The metacarpal bone undergoes, relatively to the humerus, a proportional elongation, analogous to that of the forearm.
In man, the length of the metacarpus is contained about 51⁄2 times in that of the humerus; in the bear, it is contained 4 times; in the dog, 21⁄2 times; in the horse, 11⁄3 times only.
It is well known that the proportions vary according to race, and that what we have here given are but the general indications.
The Articulations of the Anterior Limbs
The knowledge of human arthrology which we presume the reader to have previously acquired makes it unnecessary for us to enter into numerous details regarding the configuration of the articular osseous surfaces and the disposition of the fibrous bands that retain them in position. Accordingly, in the description which follows, and also in that of the articulations of the posterior limbs, we shall occupy ourselves but very briefly with the details above referred to, so as to devote ourselves especially to the indication of the movements—that is to say, of that which, while easily comprehended on recollection of former studies, presents the greatest interest from the artistic standpoint in these studies in comparative anatomy.
The Scapulo-Humeral Articulation.—The head of the humerus and the glenoid cavity of the scapula being in contact, the two bones are bound together by a rather loose articular capsule, which is strengthened by the muscles of this region which fulfil the function of active ligaments.
This articulation, so movable in every direction in the human species, is not so much so in quadrupeds; the arm in the latter, as also the shoulder, being kept in contact with the lateral region of the thorax by the numerous muscles which surround it.
Of the movements performed by the humerus, flexion and extension are the most extensive; those of abduction and adduction are much less so.
It is necessary, before proceeding further, to determine what the two principal movements which we have just mentioned really are, viz., flexion and extension.
We know that in man the displacements of the humerus which take place in the antero-posterior direction are known as movement or projection forwards, and movement or projection backwards, respectively. We do not say that the humerus is flexed or extended, because, in reality, on account of the position which the skeleton of the shoulder occupies, it is not able to flex or place itself on the line of prolongation of the scapula with which it articulates.