434 Vertebrata. 



with the distal ends of the succeeding pennae, and in many Bii-ds the loose 

 down quill is split by the developing feather in such a way, that the soft down 

 harbs remain for some time at the tip of the distal barbs of the adult feather, 

 looking like a soft external portion (Fig. 359 B). The down is really to be 

 regarded as a modified distal extremity of the feather to which it is attached. 



The contour feathers are not usually distributed regularly over the 

 whole body, but only over certain regions, the so-called feather 

 tracts {pterylce), which are regularly, but somewhat differently, 

 arranged in different birds ; there is, for instance, a pteryla along the 

 mid-dorsal line, another on the outer side of the thigh, etc. The 

 intermediate tracts (apteria), bear down feathers, which are also 

 present in the feather tracts, between the pennee, or they are entirely 

 destitute of feathers ; the apteria are covered by the contour feathers 

 of neighbouring tracts. There is an almost equal distribution of 

 feathers over the whole body in the Ratitse (where there is no down 

 unless the entire plumage be regarded as down feathers), in the 

 Penguins and a few others. 



At regular intervals, usually once a year, all the feathers are 

 thrown off, and are simultaneously replaced by new ones ; this is 

 known as moulting; in northern Birds it usually takes place in 

 the course of a few weeks in the autumn. Besides this, a 

 spring moult often occurs ; many Birds indeed, change some 

 of their feathers in spring, so that there is a partial (rarely a complete) 

 ecdysis ; in many cases, however, the distinction between winter and 

 summer plumage is dependent upon this second moult. In some, the 

 specially coloured edges of many feathers are thrown off in the spring, 

 and the appearance of the plumage is thus considerably changed; 

 in others there is an actual change in the colour of the feathers them- 

 selves, sometimes to a striking extent. This change of colour is the 

 more remarkable since the feathers consist exclusively of horny 

 material, and thus represent a dead portion of the body ; the 

 alterations are probably to be attributed to chemical changes, 

 independent of the vital functions. 



The feather is surrounded, for some time after its tip has projected from the 

 feather-follicle, by a thin tubular horny sheath, which binds the barbs 

 together; this sheath is gradually thrown off. The so-called "powder down," 

 which is found on some Birds, e.g., the Herons, is composed of feathers which grow 

 continuously from the feather-follicle (just as the teeth of certain Mammalia gi'ow 

 from persistent pulps), whilst at the same time their free ends are worn off ; the 

 powder comes from the crumbling away of the homy sheath, which is constantly 

 renewed at the proximal end. 



Attached to the follicles of the larger feathers there are usually small 

 muscles, by which, for example, the rectrices can be spread out like a fan, the 

 ordinary pennse raised, etc. 



Feathers are probably to be regarded as modified reptilian scales ; 

 indications of this are afforded by their origin from papillae. Birds 

 also possess true scales, exactly like those of the Reptiles, but 

 occurring only on the hind feet ; these scales are of various forms : 



