Milk also contains all the necessary chemical substances to make a suitable bacterial food supply. Of the nitrogenous compounds, albumen is in a readily assimilable form. Casein, the principal nitrogenous constituent of milk, exists in an insoluble condition, and cannot be directly utilized, until it is acted upon by digesting enzymes. The fat in milk does not readily decompose, and while there are a few bacteria capable of splitting this substance, the majority of organisms are unable to utilize it. Milk sugar, on the other hand, is an excellent food for most species. [Illustration: Fig. 6.--Fat Globules and Bacteria. Note the relative size of the fat globules of milk and the lactic acid bacteria.] =Sources of contamination.= Inasmuch as milk is especially exposed to the inroads of bacterial growth, and because of the fact that much of the contamination can easily be prevented, it is highly important that the milk producer and dealer should be thoroughly cognizant of the various sources of contamination. The different factors concerned in contamination may be grouped as follows: the interior of the udder; utensils, including all apparatus with which the milk is brought in contact subsequent to withdrawal from the animal; infection coming from the animal herself, from the milker, and the surrounding air. =Condition of milk when secreted.= Immediately after withdrawal from the udder, milk always contains bacteria, yet in the secreting cells of the udder of a healthy cow, germ life does not seem to be present. Only when the gland is diseased are bacteria found in any abundance. In the passage of the milk from the secreting cells to the outside, it receives its first infection, so that when drawn from the animal it generally contains a considerable number of organisms. A study of the structure of the udder shows the manner in which such infection occurs. =Structure of the udder.= The udder is composed of secreting tissue (_gland cells_) that is supported by fibrous connective tissue. The milk is elaborated in these cells and is discharged into microscopic cavities, from whence it flows through the numerous channels (_milk sinuses_) that ramify through the substance of the udder, until finally it is conveyed into the _milk cistern_, a common receptacle holding about one half pint that is located just above the teat. This cavity is connected with the outside by a direct opening (_milk duct_) through the teat. During the process of milking, the milk is elaborated rapidly in the gland cells, and their contents upon rupture of the milk cells, flow down into the cistern. The normal contraction of the muscles at the lower opening of the outer duct prevents the milk from passing out except when pressure is applied, as in milking. The inner walls of the milk duct and cistern are always more or less moist, and therefore afford a suitable place for bacteria to develop, if infection once occurs, and conditions are favorable for growth.