THE MUSCLES OF THE LOWER EXTREMITY. 181 



as shown in Fig. 24. From this origin this straight 

 muscle passes directly downwards and forwards, and 

 when it arrives at the muscles of the leg, it becomes 

 converted into a delicate, thin tendon which, passing 

 between the muscles of these parts, reaches the inner side 

 of the shaft of the tibia about one and a half centimetres 

 below its head, or rather bounding: line of its summit, 



' O 



where it is inserted on a line parallel with the long axis 

 of the tibial shaft (Fig. 50). 



The semimembranosus is a direct flexor of the leg 

 upon the thigh ; it lies nearly in the same plane with the 

 semitendinosus, the contiguous borders being joined by 

 the surrounding connective tissue. 



Mr. Garrod says of these last three muscles that 

 " some birds, as in the Eagles and Owls, have no semi- 

 tendinosus at all ; some, as the Anserine birds and 

 Penguins, have no accessory semitendinosus, in which 

 case all the fibres go straight to the tibia-head ; whilst 

 in most the above- described condition maintains ; " and 

 of the semimembranosus he remarks that " this muscle 

 is very constant in birds ; in the Grebes it is extremely 

 thin, and may sometimes be absent, as stated by Sunde- 

 vall ; but I have seen it in some fresh specimens of 

 Podiceps minor, though but very slightly developed ' 

 (Coll. Scientific Memoirs, London, 1881, p. 190). 



And of the same three, as they occur in Geococcyx, 

 I said in the P.Z.S. of 1886, that the semitendinosus 

 (Fig. G4 bis) is a marvellously well-developed muscle in 

 this form, as is also its accessory head. Its origin fills 

 about three-fourths of the nether cavity formed by the 

 posterior overarching portion of the ilium, under which 

 it arises. 



Posteriorly, the fibres forming its free margin are so 

 arranged as to create a rounded border ; the lower end 



