FEATHERS. 239 



(rami) send off barbules (radii) arranged in two ro\vs, and the bar- 

 bules (at any rate of the front row) bear barbicels and booklets, 

 which by their mutual interlocking effect the firm connection of the 

 whole vexittum. 



According to the nature of the stem and barbs, the following kinds 

 of feather can be distinguished contour feathers (pennce), with stiff 

 shaft and firm vexittum ; down feathers (plumce), with soft shaft and 

 vane, the barbs of which bear round, or knotty barbules without 

 hooklets ; and finally filoplumes (jUopluma), with slender bristle-like 

 shaft, the vane of which is reduced or absent. The pennce determine 

 the external outline of the plumage, and attain their greatest size as 

 remiges in the wings and as rectrices in the tail. The plumce form 

 the deep layer of the plumage, and are covered by the contour 

 feathers ; they serve for the retention of warmth. The filoplumes, 

 on the other hand, are distributed more among the pennce, and at 

 the angle of the mouth have the appearance of stiff bristles (vibrissce). 

 There are, moreover, many forms of feathers intermediate between 

 these principal forms. In the autumn there is a complete change of 

 feathers (autumnal moult), whereas the spring moult by which the 

 bird acquires its breeding plumage is only rarely connected with a 

 complete new formation of the plumage. As a rule, the spring 

 moult consists in a colouring of the feathers (probably by chemical 

 change in the pigments already present), and sometimes in a mechani- 

 cal breaking off of certain parts of the feathers. 



Birds have no sebaceous or sweat-glands, but there is often a 

 bilobed gland above the last caudal vertebra. This gland (the 

 uropygial gland, or oil-gland) has a simple duct, and its oily secretion 

 serves to anoint the feathers. 



The plumage is only rarely distributed evenly over the whole of 

 the body (Aptenodytes). Usually the contour feathers are arranged 

 in rows the so-called pterylce between which there are spaces the 

 apteria which are naked or only covered with down (fig. 651). The 

 form and distribution of these spaces present modifications which 

 can be used in classification. 



The grouping of the feathers on the anterior limbs and on the 

 tail determines the utility of these organs as wings and steering 

 apparatus respectively. The wing has to a certain extent the 

 form of a fan, which can be folded at two points viz., the elbow 

 joint and the carpal joint; its surface is formed by the large 

 remiges on the under surface of the hand and forearm, but partly 

 also by special folds of the skin, which stretch between the body 



