368 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



half of barbules, and fifty-four millions of cilia and 

 booklets. These feathers are not irregularly arranged, 

 but are set along definite tracts (feather tracts) the 



arrangement of which 

 (pterylosis) varies in 

 various birds, and has, 

 since the time of Nitzsch, 

 been made use of in 

 classification (Fig. 156). 

 The function of 

 feathers is not limited 

 to the diminution of 

 the specific gravity of 

 the bird, which they 

 effect by entangling air ; 

 the same process is also 

 of aid in preserving the 

 high temperature of 

 these creatures, in con- 

 sequence of the feeble 

 conductive power of air. 

 So far as the former 

 effect is concerned, we 

 have to note that the 

 Ratite birds, which 

 never soar into the air, 

 are without the barbules 

 by means of which the 

 barbs form a connected 

 vane. 



The hairs of Mam- 

 mals, like the feathers 



of birds, are epidermic in origin, but their mode of 

 development is somewhat different. As a general 

 account of the structure of hair has already 

 been given in chap, xxxiv. of Klein's " Elements of 

 Histology," it is here only necessary to give some 



Fig. 156. Pterylosis, or arrangement 

 of Feather-tracts on the under 

 surface of the body of a Cock 

 (Gallus bmikiva). (After Nitzsch.) 



