30 



VETERINARY BACTERIOLOGY 



thickened, and the bacteria, either cocci or bacilli, are imbedded in 

 a mass of gelatin. Such a mass of cells is called a zooglea. 



Size of Bacteria. The unit of microscopic measurement is 

 the micron (pi. micra or microns) and is indicated by the Greek 



Fig. 7. Types of spirilla. 



letter p. It is the one-thousandth part of a millimeter or, approx- 

 imately, the one twenty-five-thousandth part of an inch. 



Bacteria vary considerably in size, from forms OJj^ or less 

 in diameter, barely visible under the microscope, to forms 



Fig. 8. Types of filamentous bacteria: A, Leptothrix; B, Cladothrix; C, 

 Nocardia; D, Actinomyces or Streptothrix. 



100 H* or more in length. Most bacteria are bet ween ().f> and 

 5 li in diameter and 0.5 {t and 10 ^ in length. Some bacteria 

 are undoubtedly too small to be seen with the highest power < of 

 our microscopes, hence less than 0.1 jJ. in diameter. We know of the 

 existence of these organisms by their effects. The organism caus- 



