The antibrachial extremity of the humerus is flattened from without inwards. It terminates in two articular surfaces, which articulate with the radius and ulna.
The olecranon process of the ulna being slightly developed, it follows that the olecranon fossa is not large; neither is the coronoid.
General View of the Form of the Forearm and Hand
We now proceed to the study of the two regions of the fore-limbs which present the greatest variety in regard to the number of bones and also in regard to form and proportions. These two regions are the forearm and the hand.
It is first of all necessary to say that in man, when the fore-limb hangs beside the body, and the dorsum of the hand looks backwards, the two bones of the forearm are parallel, and that this position is known by the name of supination. It is also necessary to remember that there is another attitude, in which the radius, crossing the ulna, and carrying the hand with it, displaces the latter in such a way that the palmar surface looks backwards. This second position is known as pronation.
Let us now suppose that a man wishes to walk in the attitude of a quadruped. It will be necessary, in order that his upper limbs, being for the moment anterior ones, may act as members of support, to place the forearm in pronation, in order that, as is more normal, the hands may rest on the ground by their palmar surfaces. In this position the radius, being rotated on its own axis at its upper extremity and around the ulna in the rest of its extent, shall have its inferior extremity situated on the inner side of the corresponding extremity of the latter.
Such is the situation of the bones of the forearm and the attitude of the hand in quadrupeds. In short, quadrupeds have their anterior members in the position of pronation.
Fig. 22.—The Human Hand resting for its Whole Extent on its Palmar Surface: Left Side, External Surface.