534 MOVEMENTS. 



in the appreciation of weight and of resistance to contraction. Their contractility or 

 irritability is the property which enables them to contract and exert a certain amount of 

 mechanical force under the proper stimulus. All of these general properties strictly 

 belong to physiology, as do some special acts that are not neceisarily involved in the 

 study of ordinary descriptive anatomy. 



Elasticity of Muscles. The true muscular substance contained in the sarcolemma is 

 eminently contractile; and, although it may possess a certain degree of elasticity, this 

 property is most strongly marked in the accessory anatomical elements. The interstitial 

 fibrous tissue is loose and possesses a certain number of elastic fibres, and, as we have 

 seen, the sarcolemma is very elastic. It is probably the sarcolemraa that gives to the 

 muscles their retractile power after simple extension. 



It is unnecessary to follow out in detail all of the numerous experiments that have 

 been made upon the elasticity of muscles. There is a certain limit, of course, to their 

 perfect elasticity (understanding by this the degree of extension that is followed by com- 

 plete retraction), and this cannot be exceeded in the human subject without dislocation 

 of parts. It has been found by Marey, that the gastrocnemius muscle of a frog, detached 

 from the body, can be extended about one-fiftieth of an inch by a weight of a little more 

 than three hundred grains. This weight, however, did not extend the muscle beyond 

 the limit of perfect elasticity. The muscle of a frog of ordinary size was extended beyond 

 the possibility of complete restoration, by a weight of about seven hundred and fifty 

 grains. Marey also showed that fatigue of the muscles increased their extensibility and 

 diminished their power of subsequent retraction. This fact has an application to the 

 physiological action of muscles; for it is well known that they are unusually relaxed 

 during fatigue after excessive exertion, and, as we should expect, they are at that time 

 more than ordinarily extensible. 



Muscular Tonicity. The muscles, under normal conditions, have an insensible and 

 a 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 of one side of the face be 

 paralyzed, the muscles upon the opposite side insensibly distort the features. It is diffi- 

 cult 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 we can say 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, 

 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 power of resistance, immobility, and elas- 

 ticity 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 muscular acts. 

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

 we appreciate 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 seems to be, to a certain extent, 

 distinct; for, when suffering from the fatigue that follows over-exertion, it seems as 

 though we could send a nervous stimulus to the muscles, to which they are, for the time, 

 unable to respond. When we come to consider fully the subjects of muscular and ner- 

 vous irritability, we shall see that these two properties are entirely distinct, and that we 

 may exhaust or destroy the one without necessarily affecting the other. 



When the muscles are thrown into spasm or tetanic contraction, a peculiar sensation 

 is produced, entirely different from painful impressions made upon the ordinary sensitive 



