4 JUr. fans on tne fnysiology of the Egg. [u u o*, 



tracted. The chick, therefore, of hens and partridges has a 

 more perfect plumage, and a greater aptitude to locomotion, than 

 the callow nestlings of crows and sparrows. 



Such an instance of the -agency of oxygenation in the promo- 

 tion and increase of muscular power is not solitary ; the history 

 of ruminating animals will furnish a parallel example. " Their 

 cotyledons," says the ingenious author of the Zoonomia, " seem 

 to be designed for the purpose of expanding a greater surface 

 for the termination of the placental vessels, in order to receive 

 oxygenation from the uterine ones : thus the progeny of this 

 class of animals are more completely formed before their nativity 

 than that of the carnivorous classes; calves, therefore, and lambs, 

 can walk about in a few minutes after their birth ; while kittens 

 and puppies remain many days without opening their eyes." In 

 confirmation of the theory, that muscular irritability is the result 

 of a nice combination of oxygen with the animal organs, many 

 interesting facts may be adduced. Do we not find that the 

 muscular strength of an animal is (ceteris paribus) proportional 

 to the extent * and perfection of its respiration ? Birds are 

 enabled to sustain the exertion of flight, owing to their exten- 

 sive pneumatic receptacle ; and many insects, especially the 

 different species of Scarabecus, in the act of flying, disclose 

 avenues of air, which in their quiet state are closed by the cases 

 of their wings, thus procuring for themselves a larger supply of 

 the principle of muscular energy at a period when from their 

 exertion, and consequent exhaustion, they most require it : flat 

 fish, who having no swimming bladder, remain at the bottom, 

 and possess but little velocity, have gills that are quite con- 

 cealed, while those who encounter a rude and boisterous 

 stream, as trout, perch, or salmon, have them widely expanded ; 

 and, with respect to the respiration of fishes, it may be further 

 observed, that the sum of oxygen winch they receive will van/ 

 jointly as the momentum of the ivater which imparts it, a?td the 

 extent of the gills. An acquaintance with this truth at once 

 enables us to discover one of the most beautiful final causes in 

 nature. We shall no longer consider the rapid current or the 

 boisterous ocean as inimical to the strength of the animal, but 

 regard them as the powerful causes of its invigoration — " Ele- 

 menta propriis armis devicta." So also the velocity of fishes, and 

 its unwearying duration, will cease to astonish us, since it is 

 evident that such motions contribute as well to the revival as to 

 the exhaustion of muscular energy ; for although they must 

 waste the stream of irritability, yet they add to the fountain by 

 which it is supplied. Hence it follows, that whenever it is an 

 object to economize the consumption of air, as must happen in 

 crowded and confined situations, we ought to preserve our mus- 



* Narrow-shouldered men bear labour worse and pain better than others. Thus the 

 natives of North America, a narrow-shouldered race of people, will rather expire under 

 the lash than be made to labour — (Darwin's Zoonom. vol. ii. p. 14.) 



