THE LIMBS OF THE HOESE 111 



to the double origin of the muscle in man and many other mammals 

 {e.g. the dog). The name " biceps " was originally given to the muscle 

 in man, because in him it arises by two heads, one from the ischium, 

 and one from the femur ; that is to say, in the horse the two heads 

 of the human and canine muscle are represented by one head only. 

 That head of the horse's muscle which is attached to the sacrum is 

 apparently not a part of the biceps proper, but represents a gluteal 

 muscle (part of the superficial gluteal ?) that has become continuous with 

 the biceps. Hence it has been suggested that the resultant combined 

 muscle should be called m. ghotceo-hiceps. 



M. SEMITENDINOSUS. — The semitendinous muscle forms the greater 

 part of the posterior border of the thigh, where it lies between the 

 biceps and the semimembranosus muscles. It has two heads of origin. 

 One arises from the spinous process of the last sacral segment, the 

 transverse processes of the first and second coccygeal vertebrae, the 

 aponeurotic fascia of the tail, the edge of the sacro-tuberous ligament 

 where it is blended with the origin of the biceps, and the intermuscular 

 septum shared by the biceps. The other iiead springs from the ventral 

 surface of the sciatic tuber and the ridge that runs forwards from this 

 tuber. The aponeurotic tendon that succeeds the fleshy belly of the 

 muscle is partly attached to the crest of the tibia and partly blended 

 with the fascia of the leg. Its connection with the tendo-accessorius 

 of the biceps, and its consequent indirect attachment to the tuber of 

 the calcaneus, should also be noted. 



As with the biceps, so also with the semitendinosus of the horse, it 

 appears probable that the sacral origin represents portion of a gluteal 

 muscle. In man, ruminants, and the domestic carnivora the muscle 

 had only one origin, namely, from the sciatic tuber. 



Dissection. — Cut through the sacral origin of the biceps and 

 semitendinosus muscles a short distance from the sacrum. Do the 

 same with their origin from the sciatic^ tuber. Then cut across the . 

 muscles about half-way down the thigh and remove the intervening ; 

 portions. In doing this dissection, note the nerve supply (from the i 

 caudal gluteal and sciatic) of the two muscles. 

 V. 'A small lymph gland should be looked for under the biceps and on ; 

 ^ i4ke_fiourse of the caudal gluteal artery. ' j 



The sheet of fascia that continues the fascia lata underneath the ; 

 - ' , biceps and is connected with the sciatic tuber and the sacro-tuberous | 

 ,' ligament must be examined and removed. Observe the continuity of '; 

 this fascia with the tendon of the superficial gluteal muscle. j 



7"^ Next Kberate both origins of the superficial gluteal muscle and thus j 

 ( ,1 ,' expose the middle member of the group. Note that a branch of the J 

 ^^/*' ' caudal gluteal nerve bends round the edge of the piriformis muscle in 

 ,j)rder to reach the superficial gluteal. 



M. GLUTyEUS MEDius. — The middle gluteal muscle is the thickest 



and strongest muscle of /t^^g^^lgy^jj^g^o^^tirely covers the surface 



3^ 



