INTEODUCTION 



lacking in Cetacea, Sirenia, all recent Ungulata, and in most Carnivora ; but 



it is very stout in burrowing mammals. 



On the upper articular end of the humerus (Fig. 8) is a rounded head, 



adjacent to which are two prominent processes for muscular attachments, the 



greater and the lesser tuberosity, separated from each 



other by the bicipital groove (hg). From the greater 



tuberosity, and along the outer face of the humerus, runs 



the wide rough deltoid ridge {d), extending far down the 



shaft. The distal or lower end of this bone is trans- 

 versely expanded, and carries a pulley -like articular 



surface (trochlea), which is divided either by a groove 



or by a raised rim (crista intertrochlearis) into a small 



radial {ar) and an ulnar [au) articular facet. On either 



side of the trochlea projects a spur, the outer being the 



external condyle {ec), the inner the internal condyle (^c). 



Just above the latter the humerus of many primitive 



mammals is perforated by an entepicondylar {ent) foramen, 

 allowing the passage of the 

 median nerve and the brachial 

 arteiy. Above the trochlea the 

 humerus is excavated on the 

 back by a shallow or deep pit 

 which receives the olecranon 

 process of the ulna, and may 

 perforate the bone. In youth 

 the upper and lower articular 

 ends (epiphyses) are 

 from the shaft. 



The forearm (Fig. 9) consists 

 of two bones, radius and ulna. In such mammals as 

 use the fore-limb for climbing or grasping, both bones 

 have developed a rotating articulation on the proximal 

 end. In most mammals the ulna is situated behind 

 the radius. In the more specialised ungulates the lower 

 end of the ulna is lost, and the upper part fused to 

 the back of the radius. 



The upper end of the radius has a shallow, oval, 

 articular depression ; the shaft is somewhat com- 

 pressed, and the lower end expanded and provided 

 with a short, sharp, styloid process (ps) bounding the 

 inner side of the articular surfaces. On the upper 

 end of the ulna is a powerful, four-sided, olecranon 

 process {ol), in front of which is the deep sigmoid 

 notch or articulation (fs). In most mammals the 

 , , _ „ lower or distal end of the ulna is narrow, and has 



-B, Left ulna of the dog (anterior i.-i i i.'i.-i jj i. n 



aspect), ol. Olecranon ;/s, fossa ^ couvcx articular surtace which IS Doundcd externally 



sigmoldea; yZ, fossa lunaris ; c, y,^ ,><, cfvlmrl nrrippcsc! 

 capitulum ; e, epiphysis. "^ ^^^ SCyiOlO prOCeSS. ^ 



The carpus or wrist (Fig. 10) consists among all 

 mammals of two rows of small bones, between which, among primitive forms, 

 a tiny centrale may be present. These bones are as follows : — 



Fig. 8. 



Right humerus of the wom- 

 bat (Phascolomys ivovibatus), 

 anterior viev/. c, Upper 

 articular surface (caput) ; t, 

 tuberculum majus ; t, tuber- 

 culuni minus ; bg, bicipital 

 groove; rf, deltoid process; sr, 

 crista supinatoria ; ent, ent- 

 epicondylar foramen ; ec, 

 separate outer, ic, inner articular sur- 

 faces ; ar, radial, au, ulnar 

 facets. 



Fig. 9. 



A, Left radius of the dog (an 

 terior aspect). c, Upper, g 

 lower articular surface ; ps, pro 

 cessus styloideus ; e, epiphysis. 



