The humerus in quadrupeds is inflected like the letter S; in man this general form is less accentuated, the humerus being almost straight. On its body, which appears twisted on its own axis, we find the musculo-spiral groove,[8] which crosses the external surface, and is very deep in some animals. Above this groove, and on the external surface, there exists a rough surface which is the impression of the deltoid. In 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.
[8] It would be going outside our province to discuss whether the humerus is really twisted on its axis. This question, often discussed, has been solved in some recent works in the following manner: the humerus has undergone torsion at the level of its superior extremity, and not at the level of its body; this does not authorize us further to accord any definite sense to the denomination 'groove of torsion' (musculo-spiral groove). That which we must especially remember in connection with this fact, is, as we shall afterwards see, the difference of direction which the articular head presents according as the torsion has been more or less considerable: because this is established, according to the same order, in man and in quadrupeds.
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 inwards; in quadrupeds its direction is upwards and backwards. 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 correlated with the position of the scapula, the glenoid cavity of which, as we have already seen, is in man turned outwards, 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.