MYOLOGY 



one pulley may be resolved, so far as the direction of the pull is concerned, into two 

 or more units or single-joint muscles (Fig. 362). The tendons of the Flexor pro- 

 fundus digitorum, for example, pass through several pulleys formed by fibrous 

 sheaths. The direction of the pull is different 'for each joint and varies for each 

 joint according to the position of the bones. The direction is determined in each 

 case, however, by a straight line between the centers of the pulleys on either side of 

 the joint (Fig. 363). The direction of the pull in any of the segments would not 

 be altered by any change in the position or origin of the muscle belly above the 

 proximal pulley. 



The Action of the Muscle Pull on the Tendon. Where the muscle fibers are parallel 

 or nearly parallel to the direction of the tendon the entire strength of the muscle 

 contraction acts in the direction of the tendon. 



In pinnate muscles, however, only a portion of the strength of contraction is effi- 

 cient in the direction of the tendon, since a portion of the pull would tend to draw 

 the tendon to one side, this is mostly annulled by pressure of surrounding parts. 

 In bipinnate muscles this lateral pull is counterbalanced. If, for example, the muscle 



fibers are inserted into the tendon at an angle of 60 

 degrees (Fig. 364), it is easy to determine by the 

 parallelogram of forces that the strength of the pull 

 along the direction of the tendon is equal to one-half 

 the muscle pull. 



T = tendon, ra = strength and direction of muscle 

 pull. 



t = component acting in the direction of the 

 tendon. 

 </> = angle of insertion of muscle fibers into tendon. 



cos $ = cos z 60 = 0.50000 



ra 



/ = 



ra 



m 



If z < = 72 30' cos = I 

 z <t> = 41 20' cos = t 

 z (f> = 90 cos = 



z & = cos = 1 



The more acute the angle </>, that is the smaller 

 the angle, the greater the component acting in the 

 direction of the tendon pull. At 41 20' three-fourths 

 FIG. 364 of the pull would be exerted in the direction of the 



tendon and at the entire strength. On the other 



hand, the greater the angle the smaller the tendon component; at 72 30' one-third 

 the muscle strength would act in the direction of the tendon and at 90 the tendon 

 component would be nil. 



The Strength of Muscles. The strength of a muscle depends upon the number of 

 fibers in what is known as the physiological cross-section, that is, a section which 

 passes through practically all of the fibers. In a muscle with parallel or nearly 

 parallel fibers which have the same direction as the tendon this corresponds to the 

 anatomical cross-section, but in unipinnate and bipinnate muscles the physiological 

 cross-section may be nearly at right angles to the anatomical cross-section as shown 

 in Fig. 365. Since Huber has shown that muscle fibers in a single fasciculus of a 

 given muscle vary greatly in length, in some fasciculi from 9 mm. to 30.4 mm., it 

 is unlikely that the physiological cross-section will pass through all the fibers. 

 Estimates have been made of the strength of muscles and it is probable that coarse- 

 fibered muscles are somewhat stronger per square centimeter of physiological 



