GENERAL MUSCLE PHYSIOLOGY 185 



tropic (dark) disk there is a narrow light band, called the median 

 disk of Hensen. The physiological importance of these structures 

 is still unknown. 



By double refraction exhibited by many crystals a single ray of 

 light is broken up into two rays. The double refractive muscle 

 substance has an optical axis in the longitudinal direction of the 

 fibres, in which the light is broken but once. The importance of 

 this doubly refracting substance of the muscle fibrils for the 

 property of contractility is not known. 



The motor-nerve fibres are connected with the muscle fibres, 

 the axis cylinder of the nerve fibre forming a flat arborization 

 (end-plate) which lies in contact with the muscle fibres. 



Nearly all the striated muscles can be voluntarily stimulated, 

 except the heart muscle. The stimulation of most of the smooth 

 muscles is not subject to our will, except the muscle of accommo- 

 dation of the eye. 



Ti_-iiiraction of the muscle takes place when it is 

 stimulated. _Ln_a stioiulated--muscle the physiological com- 

 bustion is increased, wfrprphy ^n^roy i s se t free which pro- 

 duces the contraction and performs the work. The manner 

 in which the potential chemical energy is transformed into 

 mechanical work is still unknown. 



1. THE RESTING MUSCLE 



i . Chemical properties of a resting muscle. 



(a) Composition of muscles. The reaction of a resting 

 muscle is neutral or feebly alkaline. A muscle contains 

 25$ solids, which include: 



I. Proteids 20$. 



If a fresh frozen muscle is cut up and the extract filtered at 

 about 3, a cloudy neutral or feebly alkaline fluid is obtained. 

 This is the fluid contents of the fibres or the muscle plasma. At 

 higher temperatures it coagulates spontaneously and, the higher 

 the temperature, the more speedily it clots. The coagulation is 

 due to the formation of an insoluble proteid, myosin, from a 

 soluble proteid of the muscle plasma, myosinogen, by the action 

 of the ferment. Coagulation also occurs during rigor mortis, 

 The myosin forms about 20$ of the muscle proteids. 



The solution which is left after myosin has been formed is called 

 the muscle serum. It has an acid reaction and contains about 



