276 VERTEBRATE SKELETON 



humerus, trochanters on the femur) for the insertion of muscles. 

 Usually there are two tuberosities — radial (majus) and ulnar (minus) 

 on the humerus, the names in parentheses those of human anatomy. 

 The femur has tibial (minor) and fibular (major) trochanters and may 

 have one or (rarely) two more. The distal end of humerus and femur 

 (trochlea) is adapted for hinge motion and is divided into inner and 

 outer condyles for articulation with the zeugopodial bones, the 

 proximal parts of the condyles being the epicondyles. Many reptiles 

 and mammals have foramina near the lower end of the humerus (fig. 

 304), an ulnar (entepicondylar) foramen for the median (ulnar) 

 nerve, and a similar radial (ectepicondylar) foramen for the radial 

 nerve and brachial artery. Both foramina may coexist, usually 

 but one is present, the ulnar in «iany mammals. Neither of these 

 foramina is known in recent Amphibia or birds. 



The radius serves as the axis around which the forearm revolves 

 when the distal part of the arm is rotated, whence its name. The 

 ulna often extends proximally beyond the articulation with the 

 humerus, the projection being the olecranon on which the extensor 

 muscles are inserted. Tibia and fibula resemble the bones of the 

 fore limb, except in less rotational power and in the absence of 

 anything like an olecranon. 



The autopodium is divided into a proximal basipodium (wrist or 

 carpus, ankle or tarsus), a metacarpus (palm) ormetatarsus (instep), 

 these forming the mesopodium, the autopodium terminating with the 

 metapodiimi or acropodium, which includes the fingers and toes 

 (digits), each composed of a number of segments (phalanges). 

 The typical basipodium consists of nine or ten bones arranged in 

 three transverse rows. The proximal row includes three bones. The 

 one in line with radius or tibia is the radiale or tibiale, with uhiare or 

 fibulare in a Hke position with regard to ulna or fibula. Between 

 these two in either Hmb is an intermedium.' The distal basipodial 

 row consists, typically, of five bones— carpalia in the hand, tarsalia in 

 the foot — numbered from one to five, beginning at radial or tibial 

 side. Between the proximal and distal rows of both hand or foot 

 are one or two (rarely three) centralia. When three are present 

 (embryo Sphenodon and some Theromorphs) one is exposed on the 

 radial or tibial side. 



1 It is unccrlain whether the intermedium belongs to zeugopodium or basipodium 

 It arises between the zeugopodial cartilages and later takes a basipodial position. 



