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The quantity of milk secreted by the human female in 24 hours, during the period of lactation, is about two to three pints; the quantity removed by the infant from a full breast at one time being about two ounces.
COMPOSITION OF MILK.
Casein is the nutritive principle of milk, and constitutes its most important ingredient. It is held in solution by an alkali, but upon the addition of an acid it undergoes coagulation, passing into a semi-solid form. The presence of lactic acid, resulting from a transformation of milk sugar, causes spontaneous coagulation to take place.
The fatty matter is more or less solid at ordinary temperature, and consists of margarin and olein ; when subjected to the churning process the globules run together and form a coherent mass, the butter.
When milk is allowed to stand for a varying length of time, the fat globules rise to the surface, forming a layer more or less thick, the cream.
Milk sugar or lactose is an important ingredient in the food of the young child; it is readily transformed into lactic acid in the presence of nitrogenized ferments.
Influences Modifying the Secretion.-During lactation there is a demand for an increased amount of fluid, and if not supplied, the amount of milk secreted is diminished. Good food in sufficient quantity is necessary for the proper elaboration of milk, though no particular article influences its production.
Mental emotion at times influences the character of the milk, decreasing the amount of its different constituents.
Mechanism of Secretion.—The water and salts preëxist in the blood and pass into the gland vesicles by osmosis. The casein, fatty matter, and sugar appear only in the mammary gland, but the mechanism of their formation is not understood.
Colostrum is a yellowish, opaque fluid, formed in the mammary glands toward the latter period of utero-gestation; it consists of water, albumin, fat, sugar, and salts, and acts as a laxative to the newly-born infant.
VASCULAR OR DUCTLESS GLANDS. The Vascular Glands are regarded as possessing the power of acting upon certain elements of the food and aiding the process of sanguinification; of modifying the composition of the blood as it flows through their substance, by some act of secretion.
The vascular glands are the spleen, supra-renal capsules, thyroid and thymus glands.
The Spleen is about 5 inches in length, 6 ounces in weight, of a darkbluish color, and situated in the left hypochondriac region. It is covered externally by a reflection of the peritoneum, beneath which is the proper fibrous coat, composed of areolar and elastic tissue and non-striated muscular fibers. From the inner surface of the fibrous envelope processes or trabeculæ are given off, which penetrate the substance of the gland, forming a network, in the meshes of which is contained the spleen pulp. The splenic artery divides into a number of branches, some of which, when they become very minute, pass directly into veins, while others terminate in true capillaries.
As the capillary vessels ramify through the substance of the gland, their walls frequently disappear and the blood passes from the arteries into the veins through lacunæ (Gray).
The splenic or Malpighian corpuscles are small bodies, spherical or ovoid in shape, the zo of an inch in diameter, situated upon the sheaths of the small arteries. They consist of a delicate membrane containing a semi-fluid substance composed of numerous small cells resembling lymph corpuscles. The spleen pulp is a dark-red, semi-fluid substance, of a soft consistence, contained in the meshes of the trabeculæ. In it are found numerous corpuscles, like those observed in the Malpighian bodies, blood corpuscles in a natural and altered condition, nuclei, and pigment granules.
Function of the Spleen.- Probably influences the preparation of the albuminous food for nutrition ; during digestion the spleen becomes larger, its contents are increased in amount, and after digestion it gradually diminishes in size, returning to the normal condition.
The red corpuscles are here disintegrated, after having fulfilled their unction in the blood, the splenic venous blood containing relatively a small quantity.
The white corpuscles appear to be increased in number, the blood of the splenic vein containing an unusually large proportion.
The spleen serves also as a reservoir for blood when the portal circulation becomes obstructed.
The nervous system controls the enlargement of the spleen; division of the nerve produces dilatation of the vessels, stimulation contracts them.
The Suprarenal Capsules are triangular, flattened bodies, situated above the kidney. They are invested by a fibrous capsule sending in trabeculæ, forming the framework. The glandular tissue is composed of two portions, a cortical and medullary. The cortical being made up of small cylinders lined by cells and containing an opaque mass, nuclei, and granular matter. The medullary consists of a fibrous network containing in the alveoli nucleated protoplasm.
The Thyroid Gland consists of a fibrous stroma, containing ovoid closed sacs, measuring on the average to of an inch, formed of a delicate membrane lined by cells; the contents of the sacs consist of yellowish albuminous fluid.
The Thymus Gland is most developed in early life and almost disappears in the adult. It is divided by processes of fibrous tissue into lobules, and these again into follicles which contain lymphoid corpuscles.
The functions of the vascular organs appear to be the more complete elaboration of the blood necessary for proper nutrition ; they are most highly developed during infancy and embryonic lise, when growth and development are most active.
The Principal Excrementitious Fluids discharged from the body are the urine, perspiration, and bile; they hold in solution principles of waste which are generated during the activity of the nutritive process, and are the ultimate forms to which the organic constituents are reduced in the body. They also contain inorganic salts.
The Urinary Apparatus consists of the kidneys, ureters, and bladder.
The Kidneys are the organs for the secretion of urine; they resemble a bean in shape, are from four to five inches in length, two in breadth, and weigh from four to six ounces.
They are situated in the lumbar region, one on each side of the vertebral