ANIMAL MECHANICS. 



83 



He found also, that in twenty-nine instances the flexor 

 digitorum longus sent a slip to the tendon of the hallux, and 

 that this occurred most frequently in the cases in which the 

 flexor hallucis sent slips to the second, third, and fourth toes, 

 as if to compensate for the loss of power thus occasioned in 

 the hallux. 



If P' denote the force applied by a muscle to one extre- 

 mity of a tendon, and P denote the force conveyed to its 

 other extremity, or insertion, the forces P' and P will be 

 equal, if the tendon experiences no friction in passing from 

 the muscle to the insertion of the tendon ; but P' will exceed, 

 or fall short of P, by a force equal to the friction F, accord- 

 ing as the muscle overcomes an external resistance, or external 

 forces overcome the resistance of the muscle. We shall, 

 therefore, have 



P = P + P (15) 

 in the two cases supposed. 



In both cases P' is proportional to the cross section of the 

 muscle, or inherent force of muscular contraction ; and P, 

 which represents the external force applied, must always be 

 less than the force sufficient to break the tendon. Solving 

 the foregoing equation for P, we find 

 P = P' + F; 



from which it follows, that the safety of the system requires the 

 tendon to be made always strong enough to resist the force 

 P + P, or the sum of the muscular force and friction combined. 



Hence it follows that the cross section of the tendon will 

 always indicate a strength, sufficient not only to resist the 

 force of the muscle, but the sum of that force and the friction 

 experienced by the tendon. 



In order to ascertain the proportion of the cross section 

 (or force) of a given muscle, to the cross section (or strength) 

 of its tendon, I obtained permission from Professors Morgan 

 and Bevan, of the Royal College of Surgeons in Ireland, to 

 g2 



