ELASTICITY AND TONICITY OF MUSCLES. 471 



muscular fluid is slightly alkaline, but it becomes acid soon after death. The 

 muscle itself, during contraction, has an acid reaction. The muscular juice 

 is alkaline or neutral after moderate exercise as well as during complete 

 repose ; but when a muscle is made to undergo excessive exercise, the lactic 

 and other acids exist in greater quantity and the reaction becomes acid. 



PHYSIOLOGICAL PROPERTIES OF THE MUSCLES. 



The important general properties of the striated muscles are the following : 

 1. Elasticity ; 2. Tonicity ; 3. Sensibility of a peculiar kind ; 4. Contractility, 

 or excitability. These are all necessary to the physiological action of the mus- 

 cles. Their elasticity is brought into play in opposing muscles or sets of 

 muscles ; one set acting to move a part and to extend the antagonistic mus- 

 cles, which, by virtue of their elasticity, retract when the extending force is 

 removed. Their tonicity is an insensible and a more or less constant con- 

 traction, by which the action of opposing muscles is balanced when both are 

 in the condition of what is called repose. Their sensibility is peculiar and 

 is expressed chiefly in the sense of fatigue and in the appreciation of weight 

 and of resistance to contraction. Their contractility or excitability is the 

 property which enables them to contract under stimulation. All of these 

 general properties strictly belong to physiology, as do some special acts that 

 are not necessarily 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 presents a 

 certain number of elastic fibres ; and the sarcolemma is very elastic. It is 

 probably the sarcolemma that gives to the muscles their retractile power after 

 simple extension. 



It is unnecessary to follow out in detail all of the many 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 exten- 

 sion that is followed by complete retraction and this can not 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 -fa of an inch (0*5 mm.) by a weight of a little more than 

 300 grains (20 grammes). 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 (48 - 6 grammes). 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 physio- 

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

 during fatigue after excessive exertion, and 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 depend- 



