148 STRUCTURE OF THE FOOT IN BIRDS. 



scarcely extends before tlie bfise of the middle one; the 

 canal perforating the outer intercondyloid space is 

 bounded below by two small bars passing from the mid- 

 dle to the outer condyle, and which bars define the groove 

 for the abductor muscle of the outer toe. 



The tarso-metatarse of the diver (colymhus) is remark- 

 ably modified by its extreme lateral compression. The 

 ento and ecto-calcanea are prominent, oblong, subqua- 

 drate plates, inclining towards each other, but not quite 

 circumscribing a wide intermediate space. The broad 

 outer and inner surfaces of the shaft are nearly flat; the 

 narrow fore and back surfaces are channelled; the ante- 

 rior groove leads to the wide canal, perforating obliquely 

 the shaft above the outer intercondyloid space, from 

 which a narrower canal conducts to that interspace. The 

 middle and outer trochlea© are nearly equally developed ; 

 the inner one stops short at the base of the middle one. 



The number of toes varies in different birds ; if the spur 

 of the cock be regarded as a rudimental toe (which is not, 

 however, my view of it), it may be held to have five toes, 

 while in the ostrich the toes are reduced to two. Birds, 

 moreover, are the only class of animals in which the 

 toes, whatever bp their number or relative size, always 

 difier from one another in the number of their joints or 

 phalanges, yet at the same time present a constancy in 

 that variation. 



The innermost or back toe, i (Fig. 24), answering, as I 

 believe, to the "hallux," or innermost digit of the pen- 

 tadactyle foot, has two phalanges ; the second toe, ii, has 

 three, the third toe,^Yz, four, and the fourth toe, iv, five 

 phalanges. I believe the toe answering to the fifth in 

 lizards and other pentadactyle animals to be wanting in 

 the bird's foot; and the spur, sometimes single, sometimes 



