GENERAL CHARACTERS. 



907 



the representative of an additional radial digit. Similarly the pisiform 

 bone of the carpus, which occurs on the ulnar or postaxial border, and in 

 Mammals is usually described as one of the so-called sesamoid bones, is 

 looked upon by the same authority as the representative of a seventh 

 digit on the ulnar side. 



It is probable that the carpus and tarsus were originally formed 

 upon a common type, which persists in a more or less unaltered 

 condition in certain Amphibia and Reptilia (fig. 829). In such a 

 generalised type there is a distal row 

 of five carpalia (fig. 829), or tarsalia, 

 articulating with the metapodials. This 

 is preceded proximally by another row, 

 consisting of an intermedium, flanked 

 in the manus by a radiale and ulnare, 

 and in the pes by a tibiale and 

 fibulare, respectively articulating with 

 the two epipodial bones of the fore- 

 arm (radius and ulna) or leg (tibia 

 and fibula). The middle space be- 

 tween these two transverse rows of 

 bones is occupied by one ce?itrale, or oc- 

 casionally by several centralia. Modifi- 

 cations from this type are caused by 

 the suppression or coalescence of some 

 of these elements. All the carpals 

 and tarsals in the Mammalia have re- 

 ceived distinct names, which will be 

 noticed under the head of that class ; 

 but it may be observed here that the 

 higher Reptiles and Birds agree with 



the former in having two bones in the proximal row of the tarsus 

 — viz., the calcaneum on the fibular, and the astragalus on the tibial 

 side. 



The protuberances for muscular attachment at the proximal ex- 

 tremity of the humerus are termed tuberosities, while those of the 

 femur are known as trochanters. The latter bone in the Saur- 

 opsida may also have an inner trocha?iter on its shaft for the at- 

 tachment of the femoro-caudal muscle ; while in the Mammalia there 

 may be a third trochanter for that of the gluteus maximus, as is 

 shown in the femur of Rhinoceros represented in fig. 1226. Both 

 the humerus and femur have more or less distinct condyles at the 

 lower extremity for the articulation of the bones of the fore-arm and 

 leg — those of the humerus being often termed trochlea. Above the 

 condyles or trochlear the humerus has projecting epicondyles on 

 either side ; and there is frequently a foramen situated above either 



Fig. 829. — Dorsal aspect of the 

 right manus of a Chelonian Reptile 

 (CJiclydrd). ra, Radius ; ul, Ulna ; 

 r, z, zt, Radiale, ulnare, and inter- 

 medium; c, Centrale; i — 5, Carpalia; 

 m, Metacarpals; p, Proximal phal- 

 angeals ; 1.— v., Terminal do., 1. being 

 the pollex. (After Gegenbaur.) 



