MUSCULAR SYSTEM. 721 



each is remarkably constant : thus, the outermost toe always consists of 

 five phalanges ; the fourth toe invariably of four ; the third as constantly 

 of three ; the second, when it exists, has only two ; and, lastly, in the 

 spur or innermost toe there is but a single piece. 



(2042.) So rapidly is the process of ossification accomplished in the 

 skeleton of a bird, that it is only in very young animals the individual 



Fig. 356. 



'36 



Skull of a young Ostrich. 



bones or elements composing the cranium can be identified, as the 

 sutures speedily become obliterated : when, however, they are examined 

 under very favourable circumstances, as for example in the skull of a 

 young Ostrich, it is by no means difficult to distinguish them, and, by 

 comparing them with those of other Vertebrata, to observe the modifi- 

 cations they have undergone both in form and position. In the annexed 

 figure the principal pieces, both of the cranium and face, have been in- 

 dicated by the same figures as were used to point out the correspondent 

 bones in the skulls of the Crocodile (fig. 332) and the Serpent (fig. 335) ; 

 so that it would be needless again to enumerate them in this place. 



(2043.) The muscular system of the feathered tribes, as far as ac- 

 tivity and energy of motion is concerned, contrasts strikingly with that 

 of the Vertebrata we have as yet considered ; for, with the exception of 

 Insects, no animals in creation are comparable to Birds, either in the 

 vigour or velocity of their movements. 



(2044.) This perfection of muscular power, which is obviously essen- 

 tial to enable the bird to sustain itself in the air, and there perform the 

 varied evolutions connected with flight, is no doubt mainly connected 

 with the highly arterialized condition of the blood and the complete- 

 ness of the respiratory apparatus. Neither is it uninteresting to observe 

 that, while respiration was effected in the Insect by the admission of air 

 to every part of the system by means of tracheal tubes, in Birds like-r 



3A 



