736 



PHYLUM CHORDATA : CLASS AVES — BIRDS 



protective external sheath, and (b) an inner stratum Malpighii, which 

 becomes cornified and forms the whole feather. The process by which 

 this cylinder of cells becomes horny is remarkable ; in the upper part 

 ridges are formed, which separate from one another as a set of barbs, 

 the lower part remaining intact as the quill. When we pull off the 

 horny sheath of a young feather, we disclose a set of barbs lying almost 

 parallel with one another, yet slightly divergent. The centra! pair 

 predominate, and fuse to form the rachis ; their neighbours gradually 

 become the lateral barbs. The external sheath falls off ; the core 

 of dermis is wholly nutritive, and disappears as the feather ceases to 

 grow. 



ABB 



Fig. 437. — Parts of a feather. — After Nitzsch. 



/., Four barbs (B.) bearing anterior barbules (A.BB.) and posterior barbules 

 (P.BB.) ; //., six barbs {B.) in section, showing interlocking of barbules; 

 ///., anterior barbule with barbicels (H.). 



On the four toes and on the base of the legs there are horny epidermic 

 scales, the presence of which reminds us of the affinities between Birds 

 and Reptiles. The toes are always clawed. The thumb of Birds is often 

 clawed ; the second digit very rarely. Only in the embryo of the 

 ostrich (Struthio) is the third digit clawed. The beak is covered by 

 a horny sheath, which is annually moulted in the puffin. A moulting 

 of claws occiurs in the grouse. The dermis is thin and vascular, and 

 is rich in tactile nerve-endings or Pacinian corpuscles, especially 

 abundant in the cere. The only skin gland — the preen gland — secretes 

 an oily fluid, which some birds use in preening their feathers. Tt 

 is absent in the ostrich, emu, cassowary, and in a few Carinate 

 birds. 



