460 MOVEMENTS. 



limit of perfect elasticity. The muscle of a frog of ordinary 

 size was extended beyond the possibility of complete resto- 

 ration by a weight of about seven hundred and fifty grains. 1 

 Marey also showed that fatigue of the muscles increased 

 their extensibility and diminished their power of subsequent 

 retraction. This fact has its application to the physiological 

 action of muscles ; for it is well known that they are un- 

 usually relaxed during fatigue after excessive exertion ; and, 

 as we should expect, they are at that time more than ordi- 

 narily extensible. 



Muscular Tonicity. The healthy muscles have an in- 

 sensible and constant tendency to contract, which is more 

 or less dependent upon the action of the motor nerves. If, 

 for example, a muscle be cut across in a surgical operation, 

 the divided extremities become permanently retracted ; or 

 if the muscles be paralyzed on one side of the face, the mus- 

 cles upon the opposite side insensibly distort the features. 

 It is difficult to explain these phenomena by assuming that 

 tonicity is due to reflex action, for there is no evidence that 

 the contraction takes place as the consequence of a stimu- 

 lus. All that we can say is, that a muscle, not excessively 

 fatigued, and with its nervous connections intact, is con- 

 stantly in a state of insensible contraction, more or less 

 marked, and that this is an inherent property of all of the 

 contractile tissues. 



Sensibility of the Muscles. The muscles possess to an 

 eminent degree that kind of sensibility which enables us to 

 appreciate the pow r er of resistance, immobility, and elasticity 

 of substances that are grasped, on which, we tread, or which, 

 by their weight, are opposed to the exertion of muscular 

 power. It is by the appreciation of weight and resistance 

 that we regulate the amount of force required to accomplish 

 any muscular act. These properties refer chiefly to simple 



1 MAREY, Du mouvement dans lesfondions de la vie, Paris, 1868, pp. 289, 301. 



