Fig. 30.—Inferior Extremity of the Bone of the Forearm of the Horse: Left Side, Anterior Surface.
1, Radius; A, groove for the long abductor and the short extensor of the thumb; B, groove for the radials; D, groove for the common extensor of the digits; E, groove for the special extensor of the little finger.
We should mention that the groove E is situated, both in man and in the dog, at the level of the inferior radio-ulnar articulation; but that in the horse, as the ulna does not exist at that level, the groove is situated on the external surface of the inferior extremity of the radius. It is necessary to add that, in some horses, the ulna is, nevertheless, represented in this region by a tongue-like process of bone; and in such cases the groove is situated in front of this process, at the level of the line of coalescence, which there represents the articulation.
| Man. | Dog. | Horse. | |||
| A. | Long abductor of the thumb, and short extensor of the thumb. | A. | Long abductor of the thumb and short extensor of the thumb united as one muscle,the oblique extensor of the metacarpus.[11] | A. | Oblique extensor of the metacarpus, the homologue of the long abductor of thethumb and the short extensor of the thumb, united as one muscle. |
| B. | First and second external radials (extensor carpi radialis longior and brevior). | B. | The two radials blended superiorly, distinct inferiorly; this is the anterior extensor of themetacarpus. | B. | The radials represented by a single muscle, the anterior extensor of the metacarpus. |
| C. | Long extensor of the thumb. | C. | Long extensor of the thumb and special extensor of the index finger united superiorly. These muscles passin the following groove. | C. | The long extensor of the thumb and the special extensor of the index are absent. |
| D. | Special extensor of the index finger and the common extensor of the fingers. | D. | Common extensor of the digits and the two preceding muscles. | D. | Anterior extensor of the phalanges, the homologue of the common extensor of the digits. |
| E. | Special extensor of the little finger. | E. | Extensor of the third, fourth, and fifth digits, or the lateral extensor of the digits, thehomologue of the special extensor of the little finger. | E. | Lateral extensor of the phalanges, the homologue of the special extensor of the little finger. |
| F. | The posterior ulnar (extensor carpi ulnaris). | There does not exist on the forearm a groove for the posterior ulnar muscle, or externalflexor of the metacarpus. | |||
[11] The words printed in italics are the names used in veterinary anatomy.
It is also useful to note, with reference to the groove F, in which passes, in man, the tendon of the posterior ulnar muscle, that, when the forearm is in pronation, the radius alone being displaced, we can only see this groove on the surface which looks backwards; and that it is then separated from the groove which contains the tendon of the special extensor of the little finger by an interval equal to the thickness of the head of the ulna.[12] When the forearm is supinated, the two grooves are found, on the other hand, one beside the other: and the tendons which they contain are very naturally in contact.
[12] Édouard Cuyer, ‘Shape of the Region of the Wrist in Supination and Pronation’ (Bulletin de la Société d’Anthropologie, Paris, 1888).
In birds the forearm is flexed on the arm, and the latter being directed downwards and backwards, the former is, consequently, directed upwards and forwards. Further, because of the position of the humerus, which, as we mentioned on [p. 32], has its inferior extremity so turned that the surface which is anterior in man becomes external, the radius, instead of being outside the ulna, is placed above it. This latter is larger than the radius, but its olecranon process is very slightly developed.