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



trapezium springs from the lower half of the ulna, the adjacent part of the interosseous 

 membrane, and very slightly from the radius. 



In the Phascogale the muscle is very similar to the corresponding muscle in the 

 Cuscus, but it has not so wide an origin. 



It is very evident that this muscle in the Marsupials is a compound of the extensor 

 ossis metacarpi, and the extensor primi internodii pollicis of man. 



Extensor secundi internodii pollicis. — In the Thylacine, and Phascogale this muscle 

 (which is undoubtedly the homologue of the muscle bearing the same name in man) sends 

 tendons to three digits, vLz., the pollex, index, and medius ; in Cuscus it is split up into 

 two distinct and separate muscles — one connected with the pollex alone, and the other 

 with the medius. 



In the Thylacine (PL I. fig. 5, d.e.) it springs from the radial side of the olecranon, 

 and from the upper third of the posterior border of the ulna. In Phascogale the origin 

 is similar, with this exception, that it has a more extensive attachment to the posterior 

 border of the ulna. In both it is a thin band-like muscle, which proceeds downwards 

 under cover of the ulnar extensors, and the extensor digitorum secundus to end in a 

 single tendon. On the dorsum of the hand this tendon splits into three parts, which go 

 to the three radial digits. 



In the Cuscus (PL II. fig. 5, e.p.) the extensor secundi internodii pollicis arises by two 

 distinct heads, viz., (1) from the radial surface of the olecranon and the posterior border 

 of the shaft of the ulna in its upper third {e.p-.); (2) from the radial border of the ulna 

 in its middle third (e.p 1 .). The anconeus externus extends downwards upon the ulna 

 between these two heads (PL II. fig. 5, a.e.). Below the level of the anconeus the two 

 slips of origin unite, and the tendon which issues from the fleshy belly goes to the second 

 phalanx of the pollex. 



The extensor niedii (PL II. fig. 5, e.m.) in the Cuscus is a very delicate slip which 

 springs from the middle third of the posterior border of the shaft of the ulna. It is 

 inserted by a delicate tendon into the dorsal extensor expansion of the medius. 



The extensor secundi internodii pollicis appears to be present in the majority of 

 Marsupials. Thus Macalister describes it in the Wombat, Tasmanian Devil, 1 and Koala, 2 

 and Meckel speaks of it in the Opossum. 3 



Supinator brevis. — This muscle is feebly developed in all the three animals. In the 

 Cuscus, in which it is best marked, it is inserted into rather less than the upper fifth of 

 the anterior and inner surface of the shaft of the radius. 



Posterior annular ligament. — This ligament forms five compartments on the back 

 of the wrist in Thylacinus (PL I. fig. 5, an.l.) for the passage of the extensor tendons. 



1 Ann. and Mag. of Nat. Hist., vol. v., 4th series, p. 164. 

 e Ibid., vol. x., 4th series, p. 131. 

 3 Anat. Comp., vol. v. p. 329. 



2 



