The Muscles 57 



83. Synovial Sheaths and Sacs. The rapid movement of 

 the tendons over bony surfaces would soon produce an undue 

 amount of heat and friction unless some means were provided 

 to make the parts run smoothly. This is supplied by sheaths 

 which form a double lining around the tendons. The opposed 

 surfaces are lined with synovial membrane, the secretion from 

 which moistens the sheaths in which the tendons move. 



Little closed sacs, called synovial sacs, or bursae, similarly 

 lined and containing fluid, are also found in special places 

 between two surfaces where much motion is required. Thus, 

 there are two of these bursae near the patella. Without these, 

 the constant motion of the knee-pan in walking would produce 

 undue friction and heat. 



Experiment 21. Examine carefully the tendons in the parts dis- 

 sected in Experiment 18. Pull on the muscles and the tendons, and 

 note how they act to move the parts. This may be also admirably 

 shown on the leg of a fowl or turkey obtained from a kitchen or from 

 the market. 



Obtain the hoof of a calf or sheep with one end of the tendon of 

 Achilles still attached. Dissect it and test its strength. 



Experiment 22. The tendons which bound the space behind the 

 knee can be distinctly felt when the muscles which bend the knee are 

 in action. On the outer side note the tendons of the biceps of the 

 leg, running down to the head of the fibula. On the inside we feel 

 three tendons of important muscles on the back of the thigh which 

 flex the leg upon the thigh. 



A 84. The Mechanics of Motion. Levers. The active agents 

 of bodily movements, as we have seen, are the muscles. 

 All these movements, both of motion and of locomotion, 

 are regulated according to certain fixed laws of mechanics. 

 The bones, to which a great proportion of the muscles in 

 the body are attached, act as distinct levers. The muscles 

 supply the power for moving the bones, and the joints act 

 as fulcrums or points of support. The weight of the limb, 

 the weight to be lifted, or the force to be overcome, is the 

 resistance. 



