48 



PHYSIOLOGY OF MUSCLES AND NERVES. 



to it, when it is in a quiescent state ; the other, when 

 it is active. AVhen a quiescent muscle is brought into 

 an active condition by irritation, it assumes a form 

 which is no longer natural to it, it strives to attain the 

 latter, and shortens until it reaches its new form, which 

 is then natural to it. If the muscle is extended by a 

 weight, and is then irritated, it immediately contracts; 

 but only to that length which represents the exten- 

 sion by the attached weight, proper to its new form. 

 Let us imagine that A B, in fig. 16, is the length of 



./' 



A 



y, 



/' 



Z^ 



Fio. 16. Alteration' in Elasticity during contraction. 



3 



the muscle when quiescent and unburdened, and that 

 ^ 6 is the length of the muscle when active and un- 

 burdened. Then the muscle, if it is irritated while 

 unweighted, will shorten to the extent represented by 

 AB— Ah = hB; b B is, therefore, the height of 

 elevation of the unweighted muscle. If a weight p is 

 attached to the muscle, the latter in its inactive condi- 

 tion will be extended to a certain degree {B^ d') ; so 

 that its length will now be A B + B' d'= A' B\ On 

 being now irritated, it contracts and assumes a length 



