The first in importance is the cow herself; for while her milk when secreted is sterile, and while there are no bacteria in her blood, nevertheless the cow is the most prolific source of bacterial contamination.  In the first place, the milk ducts are full of them.  After each milking a little milk is always left in the duct, and this furnishes an ideal place for bacteria to grow.  Some bacteria from the air or elsewhere are sure to get into these ducts after the milking, and they begin at once to multiply rapidly.  By the next milking they become very abundant in the ducts, and the first milk drawn washes most of them at once into the milk pail, where they can continue their growth in the milk.  Again, the exterior of the cow’s body contains them in abundance.  Every hair, every particle of dirt, every bit of dried manure, is a lurking place for millions of bacteria.  The hind quarters of a cow are commonly in a condition of much filth, for the farmer rarely grooms his cow, and during the milking, by her movements, by the switching of her tail, and by the rubbing she gets from the milker, no inconsiderable amount of this dirt and filth is brushed off and falls into the milk pail The farmer understands this source of dirt and usually feels it necessary to strain the milk after the milking.  But the straining it receives through a coarse cloth, while it will remove the coarser particles of dirt, has no effect upon the bacteria, for these pass through any strainer unimpeded.  Again, the milk vessels themselves contain bacteria, for they are never washed absolutely clean.  After the most thorough washing which the milk pail receives from the kitchen, there will always be left many bacteria clinging in the cracks of the tin or in the wood, ready to begin to grow as soon as the milk once more fills the pail The milker himself contributes to the supply, for he goes to the milking with unclean hands, unclean clothes, and not a few bacteria get from him to his milk pail.  Lastly, we find the air of the milking stall furnishing its quota of milk bacteria.  This source of bacteria is, how ever, not so great as was formerly believed.  That the air may contain many bacteria in its dust is certain, and doubtless these fall in some quantity into the milk, especially if the cattle are allowed to feed upon dusty hay before and during the milking.  But unless the air is thus full of dust this source of bacteria is not very great, and compared with the bacteria from the other sources the air bacteria are unimportant.

The milk thus gets filled with bacteria, and since it furnishes an excellent food these bacteria begin at once to grow.  The milk when drawn is warm and at a temperature which especially stimulates bacterial growth.  They multiply with great rapidity, and in the course of a few hours increase perhaps a thousandfold.  The numbers which may be found after twenty-four hours are sometimes inconceivable; market milk may contain as many as five hundred millions per cubic inch; and