EXTEBNAL PARTS OF BIBBS. — THE FEET. 129 



aud functional efficiency. In most Passeres it is virtually provided with a special muscle for 

 independent movement, so that it may be perfectly apposable to the other t(jes collectively, 

 just as our thumb may be brought against the tip of any finger. In general, it shortens as it 

 rises on the metatarsus ; and probably in no bird in which it is truly elevated is it as long as 

 the shortest anterior toe. It is short, barely touching the ground, in most wading birds ; 

 shorter still in some swimmers, as the gulls, where it is probably functionless ; it is incom- 

 plete in one genus of gulls (Bissa), where it bears no perfect claw ; it has only one phalanx 

 and is represented only by a short immovable claw in the petrels (Procella/riicke) ; it disappears 

 in the birds named in the last paragraph but two above, and in some others. It is never actu- 

 ally soldered with any other toe, for any noticeable distance ; but it is webbed to the base of the 

 inner toe in the loons (Colymhus), and to the whole length of the toe in all the Stegcmopodes 

 (fig. 52). It may also be independently webbed; that is, be provided with a separate flap or 

 lobe of free membrane. This lobation of the hallux is seen in all our sea-dueks and mergansers 

 {FuUguUnee and Mergmos), and in all the truly lobe-footed birds, as coots (Fuliea), grebes 

 {Podieipedida) and phalaropes (Phalaropodidai). The modes of union of the anterior toes 

 with one another may be finally considered under the head of the 



Three leading Modifications of the Avian Foot. — Birds' feet are modelled, on the 

 whole, upon one or another of three plans, furnishing as many types of structure ; which 

 types, though they run into one another, and each is variously modified, may readily be appre- 

 ciated. These plans are the perching or insessorial, the walking or wading, cursorial or 

 grallatorial, and the swimming or natatorial — in fact, so well distinguished are they, that 

 cariuate 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 insessorial type, (a) In order to make a foot 

 the most of a hand, that is, to fit it best for that grasping function whioli 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 perfect freedom of the fingers and especially the appositeness of the thumb. In the most 

 accomplished insessorial foot, the front toes are cleft to the hase, or only coherent to a very 

 slight extent; the hind toe is completely incumbent, and as long and flexible as the rest. Our 

 thrushes {Twrdidce') probably show as complete cleavage i&m fi 

 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. 36, 37, 

 42, 43). There may be somewhat more cohesion of 

 the toes at base, as in the wrens, titmice, creepers, 

 vireos, etc., without, however, obscuring the true pas- 

 serine character. As regards this matter, the point is, 

 that when the toes are united at all, it is by their actual 



cohesion there, not by movable webbing. Besides the „ ,„..„,, . ^ ^ 



" ° TJ2 . Figs. 42, 43. — Typical passerine feet. 



typical passerine, there are several other modifications (The right-hand fig. is Plectraphanes lappo- 

 of the insessorial foot. (V) Thus a kingfisher shows "««"«. "»*{ size.) 



what is called a syndactyle or syngnesious (Gr. miv, sun, together ; yvr)<nos, gnesios, relating to 

 way of birth) foot (fig. 44), 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 



n 



