NORMAL FEMALE PELVIS

The present broad knowledge of the anatomy of the female pelvis has resulted in an enormous reduction in death and injury among obstetrical patients and their babies.

This knowledge of the pelvic anatomy, relating as it does, to both normal and malformed pelves, has made possible a system of taking measurements, termed pelvimetry, which gives the obstetrician a fair idea of the size and shape of his patient’s pelvis. Such information, coupled with observations upon the size of the child’s head, gives a foundation upon which to base some expectation of the ease or difficulty with which the approaching delivery is likely to be accomplished.

Since each patient’s pelvic measurements are considered from the standpoint of their comparison with normal dimensions, it is manifestly important that the obstetrical nurse have a clear idea of the structure of the normal female pelvis, and also of its commonest variations.

Viewed in its entirety, the pelvis is an irregularly constructed, two-storied, bony cavity, or canal, situated below and supporting the movable parts of the spinal column, and resting upon the femora or thigh bones. (Fig. 1, A. and B.).

Four bones enter into the construction of the pelvis: the two hip bones or ossa innominata, on the sides and in front with the sacrum and coccyx behind.

The innominate bones (ossa innominata), symmetrically placed on each side, are broad, flaring and scoop-shaped. Each bone consists of three main parts, which are separate bones in early life, but firmly welded together in adults: the ilium, ischium and pubis. The ilia are the broad, thin, plate-like sections above, their upper, anterior prominences, which may be felt as the hips, are the anterior superior spinous processes used in making pelvic measurements. The margins extending backward from these points are termed the iliac crests.

The ischii are below and it is upon their projections, known as the tuberosities, that the body rests when in the sitting position, and which also serve as landmarks in pelvimetry. The pubes form the front of the pelvic wall, the anterior rami uniting in the median line by means of heavy cartilage and forming the symphysis pubis.

The sacrum and coccyx behind are really the termination of the spinal column, the sacrum consisting, usually, of five rudimentary vertebrae which have fused into one bone. It sometimes consists of four bones, sometimes six, but more often of five. The sacrum completes the pelvic girdle behind by uniting on each side with the ossa innominata by means of strong cartilages, thus forming the sacro-iliac joints. The spinal column rests upon the upper surface of the sacrum. The coccyx, a little wedge-shaped, tail-like appendage, which ordinarily has but slight obstetrical importance, extends in a downward curve from the lower margin of the sacrum, to which it has a cartilaginous attachment, the sacro-coccygeal joint. This joint between the sacrum and coccyx is much more movable in the female than in the male pelvis.

We find, therefore, that although the pelvis constitutes a rigid, bony, ringlike structure, there are four joints: the symphysis pubis, the sacro-coccygeal, and the two sacro-iliac articulations. As the cartilages in these joints become somewhat softened and thickened during pregnancy, because of the increased blood supply, they all permit of a certain, though limited amount of motion at the time of labor. This provision is of considerable obstetrical importance, since the sacro-coccygeal joint allows the child’s head to push back the forward-protruding coccyx, as it passes down the birth canal, thus removing what otherwise might be a serious obstruction. And when, as is sometimes necessary, because of a constricted inlet, the pubic bone is cut through (the operation known as pubiotomy), the hingelike motion of the sacro-iliac joint permits of an appreciable spreading of the two hip bones and a consequent widening of the birth canal.

The pelvic cavity as a whole is divided into the true and false pelves by a constriction of the entire structure known as the brim or inlet. The inlet is not round, its antero-posterior diameter being shortened by the sacro-vertebral joint which protrudes forward and gives the opening something of a blunt, heart-shaped outline. (Fig. 2.)

As the pelvis occupies an oblique position in the body, the plane of this brim is not horizontal, but slopes up and back from the symphysis-pubis to the promontory of the sacrum. Being swung upon the heads of the femora, the relation of the pelvis to the entire body differs in the sitting and standing positions. When a woman stands upright, her pelvis is so markedly oblique in its position that she would tip backward but for strong tendons attached to the pelvis and running down the front of the thighs. Added strain upon these tendons during pregnancy may account for some of the apparently undue fatigue experienced by the expectant mother.

