154 ELEMENTS OE OENITHOLO&T. 



whicli answers to our fore-arm, these being united by a joint like 

 our elbow. These portions of the arm are further held together 

 by a very elastic and extensible fold of sMn. which is of course 

 widest opposite the elbow-joint. Another fold extends from the 

 trunk to the upper arm ; each of these folds is sometimes called 

 a patagium. Thirdly, there is a part which answers to our 

 hand, and which is termed the pinion. Within this "hand"— 

 or manus — there are, as we shall see when we study the skele- 

 ton, parts which represent, more or less imperfectly, our first 

 and second (index and middle) fingers, but they are all enclosed 

 in one solid structure invested by skin, so that no finger can 

 be externally recognized as free and separate like our own. 

 There is, however, a short and small thumb — or pollex — which 

 shows itself externally as a separate part. Thus the thumb 

 and fingers (or digits) of the hand are • enclosed as ours are 

 when we have put on a pair of boxing-gloves. The thumb is 

 a separate and distinct part of the pinion, but the other digits 

 are enclosed, together with what answers to the middle part of 

 our hand, in one continuous pad. . 



The relative proportions of the three solid parts of the wing 

 differ in different birds. Thus the upper arm is relatively, as 

 well as absolutely, very long in the Albatross ; while in the 

 Humming-birds it is relatively, as well as absolutely, very 

 short. The fore-arm is longer than usual in the Apteryx, while 

 in the Humming-birds and Swifts the pinion is relatively very 

 much elongated. 



The feathers which clothe the wing belong to three divisions, 

 two of which correspond to those of the two divisions of the tail- 

 feathers. 



There are, first, the effective feathers of fiight, which are 

 called remiges or rowers ; and there are, secondly, the covering 

 feathers or teotrices, which invest the bases of the effective 

 feathers of the wing, as the covering feathers of the tail invest 

 the bases of the effective feathers of -that part — the " rectrices." 

 The third division consists of a small group of feathers which 

 are implanted on the thumb. 



Thumb-feathers. — The feathers of the pollex are a small 

 group which constitute what is called the Alula (" little 

 wing ") or the Ala spuria (bastard wing). This little group of 

 feathers is placed on the front and outer side of the pinion, and 

 lies on the dorsum of the wing along its front or outer 

 border. 



