608 



AVES 



[CH. 



The wing is the fore-leg of the bird. One can easily recognise 

 the parts corresponding to upper arm, fore-arm and hand, but the 

 latter is highly modified and specialised for the important function 

 of carrying the long primaries or hand quills. When the wing 

 is at rest the upper arm extends backwards, the fore-arm is sharply 

 bent up on this, while the wrist is sharply bent down. When 

 the wing is expanded these are partially, but never entirely, 

 straightened out, so that a bird begins the down- stroke of the 



FIG. 302. Section through the skin of a Bird showing a developing feather. 

 Highly magnified. 



1. Epidermis. 2. Malpighian layer of the epidermis. 3. Dermis. 



4. Young feather. 5. Follicle round base of feather. 6. Dermal 



papilla which develops blood-vessels and is the organ of nutrition of 

 the feather. 



wing with the arm bent in a very similar way to that in which 

 a swimmer's arm is bent when he strikes back with it. In the 

 hand we find as a rule three digits, the first, second and third. 

 These have their first joints, the metacarpal bones, closely united 

 together. In Man the metacarpals of the various fingers are united 

 by skin and flesh which constitute the palm, but they are mov- 

 able on one another, whereas in the bird the metacarpals of the 



