MAMMARY SECRETION. 631 



secretion of the gland is thus evidently to be found in reflex action, the 

 afferent impulses passing through the sensory nerves of the nipple, the 

 secretory impulses passing through the inferior branch and the glandular 

 branch of this nerve. 



It is thus seen that, as far as we know, the mammary secretion is 

 dependent upon the amount of blood passing through the glands. 

 Changes in the general blood pressure, by modifying the blood supply 

 of the mammary gland, also influence the amount of milk secreted. 



Thus, various substances which act as stimuli to the vaso-motor 

 centre, and so produce increase of blood pressure, produce likewise an 

 increase in the amount of milk secreted. Strychnine in small amount, 

 digitalis, caffeine, and pilocarpine are all galactagogues and probably act 

 in this way, while through reduction of blood pressure, as by means of 

 chloral, the milk secretion may be considerably reduced. 



8. Milk Inspection and Analysis. — Good cows' milk is white, with a faint 

 . yellowish tint, and only bluish when diluted. If a drop of good milk is placed on 

 the thumb-nail it retains its shape instead of spreading out, as occurs when diluted 

 or unhealthy. Milk is most apt to be adulterated with water, which within cer- 

 tain limits may be detected by determination of the specific gravity. Unskimmed 

 milk possesses a higher specific gravity than that of the skimmed milk from the 

 effect of the removal of the fats, so that a milk from which all the cream has been 

 removed might, if dependence be placed upon the specific gravity alone, be con- 

 sidered as a better specimen than the pure milk. The average specific gravity of 

 normal cows' milk may be placed at about 1030 at 60° F. ; if diluted with half its 

 volume of water the specific gravity will fall to about 1014 or 1016. As a conse- 

 quence, by the determination of specific gravity a general idea may be obtained 

 as to-how much water has been added to diluted milk. The following table may 

 serve to assist in this determination : — 



With With 



Skimmed Milk. Unskimmed Milk. 

 A specific gravity of 1037 to 1083 or 1033 to 1029 indicates a pure milk. 



" " 1033 to 1029 or 1029 to 1026 " milk with 10 per cent, water. 



" " 1020 to 1026 or 1026 to 1023 " " " 20 " " 



" " 1026 to 1023 or 1023 to 1020 " " " 30 " 



" " 1023 to 1020 or 1020 to 1017 " "■ " 40 " " 



The instrument by which the specific gravity of milk is determined is usually 

 termed the lactometer, and simply consists of a hydrometer with a scale running 

 from 1000, which is the specific gravity of distilled water and is marked zero on 

 the scale, to 1034, marked 120, which is the specific gravity of a rich sample of 

 milk. In using the lactometer special attention must be paid to the physical 

 characteristics of the milk, since a little attention would readily detect skimmed 

 milk from unskimmed, although their specific gravity might be the same. In 

 milk rich in cream where the specific gravity might be abnormally low, its 

 physical appearance and the fact that it clings to the instrument would enable it 

 to be recognized, while watered or skimmed milk is bluish and does not cling to 

 the lactometer ; so, if a sample of milk should read above 110 on the lactometer 

 without manifestly being full-bodied, it would be only fair to presume that a 

 portion of the cream had been removed. Milk diluted with spring-water may be 

 recognized by the detection of nitrates in the milk. Sulphuric acid is added to 

 the milk, the precipitate filtered off, the filtrate distilled, and nitric acid looked for 

 in the distillate. This may be readily accomplished by converting, through milk- 

 sugar, the nitric into nitrous acid. A few drops of pure H ? S0 4 , potassium iodide 

 solution, and boiled starch solution are then added to the distillate; if nitrous acid 

 is present iodine is liberated from the potassium iodide solution and the starch is 

 colored blue. 



