View report page

64
For this microscopic examination I proceed as follows:— The milk is poured away
as far as possible, leaving only the sediment. I then take up into a sterile capillary pipette
a little of the sediment from the extreme apex of the conical glass. One very small
drop is placed on a sloped agar culture tube and distributed over the surface; the tube
is then incubated at 37° C. for 24 hours, and afterwards at 20° C. A. second small
drop is placed on a slide and spread into an even film; this is fixed, when dry, in
absolute alcohol and methylene blue. A third drop is placed on a slide, covered with a
cover glass, and examined unstained for crystals, cells, &c.
By this routine, carried out in exactly the same way in every case, a fair, if rough,
comparison can be made between different samples. But no exact quantitative
comparison is thus possible, and for determining the cellular content of selected samples
I have employed more refined methods. For the estimation of the number of cells in
condensed skimmed milk it suffices to take a measured amount and dilute it four or five
times with 1 per cent, caustic potash.
The cells can then be counted with a haemocytometer and the number per cubic
millimetre easily estimated. In milks containing the full cream this is not possible.
Here I take a measured amount into a centrifruge tube, dilute with weak caustic potash,
centrifugalise for 10 or 15 minutes, pipette off the supernatant fluid, fill up again with
normal saline, and again centrifugalise. The fluid is again pipetted off and Joison's
fluid is added up to the original amount of condensed milk. The mixture is now well
stirred and the cells estimated by the hsemocytometer.
The nature of the cells present is determined by a differential count on films
stained with methylene blue. In a condensed milk the cells have suffered such changes
that more refined methods of staining are almost useless. I have tried Leishman's
stain and Ehrlich's triacid stain, but they give but vague representations compared
with the fresh cell. Such differential counts are not easy; certain cells are clearly
recognisable as mononuclear and others as polynuclear, while others are plainly
squamous epithelium, but there are many cells which cannot be assigned with certainty
to any category. I have carried out differential counts of the cells in 14 cases, but I
have contented myself with estimating the percentage number of cells which in my
judgment were certainly polymorphonuclear leucocytes. In a few cases I have resorted
to animal experiment. In six consecutive samples of condensed milks, early in the series,
I injected the sediment from three teaspoonfuls of the milk subcutaneously into guineapigs.
These were samples of machine skimmed milk which I was disposed to condemn.
The Cellular and Bacterial Content of Normal Fresh Milk.
This has been amply studied, and the following facts are generally accepted:—
Fresh milk from normal cows contains from 50 to 100 up to more than 1,000 cells per
cubic millimetre. Of these some are squamous epithelium from the teats, some are
epithelial cells from the mammary gland and ducts, and some are mononuclear cells
resembling lymphocytes.
But a large proportion are cells with a lobed and twisted nucleus resembling ordinary
polynuclear leucocytes. Some observers have considered these to be derived from mammary
epithelium, but they cannot be distinguished histologically from leucocytes, and my own
opinion is that they are leucocytes. The important point is that these cells, whatever
their nature, are present in considerable numbers in milk from selected normal cows