During these movements of the humerus, there exists an essential factor--that is, the scapular balance. (It is the same as what occurs in man when he balances his arm in the antero-posterior plane.) When the humerus is flexed, the scapula moves in such a way that the superior portion projects forwards; when it is extended, the scapula, on the other hand, is inclined more backwards. But it is necessary to add that, during these displacements, the scapulo-humeral angle varies; it tends to close during the flexion of the humerus, and becomes more open during extension. The movements and the relations of the humerus and the scapula are clearly represented in Figs. 41 and 42, reproduced from the chromophotographic studies of Professor Marey--studies relative to the analyses of the movements of the horse.[13] They show clearly the movements of flexion and extension of the humerus, also the balancing of the scapula which accompanies the movements. [13] E. J. Marey, 'Analyses of the Movements of the Horse by the Chromophotograph' (_La Nature_, June 11, 1898). [Illustration: FIG. 41.--FLEXION OF THE HUMERUS: RIGHT ANTERIOR LIMB OF THE HORSE, EXTERNAL SURFACE (AFTER A CHROMOPHOTOGRAPHIC STUDY BY PROFESSOR MAREY).] [Illustration: FIG. 42.--EXTENSION OF THE HUMERUS: RIGHT ANTERIOR LIMB OF THE HORSE, EXTERNAL SURFACE (AFTER A CHROMOPHOTOGRAPHIC STUDY BY PROFESSOR MAREY).] =The Articulation of the Elbow, or the Humero-ulnar Articulation.=--In this articulation, which is constructed in the form of a true hinge, the movements of flexion and extension alone are possible. In flexion, the forearm, directed forwards, is folded on the arm, with which, in certain circumstances, it comes in contact. For example, in a horse of mettle which leaps over an elevated obstacle, the animal forcibly raises his fore-limbs by flexing them. Flexion is produced to the same extent, and even more so, and for a longer period, in felides which crouch. In extension, on the contrary, the forearm is carried backward. This movement being limited only by the contact of the tip of the olecranon with the bottom of the olecranon fossa of the humerus, the forearm is enabled, in this case, to move until it is in line with the arm. For example, during walking, when one of the anterior limbs, having reached the end of its resting stage, is considerably inclined downwards and backwards. The apex of the olecranon process--that is to say, the point of the elbow--forms a marked prominence, more salient in flexion than in extension, as in the corresponding region of the human elbow. =The Radio-ulnar Articulation.=--It is in the dog and the cat, in which the two bones of the forearm articulate by their extremities only, and remain separate in the rest of their extent, that the articulations call for special notice.