32 VETERINARY BACTERIOLOGY 



with seeming justification, believe that this is either an excreted 

 material or merely an outer swollen and differentiated portion 

 of the cell wall. Chemically this material differs in the various 

 capsulated bacteria. In some cases it is composed of mucin, a 

 slimy material made up of a protein-like substance united with 

 a carbohydrate, and resembling the mucus secreted by certain 

 body cells. In other cases the capsule is made up of pure carbo- 

 hydrates and is closely allied to certain of the vegetable gums, such 

 as gum arabic and gum tragacanth. The capsule of some bacteria 

 is partially or wholly soluble in water. When such an organ- 

 ism grows in suitable nutrient solution it renders the medium 

 slimy or gelatinous. Slimy milk, bread and whey are caused by 

 the luxuriant growth of such organisms. 



Fig. 10. Bacteria showing sheaths. 



Some of the filamentous bacteria produce a firm sheath or tube 

 outside of the cell, this sheath usually inclosing an entire filament 

 or chain of cells. In composition it probably closely resembles 

 the cell wall. In some cases it is impregnated with iron oxid, 

 It is possible that the sheath is a modified type of capsule. 



Cell Protoplasm. The living material within the cell wall 

 is called the protoplasm. Structurally it may usually be dif- 

 ferentiated into two layers, an outer thin layer lying closely 

 appressed to the cell wall, and an inner portion. The outer layer 

 or ectoplast performs one of the most important functions of the 

 cell, as this is the membrane, and not the cell wall, that determines 

 what materials in solution may enter and what may leave the 

 cell; through it must pass by diffusion all the food that enters the 

 cell. When certain bacteria, as the cholera spirillum, are placed 

 in a strong solution of some salt which does not readily pass through 



