Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/320

Rh 302 MILK so that the young readily reach them. The Insectivora have, as a rule, more nipples than are found in any other order. Thus in the tenrec (Centetes) there are as many as twenty-two, and they are rarely fewer than fourteen, spread out in pairs from the pectoral to the inguinal regions. There are ten teats in the common hedgehog, six to eight in moles and shrews, two in sloths and armadillos. In Cetacea there are two long, narrow, flat glands lying between the dermal and abdominal muscles, with the sub cutaneous blubber separating them from the skin. The peculiarity of the arrangement in these animals, where suckling is performed under water, is the large size of the central duct, which acts as a kind of reservoir, so that the young may obtain a considerable supply in a very short time. It would appear also that when suckling takes place the nose of the young is above the surface of the water. Among Ungulates, in the elephant the glands and teats are between the fore legs ; in the rhinoceros they are inguinal ; in the mare and ass the glands are two in number, and are found between the thighs, about 9 inches in front of the vulva; the tapir has two inguinal nipples, the peccary two ventral and two inguinal, the wild sow eight nipples, whilst in the domestic breeds there are at least ten, extend ing from the pectoral to the inguinal regions. Kuminants have the glands aggregated into a round mass in the inguinal region, pendulous in full function, divisible into two glands, each of which has a large reservoir. When in use the teats, one pair or two pairs being the number, in connexion with the reservoirs become so large as to receive the special name of &quot;udder.&quot; All the deer tribe, camels, the giraffe, and all kinds of cows have four teats ; most antelopes and the gazelles have two teats, whilst a few antelopes have four. As to Carnivora, the felines have usually six nipples ; the wolf, jackal, fox, dog have usually eight ; the seals and the walrus have four, the otters two, the weasels six, the bears six ; and in the kinkajou (Cercoleptes) the number is reduced to two. Amongst Quadrumana, the aye-aye (Chiromys) has only one pair of nipples, about an inch and a half in front of the vulva ; many lemurs have in addition to those a pectoral pair ; in all the platyrhine and catarhine Quadrumana there is only one pair of glands, restricted to the pectoral region. Here the teats are between the fore legs, and the young clings to the mother s breast in human fashion, but there is no protru sion of the breast as in the human being. (For further details see Owen s Anatomy of Vertebrates, vol. iii. p. 769.) In the human race the glands are two in number, form ing, along with the skin and fat, two rounded eminences, one on each side, on the front of the thorax. They extend from the third to the sixth or seventh rib, and from the side of the sternum to the axilla. In the centre projects a small conical body, the nipple. Around the nipple is a coloured circle, or areola, which is darker during pregnancy, and even in women who have borne children than in the virgin state. The surface of the nipple is wrinkled, and with a magnifying glass is seen to be covered with papillae. It is perforated by numerous openings, the mouths of the milk ducts. The tissue of the nipple contains numerous minute blood-vessels, and it has at the base muscular fibres arranged in concentric circles and in radiating bands. It has much of the character of erectile tissue, as in the corpora cavernosa of the penis, becoming turgid, firm, and prominent from excitement. The base of the gland lies on the pectoral muscle, a thin layer of fascia intervening. The surface is covered with fat, which gives it the smooth rounded outline. It is amply supplied with blood by the long thoracic artery, some other minute branches of the axillary artery, the internal intercostal artery, and the subjacent intercostals. The nerves come from the anterior and middle intercostal cutaneous branches, and the nipple is especially sensitive. The gland is com posed of numerous lobes bound together by connective and adipose tissue, and each lobe is formed of smaller lobules. Each lobe has an excretory duct, and these ducts, from fifteen to twenty in number, converge towards the areola, beneath which they are dilated so as to form sinuses from th to |-th of an inch in calibre. From these sinuses arise the ducts which open on the surface of the nipple. The general structure will be understood by referring to the accompanying figures, along with the description. FIG. 1. Half-diagrammatic view of a section through a lobule of the mam mary gland, after Klein (At/as of Histology, plate xl. fig. 1), magnified 45 diameters, a, a duct dividing into two branches; b, b, b, connective tissue surrounding and going between the ultimate pouches of the gland; c.c.c, the pouches or alveoli of the gland, the dots representing the cells lining them. FIG. 2. A portion of the same gland, magnified about 400 diameters, showing one complete and two incomplete alveoli, a, a, a, short, columnar, epithelial cells lining the alveolus, each having an oval or rounded nucleus; b, b, 6, epithelium cells, containing, next the interior of the alveolus, a milk globule ; c, c, c, c, milk globules which have been set free from epithelial cells. When a duct is traced into the gland, it is found to sub divide into smaller ducts, and these into still smaller, until the smallest ductlet is reached, round the end of which are clustered several alveoli or pouches. Each alveolus has a wall, lined with epithelium cells. In the wall of the alveolus there are capillary blood-vessels which bring the blood near the cells. By this blood the cells are nourished. There is a minute cavity in the centre of each alveolus into which cells or their products can accumulate. There can be no doubt that the formation of the milk globule takes place in these cells. Whilst milk is not being formed the cells have a granular appearance, and the lumen or central cavity of the alveolus is small ; but during secretion the cavity is enlarged and shows a few milk globules, Avhilst one or more milk globules can be seen in the interior of the cell. If the milk globule in the cell be very large, the nucleus of the cell is pressed outwards and the protoplasm of the cell is reduced to a thin covering, over the globule, at this stage presenting a striking resemblance to a fat cell containing an oil globule. Thus each milk globule is formed in the protoplasm of the epithelium cell, and even at an early stage each milk globule consists of a minute drop of fat or oil surrounded by a thin albuminous envelope. It has not been clearly ascertained whether epithelial cells, after having secreted milk globules, degenerate and fall off, or whether they have the power of ejecting the milk globules. The iluid constituents of milk (water holding