Fore 200th 2000th 4300th 6500th Strippings. milk. cc. cc. cc. cc. Expt. 1 6,500 1,700 475 220 75 5 Expt. 2 8,100 1,650 400 240 50 10 By some observers it has been claimed that it is possible to secure absolutely sterile milk in the strippings but this is rarely so. It is quite probable that such reported results are due to the fact that too small quantities of milk were used in the examinations and so erroneous conclusions were drawn. This marked diminution in numbers indicates that the larger proportion of the organisms found in the fore milk are present in the lower portion of the udder and milk ducts. When consideration is given to the structure of the udder, it is readily apparent that infection will be greater here than above. [Illustration: FIG. 8. Sectional view of udder showing teat with milk duct connecting exterior with the milk cistern. Milk sinuses are mostly shown in cross section interspersed and below the secreting tissue (Moore and Ward).] The udder is composed of secreting tissue (_gland cells_) held in place by fibrous connective tissue. Ramifying throughout this glandular structure are numerous channels (_milk sinuses_) that serve to convey the milk from the cells where it is produced into the _milk cistern_, a common receptacle just above the teats. This cavity is connected with the exterior through the milk duct in the teat, which is more or less tightly closed by the circular sphincter muscles, thus preventing the milk from flowing out. The mucous membranes of the milk duct and cistern are naturally moist. The habits of the animal render it impossible to prevent infection of the external opening at the end of the teat and there is no mechanical reason why bacteria cannot readily find their way along the moist lining membrane for some distance. If organisms are adapted to this kind of an environment, ideal conditions exist for their multiplication, as moisture, warmth and suitable food supply are present. The question arises how far up into this organ is penetration possible? Within late years numerous observations have been made on the presence of organisms in the upper portion of the udder in contact with the secreting tissue.[8] These investigations prove that bacteria are distributed throughout the whole of the udder, although numerically they are much less abundant in the region of the milk-secreting tissue than in the lower portion. Ward's conclusions are "that milk when secreted by the glands of a healthy udder is sterile. It may however, immediately become contaminated by the bacteria which are normally present in the smaller milk ducts of the udder."