The shallow, expanded portion of the pelvis above the brim is the large, or false pelvis, its walls being formed by the sacrum behind, the fan-like flares of the ilia on each side, with the incompleteness of the bony wall in front made up by abdominal muscles.

The false pelvis ordinarily serves simply as a support for the abdominal viscera, which do not occupy the true pelvis unless forced down by some such pressure as that caused by tight, or poorly fitting corsets. The false pelvis is of little obstetrical importance, its function during pregnancy being to support the enlarged uterus, while at the time of labor it acts as a funnel to direct the child’s body into the true pelvis below.

The true pelvis, on the other hand, is of greatest possible obstetrical importance since the child must pass through its narrow passage during birth. It lies below and somewhat behind the inlet; is an irregularly shaped, bottomless basin, and contains the generative organs, rectum and bladder. Its bony walls are more complete than those of the false pelvis, and are formed by the sacrum, coccyx and innominate bones. Its lower margin constitutes the outlet, or inferior strait, and being longer in its antero-posterior dimension than in its transverse measurement, its long axis is at right angles to the long axis of the inlet. (Fig. 3.) A baby’s head, accordingly, must twist or rotate in making its descent through this bony canal, for the long diameter of the head must first conform to one of the long diameters of the inlet, either transverse or oblique, and then turn so that the length of the head is lying antero-posteriorly, in conformity to the long diameter of the outlet, through which it next passes.

The posterior wall of the pelvis, consisting of the sacrum and coccyx, forms a vertical curve and is about three times as deep as the anterior wall formed by the narrow symphysis pubis. The structure as a whole, therefore, curves upon itself, resembling a bent tube with its concavity directed forward. (Fig. 4.)

Thus it becomes apparent that the structure of the pelvis requires the child’s head, not only to rotate in its passage through the birth canal, but also to describe an arc, since the part of the head which passes down the posterior wall travels farther in a given time than the part which passes under the pubis.

This twisting and curving of the birth canal must be appreciated in order to understand the mechanism of labor.

In considering the question of pelvimetry, we find that there are both external and internal measurements to be taken, all for the purpose of estimating as accurately as possible the shortest diameter of the inlet through which the baby must pass. (Fig. 5.)

According to a common system of mensuration, the first external measurement is the inter-spinous, the distance between the anterior-superior spines, those bony points which are uppermost as the patient lies on her back. This distance is normally 26 centimetres. (Fig. 6.)

The second measurement is the inter-crestal, or the distance between the iliac crests, and is normally 28 centimetres.

Baudelocque’s diameter is the third measurement and is taken with the patient lying on her side. (Fig. 7.) It is the distance from the top of the symphysis to a depression just below the last lumbar vertebra. This depression is easily located as it also marks the upper angle of a space just above the buttocks, which in normal pelves is quadrilateral. In malformed pelves this quadrangle may be so misshapen as to become almost a triangle with the apex directed either up or down. This dimension is sometimes called the external conjugate and ordinarily measures 21 centimetres.

The fourth measurement is the distance between the great trochanters, or heads of the femora, and normally is 32 centimetres.

All of these measurements, which after all are only approximate, relate to the top of the pelvis and are valuable in that they help in estimating the dimensions of the inlet, which are the important ones, and obviously cannot be measured on a live woman.

The inlet has four measurements of obstetrical importance: the antero-posterior, or true conjugate, which is the distance from the top of the symphysis pubis to the prominence of the sacrum, and is normally 11 centimetres; the transverse diameter, which is at right angles to the true conjugate and is the greatest width of the inlet, measuring from a point on one side of the brim to the corresponding point on the other, is normally 13.5 centimetres, and the two diagonal measurements, known respectively as the right and left oblique diameters, which are normally 12.75 centimetres.

