strong resemblance to scales ; and the wings are not employed to raise 

 this Bird in the air, but only to propel it through water (as fins would do) by 

 their action on the liquid. On the other hand, in the Ostrich tribe, the laminae 

 of the feathers are separated from each other, so as no longer to form a con- 

 tinuous surface ; the feathers more resembling branching hairs. Here the 

 wings are almost or completely absent; the birds of this tribe being constantly 

 upon the ground, propelling themselves by running, and approaching the 

 Mammalia in many points of their conformation. 



37. The bony frame-work of Birds presents many remarkable adaptations 

 to the same purposes. In the first place it is to be remarked, that the faculty 

 of locomotion is here entirely delegated to the extremities; and that the skele- 

 ton of the trunk must be consolidated, in proportion to the power with which 

 they are to be endowed, in order to afford their muscles a firm attachment 

 ( 22). Just as the segments of the external skeleton of the Articulata, 

 therefore, are consolidated in Insects, do we find that the vertebral column 



