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



for the pollex. The posterior group gives origin to one muscle in all, and this is the extensor 

 proprius pollicis in Arctocephalus, the extensor pollicis et indicis in Otaria and Trkhechus ; but 

 as these three are exactly alike in their mode of insertion, practically they are the same, and all 

 mi"ht be as correctly named extensor proprius pollicis. In the Phocinse the muscle is called the 

 extensor primi internodii pollicis, but only provisionally, for its origin is exactly as in all the others, 

 but its insertion is half that of the proprius and half that of the extensor primi internodii, for 

 a part of the insertion crosses the base of the 1st phalanx of the pollex, and the other half 

 is continued down the radial side like the extensor primi internodii ; thus it has a compound 

 action. 



The anterior group in the Phocinse and Otaria is similar, forming the extensor ossis meta- 

 carpi only ; in Arctocephalus two muscles come from this group owing to its division into an upper 

 and a lower half. The upper half is the extensor primi internodii pollicis, the lower the extensor 

 ossis metacarpi pollicis. In Trichechus there are three muscular elements — the extensor ossis 

 metacarpi pollicis, the extensores primi et secundi internodii, all combined at their origin, and indis- 

 tinguishable from the common mass of fibres. This affords evidence of a distinct portion of a bony 

 surface being reserved for a muscle discharging a special function with respect to a certain digit. 

 The area of origin in all these animals remaining constant and not increasing in size, whether there 

 is a single tendon or more, is probably novel. It leads to the conception that there must be 

 a method in the formation of distinct muscles out of common masses of fibres, though it may not 

 always be traceable. These muscles have the usual actions. 



Myology of the Hind Limb. 



The Ilio-Femoral Region includes the psoas and iliac muscles, with a varying set of muscles 

 in connection with the former. The following shows the various accessory muscles found in each 

 specimen. The meaning of the names is explained further on : — 



The Psoas magnus or primus is present in the large Phoca vitulina ; in the small Phoca 

 vitulina with an ilio-femoralis et lumbalis anterior; in Phoca barbata with an ilio-femoralis et 

 lumbalis anterior; in Phoca hispida with an ilio-femoralis anterior; in Macrorhinus; and in 

 Arctocepluilus. 



The Psoas minor or sccundus is found in all the above specimens and in the large Plwca 

 vitulina, on the right side, with a lumbo-femoralis posterior. 



The Psoas tcrtius is co mm on to all but Macrorhinus, and it has an ilio-femoralis posterior. 



The Eiacus is found in all the specimens. 



Before entering upon the details of the psoas, it is well to understand upon what grounds 

 the names are given, as well as to point out what peculiarities each muscle possesses, and the 

 similarities and dissimilarities in each dissection. As much interest in the anatomy of the Seals 

 centres around this group of muscles the localisation of the fibres is of importance. Upon the lumbar 

 vertebras ventrally there are two longitudinal fleshy masses, each including a psoas magnus and parvus. 

 As one of these muscles is attached in all the specimens, though with some modifications in detail, 

 to the pectineal eminence, I regard it as the equivalent of the psoas parvus of human anatomy, called 

 the secundus or minor in the text. The other muscle, lying to the outer side of the minor, has not 