Although it is very important to the expectant mother that all of these dimensions be of normal length, the length of the true conjugate, or conjugata vera, is of the gravest importance of all because it is the shortest diameter through which the child’s head must pass. If it is shorter than normal, the channel may be too constricted for the full-term baby’s head to pass through comfortably, thus making a spontaneous delivery extremely difficult, or even impossible.

The length of the all important, true conjugate is estimated by introducing the first two fingers of one hand into the vagina until the tip of the second finger touches the promontory of the sacrum. (Fig. 8.) The point at which the inner margin of the symphysis then rests upon the forefinger is measured, thus giving the length of the diagonal conjugate. This normally measures 12.5 centimetres or more, and is estimated as being 1.5 centimetres longer than the true conjugate.

The most important measurement of the outlet is the intertuberous diameter, the distance between the tuberosities of the ischii. This is the shortest diameter through which the child must pass in the inferior strait, and normally measures something more than 8 centimetres, usually about 11 centimetres. (Fig. 9.)

It is possible, by studying such measurements as these, made upon an expectant mother, and comparing them with dimensions which have been accepted as normal, to form a reasonably accurate estimate of the size and shape of her pelvis.

A delivery may be, and frequently is, accomplished through a pelvis which is not entirely normal in size or shape. But the obstetrician of to-day is closely observant of the patient whose pelvic measurements depart from the normal by more than the accepted margin of safety, and he plans for labor in accordance with the indications in each case.

Disproportion between the measurements of the mother’s pelvis and the size of the child’s head must be considered in this connection. A small pelvis may permit of the spontaneous delivery of a small child, but be too narrow for the passage of a full-sized baby, while a woman with a normal pelvis may have an extremely difficult labor because of an unusually large child.

The size and shape of the pelvis is found to vary among different races and in different individuals. And the size and contour of the inlet may be so altered by rickets, lack of proper exercise during early life, or by growths upon the pelvic bones, as to seriously interfere with normal labor.

The various kinds of malformed pelves may be loosely classified as generally contracted or small; flat; simple funnel; generally contracted funnel; and the rachitic pelves, both flat and generally contracted. There may be a contracted inlet, or a contracted outlet, or both may occur in the same pelvis.^[1]

Rachitic pelves are common among negroes and not altogether rare among white women.

The normal male pelvis is deep, narrow, rough and massive as compared with the female structure (see Fig. 1.), and the angle of the pubic arch, formed by the two pubic bones, is deeper and more acute in the male than in the female skeleton.

The normal female pelvis, on the other hand, is light, broad, shallow, smooth and large, giving evidence of the infinite wisdom and skill that entered into constructing it for the high purpose it was designed to serve.

FEMALE ORGANS OF REPRODUCTION

The female organs of reproduction are divided into two groups, the internal and the external genitals. With them are usually considered certain other structures: the ureters, bladder, urethra, rectum and the perineum, because of their close proximity (Fig. 10.); and the breasts, because of their functional relation to the reproductive organs.

Internal Genitalia. The internal organs of generation are contained in the true pelvic cavity and comprise the uterus and vagina in the centre, an ovary and Fallopian tube on each side, together with their various ligaments, membranes, nerves and blood vessels and a certain amount of fat and connective tissue.

The uterus is the largest of these organs. In its nonpregnant state, it is a hollow, flattened, pear-shaped organ about three inches long, one and a quarter inches wide, at its broadest point, three-quarters of an inch thick and weighing about two ounces.

Ordinarily it is a firm, hard mass, consisting of irregularly disposed, involuntary (unstriped or plain) muscle fibres and connective tissue, nerves and blood vessels. The arrangement of the uterine muscle fibres is unique, for they run up and down, around and crisscross, forming a veritable network. This strange arrangement of the fibres is favorable to the growth of the uterine musculature during pregnancy, and a factor in preventing hemorrhage after delivery.

The abundant blood supply to the uterus merits a word. It is derived from the uterine arteries, arising from the internal iliacs, and the ovarian artery from the aorta. The arteries from the two sides of the uterus are united by a branch where the neck and body of this organ meet, thus forming an encircling artery. A deep cervical tear during labor may break this vessel and a profuse hemorrhage occur as a result.

