Artistic Anatomy of Animals

by Édouard Cuyer

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.

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 quadrupeds, that in some of these, on the other hand, the tuberosities are on a higher level than the articular head of the bone. Between the two tuberosities is the bicipital groove.

In man, the superior extremity of the humerus, although covered by the deltoid, reveals its presence by elevating the corresponding portion of the latter. In quadrupeds, the anterior part of this extremity, although similarly covered by muscular bundles, produces a prominence under the skin. This prominence is situated at the summit of the angle formed by the opposing directions of the scapula and the bone of the arm, and constitutes what is known by the name of the point of the shoulder, or of the point of the arm.

The inferior extremity, transversely enlarged, presents an undulating articular surface, which reminds us of the trochlea and the condyle of the human humerus; on which, however, the condyle is more sharply defined from the trochlea.

In the human skeleton, the internal lip of the trochlea descends lower than the external; and also lower than the condyle. In the bear, the cat, and the dog, it is the same. In the ox and the sheep, the condyle is lower than the trochlea, but only very little lower. In the horse the arrangement is still the same, but a little more accentuated.

On the lateral parts of this extremity we find: internally, a prominence, the epitrochlea; and, externally, another, the epicondyle. It is from this latter that the crest arises, which, passing upwards, forms the posterior limit of the groove of torsion.

The two prominences, which we have just described from a general point of view, present special arrangements which it is necessary to point out. When we examine the form of the outline of the inferior extremity of the humerus in man, the bear, the cat, the dog, the ox, and the horse, we find in following this order that the extremity tends to become narrow transversely, and that the epicondyle and the epitrochlea are less and less prominent on the external and internal aspects respectively. These two processes, indeed, project backwards; the epitrochlea always remaining more developed than the epicondyle. Because of this projection backwards, the cavity situated on the posterior surface of the inferior extremity, the olecranon fossa, is very deep, more so than in the humerus of man. Its borders being thus formed by the two processes, are very prominent. In front we find the coronoid fossa, which is less deep than that of which we have just spoken.