98 GENERAL PHYSIOLOGY 



extent that it overcomes the weight of the rest of the protoplasmic 

 body. Such cases are realised in the fat-cells of the subcutaneous 

 connective tissue in man and many animals ;■ if such tissue be 

 thrown into water, it floats upon the surface. For this reason fleshy 

 men, in swimming, have to make less efl'ort to maintain themselves 

 above the water than thinner persons. Other substances in the 

 cell-body can play the same rSle as fat, particularly bubbles of gas, 

 which under certain circumstances can lower the specific gravity 

 .of the whole body of the cell enormously — a phenomenon that 

 occurs in many shell-bearing fresh-water Bhizopoda (Arcella, 

 Dijflugia). 



It follows from this fact that by the accumulation of lighter or 

 heavier substances the cell under certain circumstances can 

 actively diminish or increase its specific gravity, and, therefore, 

 can actively rise or sink in water without employing locomotive 

 organs. Under many conditions, e.g., when the vital conditions 

 become unfavourable in the place where the organism lives, such a 

 power is of great importance for the life of the organism. In all 

 cases, however, where cells are found that are lighter than water, 

 certain elements only are lighter, the whole protoplasm never. 

 The ground-mass of the protoplasm appears always to be slightly 

 heavier than water. 



3. The Optical Properties of Living Substance 



In most cases protoplasm is entirely colourless or grey; in 

 thin layers free from solid contents it is transparent, in thick layers 

 opaque. It refracts light somewhat more strongly than water. 



As regards details, the various forms of living substance behave 

 differently according to the condition of their constituents. Some 

 solid elements, such as fat-droplets, drops of water, and chlorophyll 

 grains, can be intensely coloured, so that the cells in which they 

 are present in great quantities appear yellow, red, green, etc., as, 

 e.g., in plant tissues. The power of refracting light also differs 

 with the individual constituents, that of water-droplets in the 

 vacuoles is less, that of fat-droplets greater than that of the 

 ground-substance. It would carry us too far to examine all the 

 individual cases, but it is of interest to consider somewhat in 

 detail the behaviour of one form of living substance, viz., the 

 so-called contractile substance, i.e., amoeboid protoplasm, cilia, and 

 muscle-fibres, which execute definite changes of form, called 

 contractions. 



In the first half of the century Boeck found that certain 

 elements of the cross-striated muscle-fibre are doubly refractive, 

 i.e., are able to divide a ray of light into two rays, which are 

 transmitted with different velocities. Later, Brticke, especially 

 investigated this property in detail. Still later, Engelmann (75) 



