370 



MYOLOGY 



this point the turning component gradually decreases and the pressure compon< 

 changes into a component which tends to draw the two bones apart and which 

 gradually increases as shown in Fig. 370. 



When the bone D K is in such a position that the insertion angle < = 41 20' 

 the pressure component = f I m and the turning component f / m, at 60 the two 

 components are equal, at 90 the pressure component = and the turning com- 

 ponent = I M and at 131 21' the pressure component has been converted into a 

 pulling component = / M and the turning component = f / M. 





I DO 



FIQ. 371 



If, for example, the distance from the origin to the joint D is less than the dis- 

 tance from the insertion / to the joint D, as in the Brachioradialis muscle, the 

 insertion angle increases with the flexion but never reaches 90. The turning 

 component gradually increases to a certain point and then slowly decreases as shown 

 in Fig. 371, while the pressure component gradually decreases and then slowly 

 increases. It always remains large and its action is always in the direction of the 

 joint. 



Levers. The majority of the muscles of the body act on bones as the power on 

 levers. Levers of the III class are the most common, as the action of the Biceps, 

 and the Brachialis muscles on the forearm bones. Levers of the I Class are found 

 in movements of the head where the occipito-atlantal joint acts as the fulcrum and 

 the muscles on the back of the neck as the power. Another common example is 



w 



n 



A 



w 



n 



W 



n 



the foot when one raises the body by contracting the Gastrocnemius and Soleus.. 

 Here the ankle-joint acts as the fulcrum and the pressure of the toes on the ground 

 as the weight. This is frequently, though wrongly, considered a lever of the II Class. 

 If one were to stand on one's head with the legs up and with a weight on the plantar 

 surface of the toes, it is easy to see that we would have a lever of the I Class if the 

 weight were raised by contraction of the Gastrocnemius muscle. The confusion 

 has arisen by not considering the fact that the fulcrum and the power in all three 

 classes of levers must have a common basis of action, as shown in Fig. 3/2. 



