42 BONES OF LEG AND FOOT. 



(b.) Now a bird's legs are not like ours, separate from the body from the 

 hip downward, l)ut are for a variable distance inclosed within the general 

 skin of the body. The freedom is greatest among the higher birds, and es- 

 pecially rapacious birds, that use the feet for grasping, and least in the low- 

 est swimming birds : the entire range of enclosure of the leg, is from part 

 way up the thigh down almost to the very point C, as in the case of the loon 

 and other divino- swimmers. And in no birds, is the knee, B, seen outside 

 the general contour of iXio, plumage ; it must be looked or felt for among the 

 feathers, and in most prepared skins will not be found at all. Practically, 

 it is a landmark of no consequence in determining genera and species, 

 though of the utmost importance in primary classification ; the student may 

 for awhile ignore its existence if he chooses. The iirst joint that sticks out 

 from the plumage is the heel, c ; and this is what, in loose popular terms, 

 is called "knee," upon the same erroneous notion that the wrist of a horse's 

 foreleg is called " knee." Just so people call a bird's crus the "thigh," and 

 disregard the thio-h alto2;ether. There is no need of this confusion ; and 

 even without the slightest anatomical knowledge, anyone can tell knee from 

 heel' at a glance, whatever their position relative to the Ijody ; for knees 

 ALWAYS bend fonoard, and heels always bend backivard. 



(c.) This point c corresponds to the point c in tig. 6 of the wing. There 

 we found two little carpal bones, or wrist-bones, intervening between fore- 

 arm and hand, or metacarpus ; but adult birds have no such actual bones in- 

 tervening between tibia and the next bone, d, the metatarsus. So there is 

 no tarsus proper; metatarsus hinges directly upon tibia, or foot upon leg, 

 without true ankle-bones ; that is, the foot-bone itself makes the ankle-joint, 

 with the leg, at the point c, heel. (Theoretically, however, there are tarsal 

 bones : for there is an epiphysis (§ 5Q*) at the lower end of the tibia, and 

 an epiphysis at the upper end of the metatarsal bone ; afterwards fused with 

 these bones respectively. One or the other, or both of these are held by 

 different anatomists to be tarsal bones ; more particularly, the one that fuses 

 with the metatarsus ; which last, therefore, represents both tarsus and meta- 

 tarsus, and is on this account called fxtrso-metatarsits.^) 



*This is as usually taught. But Gegenbaur has sho-wn that these so-called epiphyses are ti-ue tarsal 

 bones. He represents, in the chick at the ninth day of embryonic life, two hones, an upper and an under, the 

 former afterward anchylosing with the tibia, the latter with the metatarsus, leaving' the anlcle-joint between 

 them, as in reptiles. Morse, who has studied the embryos of several species, goes still further : he shows that 

 the upper tarsal bone of Gegenbaur is really two bones, corresponding to the tibiale and iibulare. or astrag- 

 alus and calcaneum; these subsequently co-ossify to form the upper one seen by Gegenbaur, and finally 

 co-ossify with the tibia to form the bitrochlear condyle characteristic of this bone in Aves. The distal tarsal 

 ossicle he believes to be the ceutrale of reptiles. Wyman discovers that the so-called process of the astraga- 

 lus has a distinct ossification, and Morse mteii^rets it as the intermedium, (^m. iVai. v, 1871, 524.) In the 

 light of these late discoveries, the homologies of the bird's carpus and metacai-pus become clearer. We have 

 seen (§ 55, 56, fig. C) that birds retain throughout life two distinct proximal carpal bones (called scapholunar 

 and cuneifoiTn, but better named simply radiale and ulnare), and that in early life they have a distal bone, 

 tliat was mentioned as the magnum, but appears to be centrale, corresponding to the distal tarsal ossicle, 

 just as the ulnare and radiale do to the proximal tarsal ossicles. Morse has even found in the carpus of 

 birds, two more ossicles, the homology of which remains undetermined. But what we now kno\v, renders it 

 almost certain, that the eo-called epiphyses upon the proximal ends of the metacarpals, are not epiphyses, 

 anymore than the so-called tarsal epiphyses; and that the metacarpus of birds is really carpo-metacarpue, 

 just as the metatarsus is actually tarso-metatarsus. This view is strengthened by the fact that the metacarpal 

 bones of higher vertebrates, except the first, ordinarily lack epiphyses. 



