CONTRACTILITY. 



Dr. B. You must bear in mind that in elasticity 

 there is never a real generation of power the effect 

 produced is never greater than the cause which sets it 

 in action. In the recoil of a steel spring there might at 

 first sight, appear to be an actual generation of power ; 

 but it is a well-known fact that the force with which the 

 spring recoils is precisely equal to that which is used in 

 compressing it. And so in every piece of machinery, 

 the power produced is never greater than that which 

 has been expended. But in muscular contraction 

 there is a real generation of power, for the force with 

 which a muscle contracts, bears no proportion to that 

 by which it is excited. If you prick the internal sur- 

 face of the heart in an animal recently killed, it will 

 contract with so much force as to almost bury up the 

 needle in its substance. See too, what violent agitation 

 you may produce in the muscles of a frog, simply by 

 irritating them with a pin. Are you not convinced now, 

 Emily, that contractility is altogether different from any 

 other property of matter ? 



Emily. Partially perhaps, but I am still in doubt 

 how the fibre returns to its original condition, unless it 

 is by its elasticity, 



Dr. B. This is probably produced by elasticity, but 

 this does not take away the necessity of some different 

 property to explain its contraction. 



Emily. But simple contractility merely shows the 

 fact that the power of shortening itself is inherent in 

 the muscular tissue ; it does not explain by what means 

 this effect is produced. 



Dr. B. So many theories have been published with 

 this object in view, and all so feebly supported by facts, 

 that it would be hardly worth your while to be made 

 acquainted with them. Those which have any advocates 

 now, make contraction to depend on electricity, or gal- 

 vanism, or some change in the chemical combinations 

 of the elements of the muscle. We cannot stop to dis- 

 cuss these theories suffice it to say that in the hands 



