34 CONTRACTILITY 



subject to control by impulses arriving from the nervous 

 centres, these impulses serving either to increase or to 

 decrease the degree of tonus and the rate and force of the 

 rhythmic contractions. 



Commonly, unstriated muscles are supplied by two 

 different nerves, one augmenting, the other suppressing a 

 pre-existing state of activity. Herein lies another distinc- 

 tion between the two classes, for variations in the con- 

 traction of striated muscles are brought about, so far as 

 is known, only by variation in one direction or the other 

 of a constant flow of impulses along one and the same 

 nerve. 



Heart muscle occupies an intermediate position between 

 the two classes. Structurally it exhibits a faint cross- 

 striation and continuity from cell to cell. It resembles 

 unstriated muscle in its rhythmic power, in its independence 

 of the central nervous system, and in its double nerve- 

 supply. It resembles striated muscle in the strength of 

 its contraction. 



Composition of Muscle 



If muscle-tissue be minced at 0° C, extracted with 

 NaCl solution and the mixture filtered, a filtrate is obtained 

 which consists of an opalescent fluid^ — muscle plasma. 

 This consists of two proteins, an albumin and a globuhn, 

 which have been called myosinogen and 'paramyosinogen 

 respectively. On slightly raising the temperature this 

 fluid, hke blood-plasma, undergoes coagulation, the two 

 proteins being converted into an insoluble form — -fibrin. 

 From being neutral or slightly alkaline, the reaction becomes 

 acid — a change attributable to the development of sarcolactic 

 acid. The residue which is left behind on the filter-paper 

 consists principally of what may be called the incidental 

 constituents of muscle — fibrous and nuclear material and 

 sarco lemma. 



The serum which can be squeezed out of the muscle 

 clot consists of a pigment, myohamatin (related to hsemo- 



