MUSCULAR CONTRACTILITY AND EXCITABILITY 423 



dependent on the action of motor nerves. If, for example, a muscle 

 is cut across in a surgical operation, the divided extremities become per- 

 manently retracted; or if the muscles of one side of the face are para- 

 lyzed, the muscles on 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 stimulus. All that can be said is that a 

 muscle, not excessively fatigued and with its nervous connections intact, 

 is constantly in a state of insensible contraction more or less marked. 



Sensibility of the Muscles. The muscles possess that kind of sensi-. 

 bility which gives an appreciation of resistance, immobility, and elasticity 

 of substances that are grasped, or which, by their weight, are opposed to 

 muscular effort. It is by the appreciation of weight and resistance that 

 the force required to accomplish muscular acts is regulated. These 

 properties refer chiefly to simple muscular efforts. After long-continued 

 exertion, there is a sense of fatigue that is peculiar to the muscles. It 

 is difficult to separate this entirely from the sense of nervous exhaustion, 

 but it is to a certain extent distinct ; for when suffering from the fatigue 

 that follows overexertion, it seems as though a nervous stimulus could 

 be sent to the muscles, to which they are for the time unable to respond. 

 When muscles are thrown into tetanic contraction, a peculiar sensation 

 is produced, which is entirely different from painful impressions made 

 on ordinary sensory nerves. In the cramps of cholera, tetanus, or 

 the convulsions from strychnin, these distressing sensations are very 

 marked. The general sensibility of muscles is very slight. The rather 

 indefinite property, called muscular sense, is supposed by some physiolo- 

 gists to be connected with peculiar structures neuro-muscular spindles 

 that will be described in connection with the nervous system. 



Muscular Contractility and Excitability. During life and under 

 normal conditions, muscles contract in obedience to a proper stimulus 

 applied either directly or through the nerves. In the natural action 

 of the organism, this contraction is induced by nervous influence either 

 through volition or reflex action. Still, a muscle may be living and 

 yet have lost its contractility. For example, after a muscle has 

 been for a long time paralyzed and disused, the application of the 

 most powerful stimulus will fail to induce contraction ; but when exam- 

 ined with the microscope, it is found that the nutrition of the muscle 

 has become profoundly affected and that the contractile substance has 

 disappeared. Muscular contractility persists for a certain time after 

 death and in muscles separated from the body ; and this fact has been 

 taken advantage of by physiologists in the study of the properties of the 

 muscular tissue. 



