EXTERIOR PARTS OF BIRDS 



191 



naiatoruil — in fact, so well distinguished are they, that carinate birds 

 have even been primarily divided into groups corresponding to these 

 three evidences of physiological adaptation of the structure of the 

 Avian pes. Independently of the number and position of the digits, 

 the plans are pretty well indicated by the method of union of the 

 toes, or their entire lack of union. 1. The insessorlal type, (a) In 

 order to make a foot the most of a hand, that is, to fit it best for 

 that grasping function which the perching of birds upon trees and 

 bushes requires, it is requisite that the digits should be as free and 

 movable as possible, and that the hind one should be perfectly 

 apposable to the others. Compare the human hand, for example, 

 with the foot, and observe the perfection secured by the entire 

 freedom of the fingers, and especially the appositeness of the thumb. 

 In the most accomplished insessorial foot, the front toes are cleft to 

 the base, or only coherent to a very slight extent ; the hind toe is 

 completely incumbent, and as 

 long and flexible as the rest. The 

 thrushes (Turdidcv) probably show 

 as complete cleavage as is ever 

 seen, practically as much as that 

 of the human fingers ; the cleft 

 between the inner and middle toe 

 being to the very base, while the 

 outer is only joined to the 

 middle for about the length of 

 its own basal joint. This is 

 the typical passerine foot (Figs. 

 3G, 37, 42, 43). There may 

 be somewhat more cohesion 

 of the toes at base, as in the 

 wrens, titmice, creepers, etc., without, however, obscuring the true 

 passerine character. Besides the typical passerine, there are several 

 other modifications of the insessorial foot, [h) Thus a kingfisher 

 shows what is called a syndactyl or syngnesious (Gr. o-vv, sun, together ; 

 yv7^o-tos, gnesios, relating to way of birth ; Fig. 44) foot where the outer 

 and middle toes cohere for most of their extent and have a broad sole 

 in common. It is a degradation of the insessorial foot, and not a 

 common one either ; seen in those perching birds which scarcely use 

 their feet for progression, but simply for sitting motionless, (c) The 

 zygodactyl or yoke -toed modification has been sufficiently noted 

 (Fig. 45). It was formerly made much of, as a scansorial or climhing 

 type of foot, and an absurd " order " of birds has been called Scan- 

 sores. But many of the zygodactyl birds do not climb, as the 

 cuckoos ; while the most nimble and adroit of climbers, such as 

 nuthatches and creepers, retain a typically i^asserine foot. The 



Figs. 42, 43. — Typical passerine feet. (The 

 right-hand fig. is Plectrophayies lapponicus, nat. 

 size.) 



