xv.] PRIMATES. 271 



Special Characters of the Bones of the Arm and 

 Fore-arm in the different Groups. 



Order PRIMATES. In Man the humerus is long, slender, 

 and straight, with a large globular head. Neither the tube- 

 rosities, nor the deltoid and supinator ridges, are much 

 developed. The internal condyle is prominent, but there is 

 no supracondylar foramen as a normal condition. The 

 anconeal fossa is wide and deep, and sometimes, though 

 not usually, perforated. The lower articular surface is divided 

 by a groove into a pulley-like internal portion (trochled) for 

 the ulna, and a smaller rounded portion (capitellnm\ confined 

 to the front side of the bone, for the radius. 



The whole bone is somewhat twisted on its longitudina 

 axis. Supposing it is so placed that a line drawn horizontally 

 though the axis of the head passes directly backwards, 

 another line drawn through the condyles would not cross 

 this at a right angle, as in most of the inferior Mammals, but 

 its outer end would be directed forwards. This appears to be 

 caused by a process of resorbtion and new deposition of bony 

 tissue during the growth of the embryo, producing an effect 

 resembling torsion of the axis to an extent of about 35 . 1 



The radius has the head or proximal end expanded, disk- 

 shaped, and cupped at its extremity, which is applied to the 

 capitellum of the humerus (see Fig. 87). Below this 

 expanded head the bone is comparatively slender, but 

 increases in size as it approaches the lower end, which is 

 wide from side to side : the surface next the ulna being 



o 



hollowed to receive the lower end of that bone, while the 



1 See Gegenbaur, " Jenaische Zeitschrift," Bd. IV. p. 50, also his 

 * ' Lehrbuch der Anatomie des Menschen." Leipzig, 1883. For an 

 opposite view, Albrecht, " Beitragzur Torsionstheorie, etc." Kiel, 1875 



