In the cat, there is a fourth tendon, which passes to the index-finger, so that the name common extensor of the four external digits is in this case legitimate, and the lateral extensor of the phalanges is also a common extensor, as is the anterior extensor of the phalanges, or common extensor of the digits.
In the pig, the tendon, which is single, is inserted into the external digit, for which reason it has received the name of the proper extensor of the small external digit. This muscle is, then, really the homologue of that which exists in the human species.
In the ox, it is called the proper extensor of the external digit; it is as thick as the common extensor.
Finally, in the horse, the muscle is little developed. Its fleshy body, thin and flattened from before backwards, becomes distinctly visible only below the middle of the forearm. Above, it is enclosed in a limited space, bounded in front by the common extensor of the digits, and behind by the posterior ulnar; there these two muscles approach each other so closely that from the point of view of external form they seem to be nearly in contact.
The tendon, after receiving the small fasciculus from the common extensor ([Fig. 75], 16), as well as a fibrous band emanating from the external surface of the carpus ([Fig. 75], 17), is situated at the external side of the tendon of the anterior extensor of the phalanges, and is inserted into the anterior surface of the superior extremity of the first phalanx.
This muscle extends the digit or digits into which it is inserted. It also assists in the movement of extension of the hand as a whole.
Posterior Ulnar (Extensor carpi ulnaris) ([Fig. 73], 11; [Fig. 74], 17; [Fig. 75], 18).—Designated by veterinary anatomists as the external flexor of the metacarpus,[27] or external cubital, this muscle is situated in the posterior region of the external surface of the forearm, behind the lateral extensor of the phalanges.
[27] Certain authors give it the name of ulnar extensor of the wrist. It is true that in the human being this is its action; but in quadrupeds, owing to its insertion into the pisiform, it draws the hand into the position of flexion.
It arises from the epicondyle; its fleshy body, thick but flattened, is directed vertically towards the carpus, and its tendon is inserted into the external part of the superior extremity of the metacarpus, after having given off a fibrous band, which takes its attachment on the pisiform.
It is inserted, in the cat and the dog, into the superior extremity of the fifth metacarpal; in the pig to the external metacarpal; in the ox to the external side of the canon-bone; in the horse to the superior extremity of the external rudimentary metacarpal.