474 CHORD AT A. 



crtiro tarsal joint, whereas in reptiles and birds the mainn 

 joint is an intertarsal joint. 



In both Hmbs of mammals the number of phalanges is 

 normally two in the first digit and three in each of the 

 others. 



In the various orders we shall notice that there may 

 occur fusions of certain bones, loss of others and modifica- 

 tions of others, but when once this type be learnt and 

 retained in one's mind, there is no difficulty in interpreting 

 aright the most modified mammalian limb. 



As general rules for the identification of the bones we may lay down 

 the following (see Fig. 326) : — 



1. Humerus a7id Femur. — The proximal limb-bones {humerus and 

 femur) are long bones and have an articular condyle at each end. 



Towards their proximal ends they have a "ball" which moves in the 

 socket of the limb-girdle, and two or more processes called tube^^osities 

 (humerus) or trochanters (femur). At the distal extremity they both 

 have a sigmoid condyle or trochlea. The humerus may be distin- 

 guished from the femur by its large shallow condyle, whereas the femur 

 has a rounder condyle raised on a "neck." The humerus usually has 

 a conspicuous deltoid ridge on its preaxial border. The proximal 

 end of the humerus (or femur) can always be distinguished from the 

 distal by the condylar or ball-and-socket joint in the former and the 

 sigmoid or di lateral joint in the latter. 



The humerus often has a small foramen on the inner or postaxial 

 side of the sigmoid condyle termed the entepicondylar foramen. It 

 seems to have occurred very generally amongst Eocene mammals, such 

 as Condylarthra, Tillodontia and Typotheria, and is very generally 

 found amongst Metatheria, Edentata, some Carnivora, most Insectivoray 

 Lemuroidea and Cebidce. 



This foramen should be carefully distinguished from the supra- 

 trochlear foramen in the median line above the trochlea and produced 

 by incomplete ossification. 



The third trochanter of the femur has much the same interest as the 

 entepicondylar foramen. It is on the postaxial border {cf Horse) 

 and is present in Condylarthra, Tillodontia, Typotheria, Creodonta and 

 other extinct types. It also occurs in Dasypodidce, Orycferopodidcs, 

 many Rodentia, most Insectivora, in Perissodactyla and (small) in 

 Hyracoidea. 



2. Distal limb-bones. — The distal limb-bones have a hollow arti- 

 cular facet at each end when they are fully developed. At the 

 proximal extremity they receive the condylar ends of the proximal 

 limb-bones ; at the distal end they receive the condyles of the proximal 

 carpals. 



The two most important bones are the preaxial (or the radius and 

 tibia), and the ulna and fibula are, in a great number of cases, merely 

 vestiges fused on to their respective preaxial bones, forming a single 