The uterus is covered, front and back, by a fold of the peritoneum, except the lower part of the anterior wall where the peritoneum is reflected up over the bladder. It is lined with a thick, velvety, highly vascular mucous membrane, the endometrium, the surface of which is covered by ciliated, columnar epithelium. Embedded in the endometrium are numerous mucous glands which dip down into the underlying, muscular wall.

The uterus as a whole is comprised of three parts: the fundus, that firm, rounded, head-like part above; the body, or middle portion, and the cervix, or neck, below. It is in the body and cervix that we find the long, narrow uterine cavity, divided by a constriction into two parts. The cavity of the body is little more than a vertical slit, being so flattened from before backward that the anterior and posterior surfaces are nearly if not quite in apposition. It is somewhat triangular in shape with an opening at each angle. (Fig. 11.) The lower of these openings leads into the cavity of the cervix through a constriction termed the internal os, while at the cornua, or two upper angles, are the openings into the Fallopian tubes.

The cavity of the cervix is spindle-shaped, being expanded between its two constricted openings, the internal os above and the external os below, which opens into the vagina. The external os in the virgin is a small round hole but has a ragged outline in women who have borne children.

This oblong, muscular body, the uterus, is suspended obliquely in the centre of the pelvic cavity by means of ligaments. In its normal position the entire organ is slightly curved forward, or ante-flexed, the fundus being directed upward and forward and the cervix pointing down and back. This position is affected by a distended bladder or rectum, and also by postural changes in the body as a whole. The cervix protrudes into the anterior wall of the vagina for about one-half inch and almost at right angles, since the vagina slopes down and forward to the outlet.

The upper part of the uterus is held in position by means of ligaments, the lower part being embedded in fat and connective tissue between the bladder and rectum. This more or less of a floating position makes possible the enormous increase in size and upward push or extension of the uterus during pregnancy. The pregnant uterus becomes soft and elastic as it grows. At term it is about a foot long, eight to ten inches wide, and reaches up into the epigastric region. This growth is due in part to the development of new muscle fibres and in part to a growth of the fibres already existing in the uterine wall.

After labor the uterus returns almost, but never entirely, to its former size, shape and general condition.

The Fallopian tubes are two tortuous, muscular tubes, four or five inches long, extending laterally in an upward curve, from the cornua of the uterus and within the folds of the upper margin of the broad ligament, by which they are covered. At their juncture with the uterus, the diameter of these tubes is so small as to admit of the introduction of only a fine bristle, but they gradually increase in size toward their termination in wide trumpet-shaped orifices, which open directly into the peritoneal cavity. Finger-like projections called fimbriÆ, fringe the margins of these openings.

The mucous lining of the tubes is covered with ciliated epithelium and is continuous with that of the uterus. At the fimbriated extremities of the tubes this lining merges into the peritoneum, the serous lining of the abdominal cavity.

Just here it will be well to say a word about the peritoneum because of the possibility of its becoming infected during labor and the lying-in period, and the very grave consequences of such infection. It is a delicate, highly vascular, serous membrane which both lines the abdominal cavity and covers the abdominal and pelvic organs, which press into its outer surface and are covered much as one’s fingers would be covered by pushing them into the outer surface of a child’s toy balloon. The continuity of this membrane is broken only where it is entered by the Fallopian tubes.

The ovary, the sex gland of the female, is a small, tough ductless gland, about an inch long and three-quarters of an inch wide, or about the size and shape of an almond. It is greyish pink in color and presents a more or less irregular, dimpled surface. An ovary is suspended on either side of the uterus, in the posterior fold of the broad ligament, by which it is partly covered. Its outer end is usually attached to the longest of the fimbriated extremities of the Fallopian tube, the fimbria ovarica, which has the form of a shallow gutter, or groove. The inner end of the ovary is attached to the ovarian ligament, which in turn is attached to the uterus below and behind the tubal entrance.

