34 MORPHOLOGY OP THE VERTEBBATA. 



THE TIBIA 



Is the chief and longest bone, of the hind limb. The shaft is 

 straight and expanded at both extremities, being most so at the 

 proximal end. On the outer side of the upper end is a surface 

 for union by ligament with the head of fibula. At the lower 

 third of the shaft, the styliform end of the fibula is anchylosed to 

 it. To its lower end the oscalcis and calcaneum— -the two largest 

 bones of the tarsus — are firmly and indistinguishably united. 



THE FIBULA 



Is a short styliform bone, about two-thirds the length of the tibia, 

 free at its upper end, but ancylosed to the lower one-third of the 

 tibia. The upper end of the fibula articulates with the outer 

 condyle of the femur : this is an important character in all birds. 



TARSUS. 



The proximal portion of this, a3 before mentioned, is anchylosed 

 to the lower end of the tibia (viz., oscalcis and astragalus). 



The distal portion, viz., the scaphoid cuboid and three cunei- 

 form bones, are anchylosed to the second, third, and fourth meta- 

 tarsal bones, to form the most characteristic bone in the leg of the 

 pigeon, viz., the tarso-fnetatarsal bone. This bone presents a 

 proximal end, with two articular cavities for the lower end of the 

 tibia, and at its distant end is divided into three trochlear con- 

 dyles for the second, third, and fourth phlanges. There is also 

 at the distal end a rough impression on the inner side for that of 

 ■the first metatarsal. 



THE, TARSO-METATARSAL 



Is followed by the digits, which consist of three toes directed 

 forwards, and one backwards — the hallux, which consists of two 

 phlanges ; the index, or the most internal of the three anterior 

 toes, is composed of three phlanges ; the middle, of four ; and the 

 •outermost, of five. 

 • The fifth toe is not developed, as in all other birds. 

 The toes are mechanically flexed when the pigeon is roosting 

 by the action of a muscle, which runs from the pelvis, and is 

 attached to the flexor tendons of the toes. 

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