162 THE VOYAGE OF H.M.S. CHALLENGER. 



surface of the olecranon between the inner and outer borders, as far back as the middle tubercle 

 (PL VII. fig. 4, in Arctocephalus australis); some fibres blend with the other head over the olecranon. 



The second division is nearly alike in Otaria and Trichcchus, and its origin differs from that in 

 Arctocephalus, for in the former two it arises from the back of the humerus, whereas in the latter it 

 only comes from the external border. 



The fourth or internal head 1 lies on the back of the humerus under cover of the external 

 head. It arises from the posterior surface of the shaft of the humerus beneath the origin of the 

 middle head, and from the posterior ligament of the elbow-joint. It is of a triangular form, but 

 the base is next the elbow-joint, and the apex below the inner side of the head of the bone. It is 

 inserted into the posterior ligament of the elbow-joint ; into the sides and tip of the olecranon ; and 

 into the quadrilateral surface behind the sigmoid cavity. 



In Arctocephalus gazella it arises from the posterior surface of the shaft of the humerus, from 

 the capside of the shoulder-joint, from the ligament as in Phoca, and is inserted into the .quadri- 

 lateral surface behind the sigmoid cavity of the ulna. In Otaria and Trichcchus its disposition is 

 almost the same. It is the extensor of the forearm. 



In ArctoccpJialus the musculo-spiral supplies the dorsi-epitrochlear, long, internal, and external 

 heads, the latter also has a twig from the circumflex nerve. 



The Flexor or Inner Surface. — In the Phocinse the following are the muscles — anconeus 

 internus, palmaris longus, flexor communis digitorum, flexor carpi radialis, pronator radii teres, 

 flexor carpi ulnaris, abductor minimi digiti longus. 



In Arctocephalus, instead of one palmar muscle there are three, and the abductor minimi digiti 

 is not found. Neither in the Phocinse nor Arctocephalus is there a pronator quadratus. 



The Anconeus internus, called supinator quadratus by Lucae, is nearly double the size of the 

 anconeus externus. It arises from the back of the internal condyle below the supracondyloid 

 foramen, and is inserted into the inner side of the olecranon below the long head of the triceps. 



In Arctocephalus gazella it arises, as in Phoca, from the posterior part of the internal condyle 

 above the palmaris longus, and behind the pronator radii teres. It crosses from the internal condyle 

 to the inner lip of the olecranon, and is inserted into it opposite the anterior and middle tubercles 

 of the outer surface. The long head of the triceps and the deep palmar have a bed for it in 

 their substance. It is present in Otaria and Trichcchus. It is a short extensor of the elbow-joint, 

 and also steadies it. It is supplied by the ulnar nerve. 



The Palmaris longus in the Phocinse is in two parts : — a. The first part arises from the 

 posterior half of the hollow on the internal surface of the ulna, where the olecranon and the 

 posterior border of this bone meet. It is situated at the origin below the anconeus internus, and 

 higher up the shaft than the flexor carpi ulnaris. At the junction of the upper and middle thirds 

 of the ulna it divides into two slips, one being anterior, the other posterior. The latter soon splits 

 into two fine tendons, which are inserted into the deep fascia over the wrist. The anterior slip is 

 also tendinous, and disappears beneath the palmar fascia ; upon the under surface of this it widens 

 and emerges on the opposite side as two fascial slips, which descend to the heads of the 2nd and 

 3rd metacarpal bones. Here they are attached to the sheaths of the corresponding tendons, and 

 also to the tendons of the flexor subliruis for the 2nd and 3rd digits. It is with difficulty 



1 This is Humphry's fourth division ; also Lucae's, and in his plate is the anconeus quartus. 



