32 GENERAL MICROBIOLOGY 



nutrient gelatin upon sterile, flat pieces of glass. The 

 student on becoming familiar with the difficulties of pre- 

 paring satisfactory plates with the use of the " Petri dish " 

 will appreciate those met with in Koch's first gelatin plates. 



Gelatin is a protein, i.e., a nitrogenous food material. 

 It contains as its essential elements carbon, hydrogen, 

 oxygen, and nitrogen (other elements, however, such as 

 sulphur, phosphorus, etc., may be present). Its empirical 

 formula according to Schiitzenberger and Bourgeois is 

 C7eHi24N24O29, but such a formula only gives information 

 of the chief constituents and allows one to form some idea 

 of the huge size of the molecule; no idea of the structure 

 of the molecule is given. However, by digesting with 

 dilute sulphuric acid, gelatin breaks down in the same way 

 as the proteins, yielding glycin, leucin and other fatty 

 amino-acids. 



Gelatin is an animal protein, but does hot occur as 

 gelatin in the animal tissues. It exists there as the albu- 

 minoid collagen which is the principal solid constituent of 

 fibrous connective tissue, being found also, but in smaller 

 percentage, in cartilage, bone and ligament. Collagen from 

 these various sources is not identical in composition and 

 gelatin varies correspondingly, e.g., gelatin from cartilage 

 differs from that of other sources in that it contains a lower 

 percentage of nitrogen. 



Gelatin, the body resulting from the hydrolysis of 

 collagen, is also an albuminoid. (Hofmeister regards this 

 hydrolysis as proceeding according to the equation : 



collagen + water = gelatin 



but in dealing with substances of such variable composition, 



empirical formulae of this kind have no great significance). 



Commercially, it is prepared from certain kinds of bones 



and parts of skin. These are selected, washed and extracted 



