224 THE MAMMARY GLANDS. morphosis. They retain still, as it were, the traces of organization; they belong rather to the hydrocarbon family than to the nitrogenized. may be that, for the removal of these, cell action is necessary. It from the Mal pighian sac. Whatever importance may be attached to such a suggestion, it is very Mode of remov- clear that, notwithstanding the extreme thinness of the walls al of the liquid of the tuft vessels, the relaxation in the speed of the blood current through them, and the pressure brought to bear upon them, that water could not be separated by oozing through them unless there was an additional provision. The sac into which the exudation is to take place is already full, and it may be questioned whether ciliary motion in the uriniferous tubes would exert a sufficient exhaustion to relieve the interior of the capsule from pressure; but the introduction of a liquid of a different nature into the uriniferous tube may call at once into operation the principle described at page 131 as acting in the capillary circulation of the blood, and thus the contents of the Malpighian sac are drawn forward into the uriniferous tube, just in the same manner that water is drawn from the inside of a bladder through the pores thereof by alcohol on the outside. THE MAMMARY GLANDS. The mammary glands are situated on various portions of the abdominal and thoracic surfaces of animals of the class mammalia. Description of the mammary In the higher members of this class they present the appearance of racemose glands, rudimentary in the males, but well developed in the adult females, especially after parturition. They separate from the blood the white secretion, milk. gland. In the ornithorynchus the mammary gland consists of an obtuse cone of cœcal follicles, ending upon an areolar surface. There is no nipple. The milk is expelled, both in these and the marsupials, by direct muscular pressure. In cetaceans the nipple is included in a cleft of the inIts compara- tegument, but in the higher mammalia it projects, so that, betive anatomy. ing received into the mouth of the young, and suction being made, the pressure of the air takes effect upon the surface of the gland and expels the milk. In different cases the number of mammæ differs. In the human species there are but two, placed upon the thoracic surface, and from their position favoring the care and nursing of the child. Among other animals the number seems to have a relation to the number of young brought forth at a birth, there usually being a pair for each one. Many exceptions to this rule, however, occur. Its structure. The mammary gland corresponds in anatomical structure to the parotid and pancreas. It consists of 15 or 20 lobes, each from to 1 inch in width; these are composed of lobules, and these, again, of cœcal vesicles. The excretory ducts are lined with tesselated STRUCTURE OF MAMMARY GLAND. 225 epithelium. The ducts converge toward the nipple, opening upon it by 10 or 15 apertures, and in their course dilating into ampullæ, of small capacity in women, but in the cow capable of holding a quart. Fig. 96. Development of the mammary gland. As regards its development, the mammary gland originates in the fourth or fifth Its developmonth as a papillary projection of the ment. mucous layer of the epidermis, as shown in Fig. 96, in which 1 is the rudimentary gland in the male embryo of five months, a being the horny, b, mucous layer of the epidermis; c, process of the latter, the rudiment of the gland; d, fibrous membrane round it. At 2 is the lacteal gland of a female embryo of seven months, seen from above: a, central substance of the gland; b, c, budding outgrowths, the rudiments of the gland lobes. (Kolliker.) Fig. 97, vertical section of the human mammary gland: a, a, its pectoral surface; b, b, skin on surface of the gland; c, skin of nipple; d, lobules and lobes of gland; e, lactiferous tubes passing from the lobules to the nipple. Fig. 97. Section of the human mammary gland. As pregnancy advances, the cells of the gland begin to contain fat, in a manner not unlike that which is remarked in the cells of the sebaceous follicles of the skin. When the gland becomes active after parturition, it is stated that the first-formed milk-cells break up in the lactiferous ducts into milk globules, their membrane and nucleus disappearing. The milk globules are minute particles, varying in their diameter from the Milk globules. 3000 to the 13000 of an inch. They consist of oily material inclosed in an envelope, as is shown by the fact that, though they will resist for a short time the action of sulphuric ether, they are finally dissolved by that substance. Besides these milk globules, there are other exceedingly minute fat particles present. The milk which is first secreted after delivery contains corpuscles of considerable size, and of a granulated appearance, as seen in the photograph, Fig. 98. They are called colostrum corpuscles. They are soluble in ether, and therefore Milk with colostral corpuscles. contain fat. There is reason to suppose that all the fat globules of the milk are inclosed in cyst-like pellicles of casein. In the chapter on food (Chapter II.), a general description of the character and constitution of milk has been given, together with its physio Fig. 98. Р Colostrum. 226 COLOSTRUM AND MILK. Properties of logical relations in nutrition. It may now be added that fresh milk. milk presents an alkaline reaction, which continues longer in the milk of women than in that of cows. Left to itself, and the more quickly the warmer the air, milk turns sour through the production of lactic acid, the casein undergoing coagulation. That the oil globules just spoken of are coated with a film of a coagulated protein body appears from the circumstance that it may be dissolved by acetic acid, and the included butter is then set free. milk. One of the simplest methods for the analysis of milk consists in coagAnalysis of ulating it at a temperature of 2120 with pulverized gypsum; the mass, being then evaporated to dryness, is pulverized, the butter being extracted by ether, and the sugar and soluble salts by hot alcohol. The amount of the soluble salts thus obtained may be determined by incineration; and since their amount is to that of the insoluble salts as 5 to 7, an approximate determination of the latter may be made, and thereby the weight of the sugar and casein corrected. This is the method of Haidlen. Relation of colostrum and milk. It would appear, from examinations that have been made of the secretion of the mammary gland previous to parturition, that it contains albumen in the place of casein, the casein