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from the tail to the umbilical cord. At its cephalic end is a tubercle known as the cloacal tubercle, and behind lies the coccygeal tubercle. Immediately in front of the latter the anus is formed, and between this opening and the cloacal tubercle the uro-genital canal opens on the surface by a median slit-like aperture, the primitive uro-genital opening.
The cloacal tubercle early becomes subdivided into an apical genital eminence which occupies the middle line and lies at the cephalic end of the slit-like uro-genital opening, and a basal portion which lies nearer to the umbilicus and also curves round the sides of the genital eminence. At a later time the basal part is continued to form a prominent fold on each side of the ectodermal cloacal fossa. These folds are called the labio-scrotal folds and, in the female, give rise to the labia majora. The lateral margins of the primitive uro-genital opening give origin to the labia minora, and the genital eminence becomes the clitoris. On the clitoris at a very early date a relatively large glans is marked off by a surrounding sulcus. In the male the scrotal folds grow backwards, and meeting behind the primitive uro-genital opening, fuse together. In this way the opening is pushed forwards. The genital eminence elongates rapidly owing to a growth at its basal part, and
a sulcus which is formed on its cloacal aspect gradually becomes converted into a canal by the closure of the lateral lips of the primitive uro-genital opening. Soon the urogenital opening is found to lie nearer the apex than the base of the eminence, which has now given rise to the penis. For some time the opening in the male lies at the base of the glans penis and is somewhat rhomboidal in outline. At a later time, owing to the breaking down of a dense septum of epithelial cells which appears within the glans, a sulcus and finally a canal arises within this part of the penis and thus the terminal part of the urethra is formed. When the opening at the base of the glans is closed the continuous urethral passage is established. The main portion of the urethra is entodermal in origin, but there is some evidence to indicate that the part which traverses the glans has its origin from the ectoderm.
THE MAMMARY GLANDS.
The mammæ or breasts are accessory organs connected with the female reproductive system. Each gland is situated in the superficial fascia covering the anterior aspect of the thorax, and usually extends from the level of the second or third to that of the sixth rib. The hemispherical projection formed by the gland
ies upon the superficial aspect of the pectoralis major and to a less extent upon the serratus anterior muscle. Near the summit of each mammary elevation, and usually at the level of the fourth or fifth rib, is placed the wart-like nipple or papilla mamma, which is pierced by the minute openings of the lactiferous ducts and is surrounded by a coloured circular area of skin called the areola. The skin covering the nipple is thrown into numerous wrinkles, and on the areola exhibits many minute rounded projections due to the presence of underlying cutaneous glands. These have received the name of glandulæ areolares, and are considered to represent rudimentary portions of the mamma. The colour of the nipple and areola varies with the complexion of the individual, but in young subjects they are usually of a somewhat rosy-pink colour, which changes to a deep brown during the second and third months of first pregnancy. Also, during pregnancy, the areola increases in size and its glands become more marked. The nipple contains a considerable number of unstriped muscle fibres, and becomes firmer and more prominent as a result of mechanical stimulation.
The size and appearance of the mammæ vary much, not only in the different races of mankind, but also in the same individual under different conditions. In the young child the mammæ are small, and there is little difference between those of the male and female. Their growth is slow until the approach of puberty, and then the female mammæ increase rapidly in size. At each pregnancy
the mammæ become large, and they attain their greatest development during lactation. The size of the
mamma depends partly on the amount of superficial fat and partly on the amount of glandular tissue present.
Structure of the Mamma. -The mamma is composed of a mass of glandular tissue traversed and supported by strands of fibrous connective Ampullatissue, and covered by a thick layer of fat. The glandular tissue, to which the term corpus mamma is applied, forms a somewhat conical mass whose apex corresponds to the position of the nipple while its base is loosely connected to the fascia covering the pectoralis major and serratus anterior muscles. In section the corpus mammæ is readily distinguished from the surrounding fat by its firmer consistency and by its pinkish-white colour. The corpus mammæ is composed of lobes and lobules, and its superficial aspect and edges are very uneven, the inequalities of its surface being filled up by processes of the fatty tissue which forms a covering for the gland. This fatty covering is incomplete beneath the areola, and here the lactiferous ducts pass into the nipple. The gland is composed of fifteen to twenty lobes, or lobi mammæ, which radiate from the nipple, each lobe being quite distinct from the others and possessing its own duct. The lobes are subdivided into secondary lobes and lobules, bound together and supported by a considerable amount of connective tissue which forms the stroma of the gland.
The alveoli of the gland and the secretory epithelium lining them vary much under different conditions. At puberty the corpus mammæ is composed chiefly of connective tissue stroma and the ducts of the gland. At this time the alveoli are small and few in number. During lactation, when the gland is fully functional, the alveoli are enlarged, distended with fluid, and much more numerous. The epithelial cells are cubical and filled with fat globules. When the gland is not secreting, the alveoli become small and reduced in number
while the cells of the lining epithelium, which are now small and glandular, do not contain fat globules.
The ducts, or ductus lactiferi, passing towards the nipple, become enlarged to form small spindle-shaped dilatations, called ampullæ, or sinus lactiferi; then becoming once more constricted, each duct passes, without communicating with its neighbours, to the summit of the nipple, where it opens.
In the male subject the various parts of the mamma are represented in a vestigial condition.
The presence of milk-glands is characteristic of the class mammalia, and the
Ampulla Ductus lactiferi
number of pairs of glands in each group of animals bears some relation to the number of young usually produced at each birth.
