There exists in some mammals an osseous canal, situated above the epitrochlea, and known as the _supratrochlear canal_ (Fig. 19). It is bounded by a plate of bone which at its middle portion is detached from the shaft of the humerus, and blends with the latter at both its extremities. The brachial artery and median nerve pass through the foramen.
[Illustration: FIG. 19.--INFERIOR EXTREMITY OF THE LEFT HUMERUS OF A FELIDE (LION).
1, Epitrochlea; 2, supra-epitrochlear foramen.]
[Illustration: FIG. 20.--INFERIOR EXTREMITY OF THE LEFT HUMAN HUMERUS, SHOWING THE PRESENCE OF A SUPRATROCHLEAR PROCESS.
1, Epitrochlea; 2, supra-epitrochlear process.]
A similar condition is sometimes found, as an abnormality, in man, which presents itself under the following aspect (Fig. 20): an osseous prominence more or less long, in the shape of a crochet-needle--supra-epitrochlear process--situated 5 or 6 centimetres above the epitrochlea; the summit of this process gives attachment to a fibrous band, which is inserted by its other end into the epitrochlea and the internal intermuscular aponeurosis. The fibro-osseous ring thus formed gives passage to the brachial artery and the median nerve, or in case of a premature division of this artery to the ulnar branch of the same.[9]
[9] For further details of this anomaly, see Testut, 'The Epitrochlear Process in Man' (_International Journal of Anatomy and Physiology_, 1889); A. Nicolas, 'New Studies on the Supratrochlear Process in Man' (_Review of Biology of the North of France_, t. iii., 1890-1891).
There is also found in some mammals a perforation of the thin plate of bone which, in others, separates the olecranon fossa from the coronoid. This perforation is sometimes found as an abnormality in the human humerus.
As does the sternum and the skeleton of the shoulder, the humerus of birds presents differences correlated to the functions which the thoracic limbs are destined to fulfil. Lying on the side of the thorax, directed obliquely downwards and backwards (Fig. 21), it is proportionately longer in individuals of powerful flight than in those which fly less or not at all. In the vulture it projects beyond the posterior part of the pelvis; in the cock it does not even reach the anterior border of the same. To these differences in length are added differences in volume and in the development of the processes which serve for muscular attachment, which are more considerable in birds of powerful flight.