114 THE VOYAGE OF H.M.S. CHALLENGEK. 



ischium. It passes obliquely downwards and forwards, and after being joined by the 

 second head above the knee joint, is inserted by means of a flattened tendon into the 

 anterior internal tibial crest, in front of the tendon of insertion of the semi-tendinosus. 

 The second head is fan-shaped, and arises from the lateral surface of the abdominal wall, 

 where it is attached to the aponeurosis of the abdominal muscles. The fibres converge 

 as they pass outwards, and uniting with those of the pubic head, are inserted along with 

 them into the anterior internal tibial crest. The two heads together thus arise from a 

 semicircular origin, which extends from front to back of the limb, and the fibres form, 

 so to speak, a sheath for the reception of the posterior muscles of the thigh. 



Relations. — The pubic head is concealed externally by the semi-tendinosus and cruro- 

 coccygeus. Its anterior border is in contact with the posterior border of the adductor 

 magnus. The abdominal head is quite superficial at its origin. 



Action. — The semi-membranosus co-operates mth the biceps and semi-tendinosus in 

 flexing the knee and extending the hip joint. 



Nerve supply. — A branch from the sciatic supplies the posterior or pubic head of 

 origin. The source of supply of the second or abdominal head of origin I failed to 

 determine. 



Variations. — In one specimen of Eudijptes chrysolophus I found the pelvic head of 

 origin of the semi-membranosus transferred to the last coccygeal vertebra. In others, 

 however, the muscle presented the arrangement above described. Such was also the 

 case in the specimen dissected by MM. Gervais and Alix. In Spheniscus magellanicus 

 the pelvic head of origin is attached to the whole breadth of the posterior border of the 

 pelvis. In Sp>henisci(s minor it is limited to the lower half of that border. In both 

 these species, therefore, the attachment of the semi-membranosus to the ischium is more 

 extensive than in Eudyptes chrysocome. 



Remai'hs. — This muscle in the Penguins diff'ers, so far as my own observations go, 

 from the corresponding structure in every other l)ird in the possession of a second or 

 abdominal head of origin. Nor can I find in the literature at my disposal any reference 

 on the part of anatomists to a similar arrangement elsewhere. If farther research should 

 fail to discover a similar arrangement in other species, then the possession by the semi- 

 membranosus muscle of a second or abdominal head of origin must be regarded as one of 

 the characteristic features in the anatomy of the group, seeing that it occurs in 

 every species of Penguin which I have examined. 



Meckel does not refer to the presence in the Penguins of an additional head of origin 

 to this muscle. 