Variations. Asymmetry in the development of the mammæ is very common-the left mamma being very often larger than the right. Absence of one or both mammæ is a very rare abnormality, which may or may not be associated with Pectoralis absence of the nipples. When one nipple only
is present it is usually the left. The presence of supernumerary glands or nipples is not very uncommon, and a large number of examples are recorded. The term polymasty has been applied to cases in which more than the normal number of mammæ are present, and polythely to those in which additional glands, in a vestigial condition, are represented by accessory nipples. Usually the accessory glands, or nipples, are present on the anterior aspect of the thorax, and in most instances they occur below and a little to the medial side of the normal site. When the abnormal glands are found above the normal site they generally lie further from the median plane. Much more rarely accessory glands have been found on the abdomen, in the axilla, or in some other situation, including even the dorsal aspect of the trunk. As many as three extra pairs of mammæ have been found in the same individual, and cases in which the probable representative FIG. 1052.-SECTION THROUGH A MAMMARY GLAND. of mammary glands were even more numerous Prepared after immersion in nitric acid as recom- have been recorded. Asymmetry is very common mended by Mr. Harold Stiles. (D.J. Cunningham.) in these abnormal structures. It is interesting to note that examples of polymasty and polythely occur in the male rather more frequently than in the female. In some women the accessory breasts have yielded milk during lactation; in most cases the abnormal organs are very rudimentary, and represented only by a minute nipple or pigmented areola. These cases of polymasty and polythely are supposed to represent a reversion to an ancestral condition, in which more than two mammary glands were normally present, and in which probably many young were produced at each birth. In this connexion it is interesting to observe that usually the accessory glands occur in positions normally occupied by mammæ in lower animals. In the course of the development of the mammæ in man, specialised areas of the epidermis, similar to those which give origin to the mammæ, have been observed both superior and inferior to the region in which the adult mammæ are developed. These areas appear to be present normally, but in most cases they disappear at an early stage in the history of the embryo. In some other mammals rudimentary mammæ may occur, as, for instance, in lemurs and in some cows.
A slight functional activity of the mammary glands of the male at birth and about the time of puberty is stated to be not a very uncommon occurrence.
Vessels and Nerves of the Mamma.-The breast receives its arterial supply from the perforating branches of the internal mammary artery and from the external mammary branches of the lateral thoracic. Additional supply is sometimes derived from some of the intercostal vessels. The veins coming from the gland pour their blood into the axillary and internal mammary veins. Some small superficial veins from the breast join tributaries of the external jugular.
The lymph vessels of the breast are very numerous, and form extensive lymph spaces round the alveoli of the gland. These freely anastomose with the lymph vessels of the skin and more especially with the vessels of a very large anastomotic circle deep to the skin of the nipple. The lymph vessels coming from the deep parts of the mamma for the most part join the lymph glands of the axilla. They first run directly towards the deep surface of the breast, where they enter the fascial lymph vessels contained in, or lying deep to, the fascia of the pectoralis major. These fascial vessels end for the most part in the axillary glands. It is important to remember
that while the majority of the lymph vessels first reach the lymph glands lying on the lateral border of the pectoralis major, some free vessels may "short circuit" to glands-(1) in the floor of the axilla, (2) along the circumflex vessels, (3) even to glands along the axillary vein. Yet other lymph vessels of the breast may reach first the glands in the costo-coracoid space. Some vessels from the medial part of the breast, following the course pursued by the perforating arteries, may join the lymph glands situated along the course of the internal mammary artery, but these vessels are fortunately often absent. It is also to be remembered that a few, probably irregular, communications exist across the middle line with the lymph vessels of the opposite breast; and further that lymph vessels from the infero-medial area of the breast regularly join the fascial lymph vessels of the upper part of the sheath of the rectus abdominis, and through these make communication with the lymph vessels of (1) the round ligament of the liver, (2) the peritoneum generally, and ultimately with the abdominal lymph glands. The surgical importance of the facts regarding the lymphatic drainage of the breast cannot be exaggerated.
The nerve-supply of the gland is derived from the intercostal nerves of the fourth, fifth, and sixth intercostal spaces. Along the course of these nerves sympathetic filaments reach the breast from the thoracic part of the sympathetic trunk.
DEVELOPMENT OF THE MAMMÆ.
The mammæ are developed as downgrowths of the ectoderm into the underlying mesodermic tissue. In the human embryo a thickened raised area of the ectoderm can be recognised in the region of the future mamma at the end of the fourth week. This thickened ectoderm becomes depressed in the underlying mesoderm, and thus the mammary area soon becomes flat, and finally sunk below the level of the surrounding epidermis. The mesoderm, where it is in contact with this downgrowth of the ectoderm, is compressed, and its elements become arranged in concentric layers, which, at a later stage, give rise to the connective-tissue stroma of the gland. The depressed mass of ectoderm cells soon becomes somewhat flask-shaped, and grows out into the surrounding mesoderm as a number of solid processes, which represent the future ducts of the gland. These processes, by dividing and branching, give rise to the future lobes and lobules, and much later to the alveoli. The mammary area becomes gradually raised again in its central part to form the nipple. A lumen is formed in the different parts of this branching system of cellular processes only at birth, and with its establishment is associated the secretion of a fluid resembling milk, which often takes place at this time. ampullæ appear as thickenings on the developing ducts before birth.
In those animals which possess a number of mammary glands-such as the cat, pig, etc.--the thickening of the ectoderm, which is the first indication of the development of these structures, takes the form of a pair of ridges extending from the level of the forelimb towards the inguinal region. These converge posteriorly, and at their terminations lie not far from the middle line. By the absorption of the intermediate portions the ridges become divided up into a number of isolated areas, in connexion with which the future glands arise. Somewhat similar linear thickenings of the ectoderm have also been recognised in the human embryo, and the usual positions assumed by the accessory glands when present, leads us to suspect that in all probability the ancestors of man possessed numerous mammæ arranged, as in lower animals, in lines converging towards the inguinal region.