102 



THE BONES. 



Fig. 63. 



(or external) trochanter, and great fuberositi/—corm[>Y[ses three portions, named 

 the summit, convexitij, and crest of the great tuberosity. The internal eminence — 

 the trochin, little (or interned) trochanter, or small tuherosity — also presents three 

 distinct portions, which, by their position, correspond exactly with the three regions 

 of the external trochanter ; these are so many muscular facets. 



The external and internal trochanters are separated from each other in front 

 by a channel called the bicipital groove, because the superior 

 tendon of the biceps muscle glides over it. It consists of 

 two vertical grooves, with a median ridge between them. 



The inferior extremity of the humerus has an articular 

 surface corresponding with the radius and ulna. This sur- 

 face — elongated transversely, convex from before backwards, 

 and of greater extent within than without — exhibits two- 

 trochlea separated by an antero-posterior relief. 



The median or interned trochlea, the deepest, is limited 

 internally by a kind of voluminous condyle, which corre- 

 sponds to the inner lip of the humeral trochlea of Man. 

 The external trochlea is bordered outwardly by a slightly 

 salient lip, which corresponds to the condyle of the humerus 

 of Man. Above and behind this articular surface is a wide 

 deep fossa — the olecranian (or condyloid), so named because 

 it lodges the rostrom of the olecranon in the extension move- 

 ments of the forearm. It is bordered by two eminences, 

 the external of which is less elevated than the internal. The 

 first represents the epitrochlea, and the second the epicondyle, 

 of the humerus of Man. In front, and above the inner 

 trochlea, there is another, but less spacious fossa, which 

 receives the coronoid process during extreme flexion of the 

 forearm, and which, for this reason, it would be convenient 

 to designate as the coronoid fossa. Always in front, but 

 above the external trochlea, are imprints for the attach- 

 ment of the capsular ligament of the elbow- joint and the- 

 extensor metacarpi magnus. Lastly, at the extremities 

 of the transverse axis of the inferior articular surface is 

 remarked, outwardly, an excavation for ligamentous insertion; 

 inwardly, a small tuberosity for the same purpose. 

 Structure and development. — The humerus, like all the long bones, is only 

 spongy at its extremities. It is developed from six points of ossification, one of 

 which alone forms the body, one the head and the internal trochanter, another the- 

 external trochanter, a fourth the inferior articular surface, a fifth the epicondyle, 

 and the last for the epitrochlea. The latter is sometimes absent. In the young 

 animal the humerus is less twisted, and the eminences for muscular insertion less 

 developed, than in the adult. 



In the Ass, the humerus is more twisted and curved in S form than in that 

 of the Horse, and these modifications replace the eminences on this bone in the 

 latter. Consequently, the equilibrium of the humerus is altered when it lies on a 

 horizontal plane by any one of its faces and its lower extremity. Thus, while the 

 humerus of the Ass may rest in equilibrium on a plane — its trochlea and two other 

 points of its anterior face touching it — the Horse's humerus can only do so when 

 the condyle, trochlea, and one or two points of its superior extremity touch it.. 



POSTERIOR VIEW OF THE 

 RIGHT HUMERUS. 



2, External trochanter; 

 3, articular head of 

 the bone; 4, external 

 tubercle and ridge ; 

 5, body or shaft of 

 the bone ; 10, condy- 

 loid fossa. 



