THE SKELETON IN THE FLYING LEMURS. 191 



be perforated by a miuute foramen. The internal and external condyles 

 are large, and pitted for the origin of certain muscles upon either side. 

 Above the internal condyle, anteriorly, there exists a delicate span of 

 bone of no great length. It passes, as a gently curved arch from the 

 condyle, obliquely toward the center of the shaft in a proximal 'direction 

 forming the supracondyloid foramen (figure 15), which gives passage 

 and protection to the median nerve and brachial artery. Wliei'e high 

 division of the brachial occurs, the nerve only, as a rule, passes under it, 

 but may be accompanied by the ulnar-interosseous artery. 



The distal points of the trochlea and capitellum lie in the same 

 horizontal plane to which the axis of the shaft is perpendicular. Each 

 constitutes a prominent tuberosity separated distally by a well-marked 

 valley. Both in front and behind they rise to the same transverse line 

 on the shaft, ceasing at the distal boundary of the olecranon fossa poste- 

 riorly, and at the rather shallow depression intended for the ulna 

 anteriorly. The smaller tuberosity is flat upon its internal aspect, while 

 the capitellum for the radius is fully double the size with a roundly 

 convex articular surface. The average extreme length of the limnerus 

 is about 10.2 centimeters. 



Judging from the material at hand it would appear that when tlie bones 

 of the arm are normally articulated they admit of extreme flexion to a 

 far greater degree than they do of extreme extension, that is, extension 

 to the extent of bringing the long axis of the shaft of the humerus and 

 the ulna into one and the same straight line. 



The radius is a very strong and nearly straight bone with enlarged 

 extremities and subcylindrical smooth shaft. It has an average extreme 

 length of 13.3 centimeters, or two centimeters greater than the humerus. 

 (Plate III, figure 9.) At its proximal end, the tuberosity for the in- 

 sertion of the tendon of the biceps, is represented by a short longitudinal 

 crest, terminating in a groove near which we usually discover the opening 

 of a small nutrient foramen. Above the tuberosity the bone is some- 

 what constricted to form the neck of the radius and the latter is sur- 

 mounted by the head of the bone. The head is large, oval in outline, 

 and at its summit exists the rather deep concavity for articulation with 

 the capitellum of the humerus, while its marginal articulatory surfaee 

 for the ulna is about 2 millimeters deep. The outer lower third, or 

 more, of the shaft is flattened, forming a surface to which the distal 

 fourth of the ulna is firmly attached by ligament. Anteriorly, on its 

 expanded part at this extremity, we note the five conspicuous longitudinal 

 grooves intended for the passage of the extensor tendons as they go to 

 the hand. A sharp, peg-like, styloid process projects forward at the 

 outer side of the bone, but does not extend beyond the border over which 

 the extensor tendons pass. Internal to this process is a deep, elliptical, 

 transverse facet for articulation with the first row of bones of the wrist 



