It is easy to explain this partial disappearance of the ulna. When the forearm is capable of performing the movements of pronation and supination, the ulna is completely developed, for it is in its small sigmoid cavity that the head of the radius revolves, and it is around its inferior extremity, the head, that the corresponding extremity of the radius turns. But when the movements of rotation of the forearm do not exist, the inferior extremity of the ulna becomes functionally useless and disappears. As to its rôle in the movements of the region of the wrist, that is nil, for we may remember--we will observe it again when we come to treat of the articulations--that the hand articulates with the radius alone (radio-carpal articulation); this is the reason that, when the forearm possesses the fullest mobility, the hand follows the movements which that bone makes around the ulna. [Illustration: FIG. 27.--SUPERIOR EXTREMITY OF THE BONES OF THE FOREARM OF THE HORSE: LEFT LIMB, SUPERIOR SURFACE. 1, Radius; 2, ulna; 3, olecranon process; 4, coronoid process.] It is not so with the articulation at the elbow-joint; there it is the ulna, which, with the humerus, forms the essential parts (humero-ulnar articulation); its olecranon process limits the movement of extension of the forearm. It is for this reason that, even in those quadrupeds in which the ulna is atrophied, the olecranon process presents a relatively considerable degree of development. [Illustration: FIG. 28.--INFERIOR EXTREMITY OF THE BONES OF THE FOREARM OF A MAN: LEFT SIDE, POSTERIOR SURFACE, POSITION OF SUPINATION. 1, Radius; 2, ulna; A, groove for the long abductor and short extensor muscles of the thumb; B, groove for the radial muscles; C, groove for the long extensor of the thumb; D, groove for the special extensor of the index finger and of the common extensor of the fingers; E, groove for the proper extensor of the little finger; F, groove for the posterior ulna.] [Illustration: FIG. 29.--INFERIOR EXTREMITY OF THE BONES OF THE FOREARM OF THE DOG: LEFT SIDE, ANTERIOR SURFACE, NORMAL POSITION--THAT IS, THE POSITION OF PRONATION. 1, Radius; 2, ulna; A, groove for the long abductor and for the short extensor of the thumb; B, groove for the radials; D, groove for the long extensor of the thumb, the special extensor of the index-finger, and the common extensor of the fingers; E, groove for the special extensor of the little finger.] We know that on the posterior surface of the inferior extremity of the bones of the human forearm are grooves in which pass the tendons of the posterior and external muscles which, belonging to this region, are directed for insertion towards the hand.