gradually appearing as the period of parturition approaches, but not reaching its maximum until a few days after that event. Colostral milk differs essentially from the subsequent ordinary secretion, as the following table shows: Constitution of Colostrum and Milk. (From Simon.) The specimens here presented were obtained from the same individual: and from the table it appears that the colostrum contains a much larger proportion of solid material than the milk. The quantity of fat is nearly double; the quantity of sugar is likewise much greater, but the relative quantity of casein is less, this being in accordance with the statement that the production of that substance approaches gradually to a maximum which is not attained till a few days after parturition. The composition of milk varies with many circumstances. Thus, Variability in among cows, it is well known that there are certain breeds its composition. which yield a milk in which butter predominates; in others, a milk in which there is an excess of casein. It is in reference to this that such are, among agricultural people, often described as good butter cows, or good cheese cows, as the case may be. Such variations are likewise often popularly referred to peculiarities in the color of these animals; and, indeed, there is a general impression of the same kind as respects the milk of women, that that of fair women is inferior to that of brunettes. L'Heritier, who has examined into this matter, selected two females of the same age, 22 years, and caused them to adopt the same diet and the same mode of life. The one was a blonde, the other a bruThe following table exhibits the most marked of his results: nette. Milk of Women of different Temperaments. (From L'Heritier.) The average of the various analyses he made shows the same general result, though not so strikingly, the number being for the solid constituents, in the case of the blonde, 120, and for that of the brunette, 134. per As would be expected, the constitution of the milk varies greatly with the diet. Simon found that in the case of a very poor woman, Influence of who had been almost deprived of the necessaries of life, the diet on milk. quantity of solid material was only 8.6 per cent. On giving her a nutritious meat diet it rose to 11.9 per cent. Being again reduced, by circumstances, to the utmost destitution, the solid residue sank to 9.8 cent.; and on once more being supplied with a nutritious meat diet, the percentage rose to 12.6. These results illustrate in a striking manner, as will be presently seen, the function of the mammary gland. Simon also found, in this particular case, that the relative quantities of casein and sugar do not greatly vary with these extreme dietary variations, but that the absolute quantity of butter does. On the two occasions of starvation, it was as low as 8 parts in 1000 of milk, and on the two of full nutritious diet, it rose to 34 and 37 respectively. From this it seems to follow that while the amount of butter in milk is determined by the quantity and quality of the food, the amounts of casein and sugar are, to a considerable degree, independent thereof, and hence I believe their origin is to be attributed to changes taking place in the system, and that these substances are more immediately furnished from metamorphoses of its structures. Origin of the casein and of the butter. The casein and the sugar are reciprocally related to each other, the quantity of casein steadily increasing from the time of par- Relative quanturition until a fixed proportion is attained. At parturition tity of casein the quantity of sugar is at its maximum, a gradual decline then occurring until its proportion likewise becomes nearly constant. and sugar. 228 ACTION OF THE MAMMARY GLAND. Saline substances administered by the stomach or rectum do not alExtraneous ways appear in the milk; thus the ferrocyanide of potassisalts in milk. um, which may be quickly detected in the urine, can not be found in the milk. It is curious, that when iodide of potassium has been administered to the mother, in doses, for example, of three grains thrice a day, it can be readily detected in the urine of the infant by the usual test of starch and nitric acid. Diurnal quan The diurnal quantity of milk yielded by the human female has been estimated at from 32 to 64 ounces. This estimate is made. tity of milk. by determining the weight of the infant before and after suckling. Although a certain proportion is present in the gland, the secretion appears to take place for the most part with great rapidity. On the application of the infant the blood flows suddenly, and the milk pours into the ducts, constituting what is termed the draft. We now enter on a consideration of the function of the mammary Mode of action gland, with a view of determining whether it acts in virtue of the mamma- of its special construction, whether it fabricates in itself, by ry gland. the agency of cells, the proximate constituents of milk, or whether it merely strains them from the blood in which they pre-exist. Influence of Due weight should here be given to the fact that, unlike the excretions of the lungs, the kidneys, or even the liver, the milk contains a very large percentage of histogenetic or formative bodies. Its casein can not be considered as in the career of retrograde transformation, since in the body of the infant it is presently changed into albumen. Such a fact might even lead us to suspect that we should detect some essential structural and functional differences between the mamma and other glands. The influence of special structure is, however, disposed of by the numerous well-authenticated cases now on record, in which porspecial struc- tions of the skin, or the stomach, the navel, intestines, the axilla, and glands in the groin have assumed a vicarious action, and secreted milk; and though it has been said of the latter instance that it may be nothing more than an obscure manifestation of an attempt in the human species at a repetition of the mammary gland in a region near which it is normally present in the lower mammals, such a remark has no application in the other cases. We may therefore infer that the proximate constituents of the milk are not manufactured by reason of any special structure of the gland which secretes them, since other structures can assume a vicarious action. ture. This therefore narrows our inquiry down to the point, Does the mammary gland merely filter off from the blood substances already existing in it, or, those substances not so pre-existing, are they made in this organ by cells? Of the proximate elements of milk, many, such as the entire group |