384 THE MECHANICAL FUNCTIONS. 



07iocrotalus, for instance, or white Pelican, which is five 

 feet in length, was found by the Parisian Academicians to 

 weigh only twenty-three ounces, while the entire bird 

 weighed nearly twenty-five pounds. The cavities in the 

 bones communicate with large air cells, which are distri- 

 buted in various parts of the body, and which contribute 

 still farther to diminish its specific gravity: and by means 

 of canals which open into the air passages of the lungs, this 

 air finds a ready outlet when it becomes rarefied by the as- 

 cent of the bird into the higher regions of the atmosphere.* 

 The conditions in which a bird is placed with regard to 

 the density of the surrounding medium, as well as their mode 

 of progression, are so opposite to those of fishes, that we 

 should expect to find great corresponding differences in 

 their conformation. These two classes of vertebrata, accord- 

 ingly, are remarkably contrasted with respect to the struc- 

 ture of their skeletons. In fishes we have seen that the chest 

 and all the viscera are carried as far forwards as possible; 

 the respiratory organs and the centre of circulation being 

 close to the head, the neck having disappeared, and the 

 trunk being continued into the lengthened tail, in which the 

 chief bulk of the muscles are situated. In birds, on the con- 



* This air, being" contained in the interior of the body, which preserves 

 a very elevated temperature, must be constantly in a state of greater rarefac- 

 tion than the cooler external air; a condition which must contribute in some 

 slight degree to render the whole body lighter than it would otherwise have 

 been. It appears to me, however, that considerably greater importance has 

 been attached to this circumstance than it really possesses. Many have gone 

 so far as to represent the condition of a bird as approaching to that of a bal- 

 loon filled with a lighter gas than atmospheric air: and have been lavish in 

 their expressions of admiration at the beauty of the contrivance which thus 

 converted a living structure into an aerostatic machine. A little sober con- 

 sideration will suffice to show that the amount of the supposed advantages 

 resulting to the bird from the diminution of weight, occasioned by the dif- 

 ference of temperature between the air included in its body and the exter- 

 nal atmosphere, is perfectly insignificant. Any one who will take the trou- 

 ble to calculate the real diminution of weight arising from this cause, under 

 the most favourable circumstances, will find that, even in the case of the 

 largest bird, it can never amount to more than a few grains. 