The ovary consists of two parts, the central part or medulla, composed of connective tissue, nerves, blood and lymph vessels, and the cortex, in which are embedded the vesicular Graafian follicles containing the ova. At birth each ovary contains upwards of 50,000 of these ova, which are the germ cells concerned with reproduction and the process of menstruation.

These ovarian glands perform two vital functions, for in addition to their prime function of producing and maturing the germinal cell of the female, they provide an internal secretion which exercises an immeasurably important, though imperfectly understood, influence upon the general well-being of the entire organism.

The vagina is an elastic, muscular sheath or tube, about four inches long, lying behind the bladder and urethra and in front of the rectum. It leads interiorly up and backward from the vulva to the cervix, which it encases for about half an inch. The space between the outer surface of the cervix that extends into the vagina, and the surrounding vaginal walls, is called the fornix. For convenience of description, this is divided into four sections or fornices: the anterior, posterior and lateral fornices.

Between the posterior fornix and the rectum a fold of the peritoneum drops down and forms a blind pouch known as Douglas’ cul-de-sac. At this point the delicate peritoneum is separated from the vagina by only a thin, easily punctured, muscular wall. This is a fact of grave surgical significance, for unless instruments and nozzles introduced into the vagina are very gently and skillfully directed, they may easily pierce this thin partition. Septic material may thus gain entrance to the peritoneal cavity and peritonitis result.

The bore of the vaginal canal ordinarily permits of the introduction of one or two fingers. It is somewhat flattened from before backward, and on cross section resembles the letter H. During labor this canal becomes enormously dilated, being then four or five inches in diameter, and permits the passage of the full term child.

The vagina is lined with a thick, heavy, mucous membrane which normally lies in transverse folds or corrugations called rugÆ. These folds are obliterated and the lining stretched into a smooth surface as the canal dilates during labor.

Attention must be drawn to the fact that the vagina, cervix, uterus and tubes form a continuous canal from the vulva to the easily infected peritoneum, a fact which makes absolute surgical cleanliness in obstetrics virtually a matter of life or death to the patient.

This muscular tube is lined throughout its entire length with mucous membrane, which, though continuous, changes somewhat in character along its course. The epithelial cells of the lining of the tubes and body of the uterus have hair-like projections, cilia, which maintain a constant waving motion from above downward. The effect of this sweeping current is to carry down toward the outlet any object or secretion which may be upon the surface of the lining of the tubes or uterine cavity. The unfertilized ovum is thus swept down to meet the germ cell of the male and become fertilized.

Along this variously constructed canal, at different periods in the life of the individual, pass the matured ovum, the menstrual flow, the uterine secretions, the fetus, the placenta and lochia, (the discharge which occurs during the puerperium).

Although the bladder and rectum are not organs of reproduction, they are contained in the pelvic cavity and lie in such close proximity to the internal genitalia that at least a passing word must be devoted to their description.

The bladder is a sac of connective tissue which serves as a reservoir for the urine and is situated behind the symphysis pubis and in front of the uterus and vagina. Urine is conducted into the bladder by the ureters, two slender tubes running down on each side from the basin of the kidney across the pelvic brim to the upper part of the bladder, which they enter somewhat obliquely, at about the level of the cervix. It is thought that pressure of the enlarged pregnant uterus upon the ureters at this point may be one factor in the causation of pyelitis, a frequent complication of pregnancy. The bladder empties itself through the urethra, a short tube which terminates in the meatus urinarius, a tiny opening in the vulva.

The rectum, the lowest segment of the intestinal tract, is situated in the pelvic cavity behind and to the left of the uterus and vagina. It extends downward from the sigmoid flexure of the colon to its termination in the anal opening. The anus is a deeply pigmented, puckered opening situated an inch and a half or two inches behind the vagina. It is guarded by two bands of strong circular muscles, the internal and external sphincter ani. The skin covering the surface of the body extends upward into the anus where it becomes highly vascular and merges into the mucous lining of the rectum. Pressure exerted during pregnancy by the enlarged uterus is felt in both the rectum and bladder, frequently causing a good deal of discomfort and almost painful desire to evacuate their contents.

