EEPOET ON THE SEALS. 175 



In Otaria "it takes origin from the outer surface of the olecranon, and from the ulna to as far 

 as about the middle of the latter, and is inserted into the prominent and anterior or outer corner of 

 the metacarpal of the pollex." 



In Trichechus it is combined with the extensores primi and secundi internodii pollicis. This 

 muscle in the Phocinae covers that part of the ulna which, in Arctocephalus, gives origin to the 

 extensor ossis metacarpi pollicis and the extensor primi internodii pollicis, in Otaria gives origin 

 to the same muscles, and in Trichechus to the extensor ossis metacarpi, extensor primi internodii, 

 and extensor secundi internodii pollicis. It is supplied by the posterior interosseous nerve. 



The Extensor primi internodii pollicis arises from the posterior third of the outer surface of the 

 olecranon; from the posterior border of the ulna in its upper half; and very slightly from the 

 shaft where the extensor ossis metacarpi begins to cross the radius. One inch above the wrist it 

 forms a tendon, which passes beneath those of the extensor communis digitorum secundus, and 

 enters the third division of the annular ligament with and below the extensor communis primus. 

 It is, inserted into the base of the 1st phalanx of the thumb on the anterior or radial side; and 

 into the head of the metacarpal. 



In Arctocephalus the extensor digitorum and the extensor carpi ulnaris must be turned up 

 before the origin of this muscle is seen. It arises from the external surface of the olecranon 

 between the anterior and middle tubercles; from the edge of the quadrilateral surface in front 

 of the anterior tubercle; from the sigmoid cavity of the ulna; from the concave outer sur- 

 face of the ulna, as far as the origin of the extensor ossis metacarpi pollicis ; from the external 

 surface of the ligament of the capsule of the elbow-joint ; from the interosseous ligament ; and from 

 the upper half of the posterior quarter of the breadth of the shaft of the radius. Its tendon 

 crosses the radius, and goes through the same division as the extensor ossis metacarpi pollicis. 

 Opposite the head of the 1st metacarpal it is bound to the palmar fascia, and is inserted into the 

 radial side of the base of the 1st phalanx of the pollex, being joined to the extensor proprius 

 pollicis. 



Lucae gives the insertion into the base of the 1st metacarpal, but Humphry gives a different 

 insertion into the back of the 1st phalanx of the pollex, which is the phalanx for the insertion 

 of the extensor primi internodii pollicis. 



In Otaria this muscle is wanting In Trichechus it is inseparably united with the extensor 

 ossis metacarpi pollicis. That part of the ulna upon which this muscle is implanted in the Phocinae 

 gives origin in Arctocephalus, Otaria, and Trichechus to the extensor proprius pollicis. It is 

 supplied by the ulnar nerve. 



According to those works on the Pinnipedia that I have had an opportunity of reading, the 

 outer surface of the ulna in the Phocinae and Arctocephalus gives origin only to muscles for the 

 pollex. The surface to which the fibres are attached, generally speaking, is the upper two-thirds of 

 the shaft. In Arctocephalus the outer surface of the ulna is divided into two by a ridge commencing 

 from the middle tubercle on the outer side of the olecranon, and becoming indistinct at the junction of 

 the upper two-thirds and the lower third of the shaft (PI. VII. fig. 4). In the Phocinae there is a slight 

 ridge dividing the external surface of the olecranon into two, which comes close to the posterior border 

 of the ulna f of an inch below the junction of the posterior border with the olecranon, runs down the 

 shaft close to it, and ends at the middle of the shaft. These ridges separate the muscle fibres, 

 which clothe the external surface of the shaft, into two groups of the same functional importance 



