442 MANUAL OF PHYSIOLOGY. 



<;f refraction, which give the appearance of dark and light 

 bands. 



In the muscle fibre are found long granular masses like proto- 

 plasm ; these are the nuclei of the contractile substance. They 

 must not be confounded with the nuclei of the sarcolemma, which 

 are much more numerous along the edge of the fibre, or with the 

 other short nuclei seen in such numbers between the fibres, which 

 indicate the position of the capillary vessels. 



It is stated that each striated muscle fibre has a nerve fibre 

 passing directly into it, but the exact details of the mode of union 

 in mammalia are not yet satisfactorily made out. 



PROPERTIES OF MUSCLE IN THE PASSIVE STATE. 



Consistence. The contractile substance of muscle is so soft as 

 to deserve rather the name fluid than solid ; it will not drop as a 

 liquid, but its separated parts will flow together again like a half- 

 melted jelly. In this respect it resembles the protoplasm of ele- 

 mentary organisms, the buds from which are so soft that they can 

 unite around foreign bodies, and yet have sufficient consistence to 

 distinguish them from fluid. 



Chemical composition. The chemical composition of the con- 

 tractile substance of muscle in the living state is not accurately 

 known. The death of the tissue is accompanied by certain 

 changes of a chemical nature which give rise to a kind of coagu- 

 lation, resulting in the formation of two substances, viz., muscle 

 serum and muscle clot or myosin. This coagulation can be post- 

 poned almost indefinitely in the contractile substance of the 

 muscles of cold-blooded animals, by keeping the muscle after its 

 removal at about 5 C. In this way a pale yellow, opalescent, 

 alkaline juice may be pressed out of the muscle and separated on 

 a cold filter. This substance turns to a jelly at freezing point, 

 and on being allowed to come to the ordinary temperature of the 

 room it passes through the stages of coagulation seen in the 

 contractile substance of dead muscle, and gives the same fluid 

 serum and clot of myosin. Since a frog's muscle can be frozen 

 and thawed without the tissue being killed, it is supposed that 



