OSTEOLOGY AND ARTHROLOGY 29 



some species this rugosity is very prominent, and is called 

 the tuberosity of the deltoid ; it is prolonged downwards by 

 a border which forms the anterior crest of the musculo-spiral 

 groove and limits this latter in front. The external border 

 of the bone, or posterior crest of the groove, limits it behind. 



The superior extremity is enlarged, and remarkable in 

 three portions which it presents ; these are : an articular 

 surface and two tuberosities. 



The articular surface, or head of the humerus, smooth and 

 round, is in contact with the glenoid cavity of the scapula. 

 This head in the human skeleton is directed upwards and in- 

 wards ; in quadrupeds its direction is upwards and back- 

 wards. The inferior extremity, having in both one and 

 the other its long axis directed transversely, and the point 

 of the elbow looking backwards in all, the result is that the 

 head of the humerus is not situated vertically above the 

 same regions ; in the first, it is almost directly above the 

 internal part of this extremity ; in the latter, it is situated 

 above its posterior surface, or the point of the elbow in 

 the complete skeleton This difference of direction is corre- 

 lated with the position of the scapula, the glenoid cavity 

 of which, as we have already seen, is in man turned out- 

 wards, whereas in quadrupeds it looks downwards. In 

 the latter case the scapula consequently rests on the head 

 of the humerus ; and this position is most favourable for the 

 performance of the functions which the anterior limbs have 

 to fulfil in these latter. 



Of the tuberosities of the head of the humerus, one is 

 situated on the external aspect — it is the great tuberosity, or 

 trochiter ; the other is placed internally — it is the small 

 tuberosity, or trochin. The great tuberosity is divided into 

 three parts — summit, convexity, and crest ; these different 

 parts give insertion to the muscles of the shoulder. We 

 recollect that the facets (anterior, middle, and posterior) of 

 the great tuberosity of the humerus in man give attachment 

 to the muscles of the same region. The head of the humerus 

 in the human body projects above the tuberosities. We 

 shall see afterwards, when dealing with some special quadru- 

 peds, that in some of these, on the other hand, the tubero- 



