1894.] MYOLOGY OF EODENTS. 285 



and E. epixanthus ^ the same arrangement is found. This 

 corresponds to the arrangement that Mivart and Murie° have 

 described in the Agouti, but presents a clearer illustration o£ it 

 than is found in that animal. 



In Sciurus and Pteromi/s the superficial head is small, and instead 

 of rising from the caudal vertebrae comes from the deep surface of 

 the gluteus maximus. Xerus, Spermophilus, Arctomys, and Castor 

 as well as all the other rodents have the typical arrangement. In 

 Myopotamus a strong tendon runs to the head of the fibula. 



Semitendmosus. — The semitendiuosus rises by two heads, the 

 most superficial of which springs from the sacral and anterior 

 caudal spines and slightly from the lumbar fascia. The deep head 

 rises from the tuber ischii deep to the biceps. These two parts 

 unite in the upper part of the thigh, and are inserted into the 

 cneraial crest of the tibia and the fascia of the leg below this. 



This arrangement applies to all the animals dissected, except 

 the Hystricidaj and Pteromys. 



In the Hystricidaj the muscle rises only from the sacral and 

 caudal spines, but in Hystrix cristata and Erethizon dorsaius ^ a 

 small slip is given to reinforce the biceps from this. In Sphin- 

 f/urus no slip goes to the biceps. In Pteromys the muscle rose 

 from the tuber ischii ; but I am inclined to regard this as an 

 individual variation, because all the other Sciuromorpha, including 

 Sciurus, have both heads. 



Semimembranosus, — This muscle consists of two parts, which are 

 sometimes distinct, at others blended. The main part of the 

 muscle rises from the tuberosity and adjacent part of the ramus of 

 the ischium, and is inserted by a rounded tendon into the internal 

 tuberosity of the tibia. It is supplied by the great sciatic nerve. 



The second portion is often included in the description of the 

 adductors, with which it is frequently closely blended ; its insertion 

 is always into just above the internal condyle of the femur where, 

 in Man, the adductor tubercle is situated ; this insertion is separated 

 from that of the adjacent adductors by the femoral artery. The 

 origin of this part of the muscle is not constant — sometimes it rises 

 separately from the sides of the caudal vertebrae, sometimes from 

 the tuberosity of the ischium in common with the other head of 

 the muscle, and sometimes from the ramus of the ischium, as part 

 of the adjacent adductor magnus. Whatever its origin it is always 

 supplied by the great sciatic nerve and never by the obturator 

 which supplies the adductors. 



In the Dipodidae the muscle rises from the tuberosity of the 

 ischium, and is inserted into the lower part of the back of the 

 femur and the internal tuberosity of the tibia ; the oblique 

 condylar slip is separate and also rises from the tuber {Dipus 

 cegyptius) . 



In the Octodontidae the origin is from the tuber and ramus of 



^&' 



1 Journ. Anat. 1888, p. 126. 



2 P. Z. S. 1866, p. 383. 



3 P. Z. S. 1882, p. 271. 