The blood vessels in the anal lining just within the external sphincter sometimes become engorged and inflamed, even bleeding during pregnancy, as a result of the pressure exerted by the greatly enlarged uterus. The distended blood vessels, which in this condition are called hemorrhoids, not infrequently protrude from the anus and become very painful.

After having considered the structure and relative positions of the pelvic organs one is able to picture more clearly the arrangement and disposition of the uterine ligaments, all of which are formed by folds of the peritoneum. They are twelve in number, five pairs and two single ligaments, namely: two broad, two round, two utero-sacral, two utero-vesical, two ovarian, one anterior and one posterior ligament.

The broad ligaments are in reality one continuous structure formed by a fold of the peritoneum, which drops down over the uterus, investing the fundus, body, part of the cervix, and part of the posterior wall of the vagina. It unites on each side of the uterus to form a broad, flat membrane which extends laterally to the pelvic wall, dividing the pelvic basin into an anterior and posterior compartment, containing respectively the bladder and rectum. Between the folds of the broad ligament are situated the ovaries and ovarian ligaments, the Fallopian tubes, the round ligaments and a certain amount of muscle and connective tissue, blood vessels, lymphatics and nerves.

The round ligaments, one on each side, are narrow, flat bands of connective tissue derived from the peritoneum and muscle prolonged from the uterus, and containing blood and lymph vessels and nerves. They pass upward and forward from their uterine origin just below and in front of the tubal entrance, finally merging in the mons veneris and labia majora.

The utero-sacral ligaments, of which there is one on each side, arise in the uterus and, extending backward, serve to connect the cervix and vagina with the sacrum.

The utero-vesical ligaments, one on each side, extend forward and connect the uterus and bladder.

The ovarian ligaments, as previously described, are attached to the uterine wall and to the inner end of the ovary, one on each side.

The anterior ligament is a portion of the peritoneum which dips down between the bladder and uterus, forming a pouch. It is known also as the uterine-vesical pouch, or the vesico-uterine excavation.

The posterior ligament is formed in much the same manner by a portion of the peritoneum dipping down behind the uterus, in front of the rectum, and forming the recto-vaginal pouch. This is the Douglas’ cul-de-sac previously referred to.

External Genitalia.—The vulva, or external genitalia, are situated in the pudendal crease which lies between the thighs at their junction with the torso, and extends posteriorly from the pubis to a point well up on the sacrum. (Fig. 13.)

The mons veneris is a firm cushion of fat and connective tissue, just over the symphysis pubis. It is covered with skin which contains many sebaceous glands and after puberty is abundantly covered with hair.

The labia majora are heavy ridges of fat and connective tissue, prolonged from the mons veneris and extended down and back almost to the rectum, on each side, forming the lateral boundaries of the groove. They are lined with mucous membrane and covered with skin and hair, the latter growing thinner toward the perineum until it finally disappears.

The labia minora are two small cutaneous folds lying between the labia majora on each side of the vagina. Like the larger folds, they taper toward the back and practically disappear in the vaginal wall. Their attenuated posterior ends are joined together behind the vagina by means of a thin, flat fold called the fourchette. The labia minora divide for a short distance before joining at an angle in front, thus forming a double ridge anteriorly. In the depression between these ridges is the clitoris, a small, sensitive projection composed of erectile tissue, nerves and blood vessels and covered with mucous membrane. The meatus urinarius is just below the clitoris and between two small folds of the mucous membrane.

The vestibule is the triangular space between the labia minora, and into it open the meatus urinarius, the vagina and the more important vulvo-vaginal glands.

The vaginal opening is below the vestibule and above the perineum. It is partially closed by the hymen, a fold of mucous membrane disposed irregularly around the outlet, somewhat after the fashion of a circular curtain. The hymen is ragged or more or less scalloped in outline, and varies greatly in size in different women, in some instances extending so far over the opening as nearly or quite to close it.

The fossa navicularis is a depressed space between the hymen and fourchette, so named because of its boat-like shape.

