PLATE XXIX. 



THE BACK OF THE THIGH, HAMSTRINGS, AND SMALL 



SCIATIC NERVE. 



The dissection of the back of the thigh shows the small sciatic and hamstrings. It 

 is a little less than two-thirds natural size. The gluteus maximus has been drawn 

 slightly upwards in order to expose the origin of the hamstrings from the tuber ischii. 



\\hen the origin of the biceps from the tuberosity is examined, it will be rtoticed 

 that a large number of the tendinous fibres of origin are continuous with those of the 

 great sacro-sciatic ligament. In ruminants this ligament is 'replaced by a thick strong 

 tendon, and it may be taken that the ligament is mainly the continuation of the tendon 

 to a higher origin, which has been cut off by the outgrowth of the tuber ischii. In 

 some of the marsupials the biceps has two origins a tendinous origin from the tuber 

 with the semitendinosus and another muscular slip which springs from the second and 

 third caudal vertebra under cover of the agitator cauda (posterior part of gluteus 

 maximus), and passes down superficial to, and a little to the outer edge of, the tuber, 

 and then joins the main mass of the muscle at its insertion. The use of the second 

 origin is to associate the movements of the hind limb with a heavy tail. 



The hamstrings are good examples of long muscles passing over more than one 

 joint acting on both, and so tending to co-ordinate their movements, and enabling 

 them to be produced with the least expenditure of power. They are also too short 

 to allow complete flexion of the hip when the knee is extended : this may be tested 

 by noticing the strain required to touch the toes with the fingers when the knee is 

 straight ; again the gastrocneruius is too short to allow complete flexion of the ankle 

 when the knee is extended ; also the flexors of the fingers are too short to permit 

 full extension of the fingers when the wrist is over-extended. If the muscles be 

 measured in these different positions, the length will not be found to alter, the varying 

 positions of the joints being due to a slight advance of one and retreat of the other; 

 hence, in walking, the rectus, gastrocnemius, and hamstrings are not alternately con- 

 tracted and stretched, but remain equal in length, the advance of one joint compensating 

 for the retreat of the other. Replace these muscles by ligaments and see what would 

 occur. Suppose the thigh bent by he ilio-psoas ; the hamstrings ligament would at 

 once flex the knee, being too short to allow its remaining extended. Now suppose the 

 limb bent at the ankle by the weight of the body producing flexion ; the gastrocnemius 

 ligament would at once slightly flex the knee, and this would also tend to bring the 

 centre of gravity back to a more stable condition. Thus muscles passing over more than 

 one joint partake of the nature of elastic ligaments ; but being built up of contractile 

 substance, if this contracts at the same time they would act far more effectually than 

 ligaments could do ; and when acting in this fashion as ligaments they are combiners 

 and diffusers, not producers of the movement, the short muscles producing and the long 

 diffusing it : thus they give the short muscles ' an indirect action on the joints over 

 which they do not pass,' and so increasing their range of action. Being also elastic, 

 they give when taxed beyond their strength, instead of rupturing as ligaments would do. 1 

 ! Professor CIcland, Journal of Anat. and Phys. vol. i. p. 85. 



