306 



ELEMENTARY PHYSIOLOGY 



synovial sheaths, i.e. quite covered for some distance by 

 a synovial bag forming a double sheath, very much in the 

 same way that the bag of the pleura covers the lung and 

 the chest- wall. 



Usually, the direction of the axis of a muscle is that of 

 a straight line joining its origin and its insertion. But in 

 some muscles, as the mtperior oblique muscle of the eye, 

 the tendon passes over a pulley formed hy ligament, and 



FiQ. 92. 



-The noNEs OF the Upper Extremity with the Bicbps 



Muscle. 



The two tendons by which this musole is attached to the scapula are 

 •een at a. P, indicates the .attachment of the muscle to the radius, and 

 hence the point of action of the power ; F, the fulcrum, the lower end of 

 the humerus on which the upper end of the radius (together with the 

 ulna) moves ; W, the weight (of the hand). 



completely changes its direction before reaching its inser- 

 tion. (See Lesson TX.) 



Again, there are muscles which are fleshy at each end, 

 and have a tendon in the middle. Such muscles are 

 called dignairic, or two- bellied. In the curious muscle 

 which pulls down the lower jaw, and especially receives 

 this name of di'jastrir, the middle tendon runs through a 

 pulley connected with the hyoid bone ; and the muscle, 



