xvi. 3 FEATHERS 435 



tuft of barbs or an extra shaft, the aftershaft, perhaps in some way 

 representing the down-feather. 



The barbs or rami that make up the vane are held together by rows 

 of barbules (radii) running nearly at right angles to the barbs and 

 carrying hooks (hamuli) by which the barbules of one radius become 

 fixed to grooves in those of the next (Fig. 250). Anyone who has played 

 with a feather knows that these connexions can be broken down so that 

 the barbs become separate, but can be joined again by 'preening' the 

 whole feather. 



The feathers are provided with muscles at the base and the control 

 of their position is important for the regulation of heat loss, for flight, 

 and in many other activities, for instance sexual display. Like the hairs 

 of mammals the feathers are also used as organs for the sensation of 

 touch, nerve-fibres being wound round the base of the papilla. In owls 

 and other night-birds special vibrissae, analogous to those of mam- 

 mals, are present. Various specialized feathers are used for eyelashes, 

 ornament, and other purposes, and in some birds patches of special 

 feathers without rhachis break up to make a greasy 'powder down'. 



The feathers are not spread uniformly over the body but are localized 

 to certain tracts, the pterylae, separated by bare areas, apteria. Among 

 the contour feathers it is usual to recognize the remiges of the wing 

 and rectrices of the 'tail'. The former are divided into primaries on 

 the hand and secondaries on the forearm (Fig. 256). Each large feather, 

 whether in wing or tail, is usually covered above and below by several 

 rows of upper and under coverts. In many birds there is a peculiar gap 

 in the secondary feathers of the wing, the fifth remex feather being 

 absent (diastataxis); the condition in which this feather is present is 

 called eutaxis. The feathers have a remarkably flexible structure, so 

 that they adopt different shapes with different positions of the wing. 

 The shape of the quill and barbs varies between feathers and parts of 

 a feather, for instance the barbs at the tip of the primary feathers 

 provide a stream-lined cross-section, like that of certain aeroplane 

 propellers (p. 453). The small covert feathers at the front of the wing 

 stand up vertically, but have a right-angle bend, thus providing the 

 wing camber. 



The rectrices vary greatly, being almost absent in birds that live 

 near to the ground, such as the wrens, but very large in fast-moving 

 birds that change direction quickly (swallows). In these latter the outer 

 rectrices are enlarged for steering purposes. The rectrices may be put 

 to special uses, as in the woodpeckers, where they make a rigid brace, 

 or in the peacocks, whose display feathers are the tail coverts. 



