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



Professor Humphry mentions two divisions in Phoca vitulina — (1) the first or pectoral proper 

 includes the presternal and sternal ; (2) the second is the same as the abdominal. 



In Arctoccplialus gazella there are also three — (1) the Presternal, (2) the Sternal, (3) the 

 Thoracico-abdominal. The last is so named because it covers the thorax and abdomen, and is not 

 wholly abdominal as in Phoca vitulina. 



Dr. Murie in the Otaria gives three divisions — (1) a first division (which embraces the presternal 

 and sternal parts in Phoca vitulina and in Arctoccplialus), (2) a second (representing the abdominal 

 part in Phoca vitulina and the thoracico-abdominal in Arctoccplialus), (3) a third (not found in 

 Arctocephalus and Phoca vitulina, and called the sterno-scapular). 



The same author in the Trichcchus gives three divisions — (1) a thick fleshy pectoralis major 

 (equivalent to the presternal and sternal parts in Phoca vitulina and Arctoccplialus), (2) a second 

 layer or pectoralis minor (which is the abdominal part in Phoca vitulina and the thoracico-abdominal 

 in Arctoccplialus), (3) a third layer (called the sterno-scapular by Dr. Murie in Otaria). 



The muscle fibres are not arranged alike in the Phocinas and Arctoccplialus, but form muscular 

 layers of very different shapes. The presternal and sternal parts in comparing their form can be 

 combined, and this gives two masses for examination. The presternal and sternal parts in the Phocinae 

 form a large triangular layer, with the anterior and middle fibres transverse, and the posterior 

 obliquely directed forwards ; the base of the triangle springs from the whole of the presternum, 

 meso-sternum, and xiphi-sternum. In Arctoccplialus and in Otaria the same mass consists only of 

 transverse fibres, and stretches from the presternum and meso-sternum directly outwards to the 

 flipper. Judging by the drawing of the Trichcchus, the configuration of the same division is more 

 like that of the Phocinae, for it approaches the triangular shape, and the posterior fibres are not directly 

 transverse as in Arctoccplialus and Otaria, but obliquely directed forwards as in the Phocinae. The 

 abdominal part in the Phocinas approaches the triangular shape, and consists of an inner or mesial 

 belt of fibres directed forwards and outwards, and an outer or lateral belt running along the lateral 

 abdominal wall and a number of intermediate muscular bars or fingers filling in the triangle. 

 All these fibres go to the axilla. In Arctoccplialus the thoracico-abdominal part is a large badly 

 formed triangle nearly like that of Otaria and Trichcchus. 



Humphry describes the second division (i.e., abdominal part) of the pectoral muscle in Phoca 

 vitulina as arising " from the linea alba, the pubes, and also from the margin of the ilium, covering 

 the fibres of the external oblique which were seen running transversely between the iliac and pubic 

 portions." This being a most interesting point in the anatomy of the pectoral muscle, I made a 

 series of dissections to ascertain the exact condition, and in the large Phoca had special opportunity 

 of investigating this among many other points, and wish to emphasise what was ascertained. A 

 group of fibres did come from the linea alba, also one from the back of the leg, and an intermediate 

 number of digitations from the fascia on the external oblique muscle whose hindmost ends did 

 not pass a line drawn from 3 inches behind the xiphi-sternum to the inner side of the patella, 

 so none reached the pubes. In this animal the digitations of some of the intermediate group of 

 fibres of the abdominal part reached the outer side of the rectus sheath. 



As no other writer describes a presternal part, but includes it with the sternal, I give my 

 reason for so doing. In the Phocinas some were fresh specimens, and in these there was a slight 

 separation of the fibres at the junction of the presternum and meso-sternum ; but in Arctoccplialus, 

 the specimens being at least ten years old and preserved in brine, which had hardened the flesh and 



