Physical Properties of Protoplasm 159 



is removed. Beautiful but misleading diffraction phenomena are 

 to be observed when a piece of muscle cell is stretched. If the point 

 of a very minute needle be pushed into a muscle cell, it can be moved 

 in one direction about as easily as another. 



The optical image of striped muscle is very misleading. Dissec- 

 tions have shown that the dark bands seen in living muscle are pro- 

 duced by concentrated areas of muscle substance which absorb enough 

 transmitted light of low intensity to appear as dark bands in the 

 optical image. I have been unable to dissect out definite fibrils. 

 The substance lying between the concentrated regions and appearing 

 as light bands is a highly viscous, elastic gel and has no physical 

 properties that serve to distinguish it from the surrounding sarco- 

 plasmic gel. By cutting the dark band to pieces, small masses of 

 highly concentrated muscle substance, frequently less than one 

 micron in diameter, are partially freed from the dilute enveloping 

 gel and in light of low intensity show well-defined diffraction halos. 

 The appearance of dark bands in the optical image, then, is produced 

 by absorption of light waves by the concentrated muscle substance; 

 the light bands, by the low absorptive power of the diluter inter- 

 mediate gel, and the diffraction of the light waves by the edges of the 

 concentrated substance. Striking changes in the optical image that 

 are well known can be produced by increasing the intensity of illumi- 

 nation. The dark band becomes cloudy and more or less opalescent 

 and the light band may show an intersecting dark line or well-defined 

 diffraction fringes just outside the geometrical shadow of the con- 

 centrated substance. Hence, absorption, diffraction, refraction and 

 dispersion are involved in the formation of the optical image of striped 

 muscle and the former two particularly when the illumination is of a 

 relatively high intensity. 



The nuclear substance is a gel that is for the most part compara- 

 tively dilute but contains more concentrated areas in the form of 

 granules and an imperfect network. The appearance of a network 

 in the optical image is due not to definite fibrils but to more con- 

 centrated parts of the gel that grade into the diluter nuclear substance. 



On the outer surface of the muscle cell is found a highly trans- 

 lucent membrane, the sarcolemma, which is extremely elastic and 

 measures about one micron in thickness. It is stuck to the whole 

 outer surface of the muscle cell and is viscous and cohesive enough 