The Bartholin glands, probably the largest and most important of the vulvo-vaginal glands, are situated one on each side of the vagina and open into the groove between the hymen and labia minora. Reference is made to these glands because of the danger of their becoming infected. A gonorrheal infection of these glands is particularly troublesome.

The perineum is a pyramidal structure of connective tissue and muscle which occupies the space between the rectum and vagina, and by forming the floor of the pelvis serves as a support for the pelvic organs. The lower and outer surface of this mass, representing the base of the pyramid, lies between the vaginal opening and the anus and is covered with skin. As the anterior part of the perineum is incorporated in the posterior wall of the vagina, the entire structure becomes stretched and flattened when the vagina is dilated during labor by the passage of the child’s head.

Unless very carefully guarded at the time of delivery, and often even then, the perineum gives way under the great tension undergone at that time, and a tear is the result. The injury may be only a slight nick in the mucous membrane or it may extend to, or into the levator ani, the most important muscle of the perineal body, or if a “complete tear” will extend all the way through the perineum and completely through the sphincter ani. Such a tear is lamentable, as a break in the ring-shaped sphincter muscle guarding the anal opening robs a woman of control of her bowels, and is repaired with difficulty.

BREASTS

The breasts are large, specially modified skin glands of the compound, racemose or clustering type, embedded in fat and connective tissue and abundantly supplied with nerves and blood vessels. They are situated quite remotely from the pelvic organs, but because of the intimate functional relation between the two, the breasts of the female may be regarded as accessory glands of the generative system. They exist in the male, also, but only in a rudimentary state.

Although the breasts sometimes contain milk during infancy, their true function is to secrete, in the parturient woman, suitable nourishment for the human infant during the first few months of its life.

These glands are symmetrically placed, one on each side of the chest, and occupy the space between the second and sixth ribs extending from the margin of the sternum almost to the mid-axillary line. A bed of connective tissue separates them from the underlying muscles and the ribs. (Fig. 14.)

They vary in size and shape at different ages, and with different individuals, particularly in women who have borne and nursed children, when they tend to become pendulous. But in general they are hemispherical or conical in shape with the nipple protruding from one-quarter to one-half inch from the apex. The nipples are largely composed of sensitive, erectile tissue and become more rigid and prominent during pregnancy and at the menstrual periods. Their surfaces are pierced by the orifices of the milk ducts, which are fifteen or twenty in number. (Fig. 15.)

The breasts are covered with very delicate, smooth, white skin, excepting for the areolÆ, those circular, pigmented areas one to four inches in diameter, which surround the nipples. The areolÆ are darker in brunettes than in blonds, and in all women grow darker during pregnancy. The surface of the nipples and of the areolÆ is roughened by small, shot-like lumps or papillÆ known as the tubercles of Montgomery. This roughness becomes more marked during pregnancy, since the papillÆ grow larger and sometimes even contain milk.

The secretory apparatus of the breasts is divided into fifteen or twenty lobes, these in turn being divided into clusters of lobules. The lobules in turn are composed of tiny, secreting cells, called acini, in which the milk is elaborated from the blood. The acini are minute globules lined by a single layer of cells and enveloped by a very delicate membrane. Tiny ducts carry the milk from the acini to the main duct of the lobule, around which the acini cluster. These ducts empty the milk into the larger duct of the lobe, which runs straight to the nipple and opens upon the surface. Just before reaching the surface, each of these lactiferous sinuses expands into an ampulla, a minute reservoir for collecting the milk, which is secreted during the periods between nursings.

These clusters of acini uniting to form lobules with tiny ducts leading into the main duct of each lobule, closely resemble a bunch of grapes. The separate grapes correspond to the acini, their small stems correspond to the tiny ducts of the glands which lead to a larger one, and the central stem of the grape cluster, to the milk duct that opens upon the nipple.

The secretory tissue really constitutes a small part of the breasts until they begin to function. But during lactation the acini become enormously developed and enlarged. After lactation ceases, the acini assume a more or less tubal form, many of them undergoing atrophic changes.