10 THE BODY AT WORK 



he obtains the same pattern with several different methods, 

 he infers that the appearance which he sees is that of a structure 

 existing in the living cell ; but he is never quite sure that it is 

 not an arrangement produced by reagents after death. 



The tendency of protoplasm to dispose itself in the form 

 of a network or spongework is of the greatest interest in its 

 bearing upon the theory of its activity in effecting chemical 

 change. The body itself, as we shall find later, is a network 

 of tissues enclosing lymph. The lymph in the tissue-spaces 

 contains foods and waste products in solution. The tissues are 

 constantly taking from it the former, and discharging into it 

 the latter. Every cell is, microscopically, a tissue. The 

 strands of its protoplasm are perpetually sorting foods from 

 its cell- juice, adding to its cell- juice waste products. By 

 diffusion, foods, including oxygen, pass from lymph to cell ; 

 waste products, including carbonic acid, pass from cell to 

 lymph. If water be added to gum, the gum swells. The 

 mixture is homogeneous. Diffusion takes place slowly through 

 the mucilage. When water is taken up by protoplasm, the 

 protoplasm swells ; but the mixture is not homogeneous. 

 The protoplasm expands as a wet sponge expands, although 

 the relation of the enclosing reticulum to the water which it 

 encloses is far more complicated. It is, as it were, a sponge 

 made of gum. Some water is combined with the protoplasm ; 

 the remainder fills its spaces. There is an active surface 

 relation between the free water and the protoplasmic threads. 

 As water rises in a capillary tube, as it passes from the inside 

 to the outside of a flannel shirt, so it circulates within the cell. 



Irritability is a property commonly attributed to proto- 

 plasm, but it is a little doubtful whether there be not 

 again some danger of an illogical use of terms. An amoeba, 

 one of the unicellular organisms found in ponds, has the power 

 of moving. If a piece of a water-plant the stalk of duck- 

 weed is a suitable object be examined with the microscope, 

 these little animals are usually to be found upon its surface. 

 They feed upon algae more minute than themselves. When they 

 come in contact with something suitable for food, their body- 

 substance flows around it. The food is coagulated. So much 

 of it as is digestible is digested ; the remainder is extruded. 

 Constantly parts of the body-substance are protruded, other 



