DISSECTION OF THE ANTERIOR LIMB. 19 



nerves from the musculo -spiral will be found to pierce the muscle, or 

 emerge at its loAver edge, and become distributed to the outer side of the 

 fore-arm. These should, as far as possible, be preserved. 



The Caput Medium (Plates 7 and 8) arises, by a short aponeurotic ten- 

 don, from a curved line beginning on the deltoid tubercle and continued 

 upwards to the external tuberosity. It is inserted into the olecranon. 



Action. — ^Like the other divisions of the triceps, this muscle is an 

 extensor of the elbow-joint. 



Directions. — By raising the lower edge of the last muscle and dissect- 

 ing upwards, the anconeus will be partly exposed ; but to effect a com- 

 plete and natural separation of the two muscles, is a matter of some 

 difficulty. 



The Anconeus (Plates 7 and 8) is a small muscle which lies above the 

 olecranon fossa, and there covers the synovial membrane of the joint, 

 a pad of fat being interposed. It arises from the margin of the fossa, 

 and is inserted into the oleci'anon on its outer and anterior aspect. 



Action. — To assist in extending the elbow, and at the same time to 

 raise the synovial membrane and prevent its injury between the bones. 



Directions. — If the caput medium be now severed at its origin, and 

 turned backwards, the musculo-spiral nerve and some branches of the 

 deep humeral artery will, as already described, be found turning round 

 the humerus in the musculo-spiral groove, which is mainly filled by the 

 brachialis anticus muscle. 



The Brachialis Anticus muscle (Plate 8), also known as the humeralis 

 obliquus or externus, is lodged in the fiu-row of torsion on the shaft of the 

 humerus. The muscle has its origin on the posterior aspect of the shaft 

 of the humerus below its articular head. Its tendon, which cannot be 

 followed at present, passes in front of the elbow-joint, and is afterwards 

 reflected under the internal lateral ligament of the joint, to be inserted 

 into the radius and ulna. 



Action. — To flex the elbow-joint. 



the fore-arm. 

 Surface-marking. — At, the elbow-joint the olecranon process of the 

 ulna is distinctly seen ; but the shafts of the bones of the fore-arm ai'e 

 clothed with muscles, except at the lower third of the inner border of 

 the radius, where the bone is subcutaneous. On the outer side of the 

 front of the elbow-joint a large muscular mass is formed by the extensor 

 metacarpi magnus and the anterior extensor of the digit (extensor 

 pedis). In the living animal (in which it is preferable to study these 

 surface-markings) this is more distinctly visible, and the tendons of these 

 muscles and that of the lateral extensor (extensor suffi-aginis) may be 

 distinctly traced. On the inner side of the elbow-joint one may feel the 

 tendon of insertion of the biceps; and just behind the tendon the posterior 



