THE LIMBS OF THE HORSE 37 



fibrosus) that leaves the biceps near its insertion and merges into the 

 surface investment of the radial extensor. In the distal third of the 

 forearm the tapering fleshy part of the muscle gives place to a strong, 

 flattened tendon that, traversing a groove at the distal end of the 

 radius and crossing the dorsal aspect of the carpus, is inserted into the 

 muscular imprint on the dorsal (anterior) medial part of the base of the 

 third metacarpal bone. The tendon is bound down in the groove on 

 the radius by the dorsal carpal ligament, and here, as well as over the 

 carpus, is surrounded by a synovial sheath. 



M. extensor digitorum communis. — Smaller than the preceding, 

 and lateral to it in position, the common extensor of the digits arises 

 from the anterior and most distal part of the lateral epicondyle of the 

 humerus, the coronoid fossa (in common with m. extensor carpi radialis), 

 the lateral collateral ligament of the elbow joint, the lateral aspect of 

 the proximal part of the radius, and the adjacent part of the ulna. 

 Some fibres also arise from the intermuscular septum between this 

 muscle and the radial extensor of the carpus. 



Commonly, the most lateral part of the muscle can be isolated as the 

 muscle of Phillips, and occasionally another and smaller bundle of fibres, 

 medial to the preceding, may be separated from the main muscle — the 

 muscle of Thiernesse. If the last-named is separable, it is provided with 

 a thin tendon which soon joins the main tendon of the muscle. The 

 tendon of the muscle of Phillips accompanies the main tendon into the 

 metacarpus and there joins that of the extensor digiti quinti. 



The main tendon of the common extensor leaves the tapering distal 

 part of the muscle, plays down a groove at the distal end of the radius, 

 crosses the extensor aspect of the carpus and enters the metacarpus. 

 At a later stage of the dissection it will be followed to the extensor 

 process of the third phalanx. 



Like the tendon of the radial extensor, that of the common digital 

 extensor is secured in a groove on the radius by the dorsal carpal 

 ligament, and is provided with a synovial sheath here and over the 

 carpus. Since this arrangement is common to the extensor tendons of 

 the region, the tendon sheath should be slit open and an examination 

 made of the mode of disposition of its synovial lining. The sheath may 

 be regarded as a closed bag closely applied to the surface of the tendon 

 as well as to the wall of the fibro-osseous canal in which the tendon lies. 

 The layer upon the tendon is connected with that lining the canal by a 

 thin membrane known as the mesotenon. 



To establish the strict homology of the three parts of the horse's 

 common extensor is not without difficulty. There are those who see in 

 the muscle of Phillips the equivalent of that part of the dog's common 



