CH. XXIVJ STRUCTURE OF FEATHERS 607 



bone and in the structure of the hinder part of their skulls, of 

 their breast-bones and of their ankle-joints. As in Reptiles, the 

 number of neck vertebrae is variable. Like Reptiles, Birds have 

 nuclei in the red corpuscles of the blood, and the sole remaining 

 complete systemic arch goes to the right (Fig. 310), like the 

 principal arch in Reptiles. On the other hand, they are "warm- 

 blooded," that is to say, the temperature of the body remains 

 practically the same whether the surrounding air gets hot or cold ; 

 it is in fact higher than that of any mammal : the ventricle of the 

 heart is completely divided into two, and in addition to the wings 

 and feathers, the structure of the leg and hip-bones and of the 

 brain, distinguishes them from any living Reptile. 



Strange as the statement may appear, it is true, nevertheless, 

 that the feathers are really scales like those found in 

 Lizards, but immensely developed and with the edges 

 frayed out. Like' scales, they are epidermal, that is, developments 

 of the outer or horny layer of skin. The area which is about to form 

 the feather becomes raised into a little finger-shaped knob by the 

 growth of the deep layer of the skin or dermis which carries the 

 blood-vessels. The little knob thus formed is in turn sunk in a 

 pit called the follicle, the skin immediately surrounding it being 

 depressed. Thus the lowest part of the feather is a little hollow 

 tube of horny cells formed round the knob of dermis, but the 

 upper part, like the scale of a lizard, is formed only on one side of 

 the knob, and as this part is pushed away by the growth of the 

 deeper parts it becomes frayed out so as to form the vane of the 

 feather (Fig. 302). In the latter we can distinguish a central stem 

 or rachis, and two rows of lateral branches or barbs, which are 

 kept in position by a number of secondary processes or barbules. 

 The barbules bear little hooks which interlock with one another. 

 Down consists of small feathers growing between the bases of 

 the larger ones. In these the barbules are absent, so that the 

 barbs are not held together but float freely about, forming a kind 

 of fluff. When a bird is plucked it is seen that the feathers are 

 confined to certain tracts (pterylae) separated by others called 

 apteria devoid of feathers or covered only with down feathers. 

 Thus in most birds the mid-ventral and mid-dorsal lines are 

 apteria. The colour of the feathers is partly due to coloured 

 substances or pigments in the epidermal cells and partly to minute 

 structural detail which causes interference of the light waves re- 

 flected from them. 



