424 AVES. 



the whole class and is found even in members of the same family. 



Covering the bases of the remiges and filling up the gaps be- 

 tween them are smaller pennae called wing-coverts (tectrices). A 

 small number of pennae attached to the humerus are called 

 scapulars (parapterum), and some feathers attached to the 

 thumb constitute the bastard wing (alula). In some birds the 

 wings become so much reduced that the power of flight is almost 

 or quite, as in Ratitae, penguins, etc., lost. 



The great tail-quills are called rectrices (Rt), because during 

 flight they are used for steering. There are, as a rule, twelve 

 (sometimes ten or twenty and more) rectrices attached to the 

 last caudal vertebrae in such a way that they can be moved 

 singly, and unfolded laterally like a fan, as well as be all raised 

 or depressed together. The roots of the rectrices are covered by 

 a number of coverts, which in some cases attain an extraordinary 

 size and shape and constitute an ornament to the bird (pea- 

 cock). When the power of flight is absent the tail loses its sig- 

 nificance as a steering apparatus and the rectrices are reduced 

 or absent. In such cases, however, some of the coverts may 

 attain a considerable development as ornamental feathers. 



Birds have no sebaceous or sweat glands, but as mentioned 

 above there is an oil-gland on the rump. 



The hind limbs, which are principally used in movement 

 upon firm ground, present much diversity, according to the 

 mode of locomotion of the bird. In the first place walking feet 

 (pedes gradarii) and wading feet (pedes vadantes) are to be dis- 

 tinguished (Fig. 236). In the former the legs are much more 

 completely feathered, being covered at least as far as the tarsal 

 articulation ; but they vary considerably. As a rule four toes 

 are present (digit No. 5 being absent), and the first toe is directed 

 backwards, but the following varieties may be distinguished 

 (Fig. 236): All four toes are directed forwards (p. adhamantes), 

 e.g. Cypselus (a), sometimes the inner toe can be turned for- 

 wards and backwards (Colius) ; two toes directed forwards and 

 two (1 and 4) backwards (p. scansorii), Picus (b), sometimes the 

 outer toes of this type of foot can be turned both forwards and 

 backwards (Cuculus) ; three toes directed forwards and one 

 back, the anterior toes being free to their roots (p. fissi). Turdus 

 (d) ; three toes directed forwards, the inner toe backwards, the 

 middle and outer toes united at their roots (p. ambulatorii), 



