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constitute the bile-ducts within the liver. Adjacent trabeculæ become arranged into the form of a lobule, each with a vascular channel in its interior, which communicates with the vascular network in the surface of the lobule by capillary intervals between adjacent trabecula.
The central vein becomes a tributary of a hepatic vein, and the capillary network becomes the terminal distribution of branches of the portal vein.
The proximal portion of the original hollow diverticulum becomes the bile-duct, and the gall-bladder and cystic duct are formed by an evagination from it.
As the liver increases in size, it begins to project down from the inferior part of the septum transversum into the ventral mesentery, so that now, instead of being situated within the septum, it looks like an appendage of its inferior surface. In other words, the septum begins to differentiate into two parts-an inferior, the liver, and a superior, which constitutes the
Inferior mesenteric artery
Superior mesenteric artery Duodenum
Inferior mesenteric artery
FIG. 977.-Two DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE
The figure to the right shows the rotation of the intestinal loop around the
greater portion of the diaphragm, both of these having been at first one continuous mass. In the course of development the separation of the two becomes more marked, and finally is complete everywhere except at the coronary and lateral ligaments behind, and at the falciform ligament in front, where they are still connected.
As the liver separates off from the future diaphragm, and descends into the abdomen, it lies between the layers of the ventral mesentery—a fold which connects the stomach and duodenum with the anterior abdominal wall. This is divided by the liver into two partslower, stretching from the front (lesser curvature) of the stomach to the liver, which becomes the Stomach lesser omentum; and an upper, stretching from the liver to the diaphragm and anterior wall of the abdomen, which forms the falciform ligament. 2. Pancreas. The pancreas is developed at a very early period in man (being present in embryos of 5 mm.) from two outgrowths from the alimentary canal, a dorsal and a ventral.
The dorsal rudiment is an outgrowth from Duodenum the dorsal aspect of the intestine, anterior to the origin of the hepatic outgrowth. The ventral rudiment grows at a later stage from the root of the hepatic bud in the form of two ventral offshoots, one on either side. That on the left side, however, soon disappears.
FIG. 978.-DIAGRAM OF THE ORIGIN OF THE
Through the rotation of the duodenum around its long axis, the dorsal and ventral rudiments approach one another and become fused, and their ducts open on the left side of the duodenum. The connecting stalk between the ventral rudiment and the hepatic bud becomes the main duct of the pancreas, while the connexion of the dorsal outgrowth with the duodenum remains as the accessory pancreatic duct. In embryos of the fifth week, a large dorsal pancreatic rudiment is present, and also a smaller ventral rudiment, which opens into the duodenum in common with the bile-duct, and lies on the right of
the vena porta. In the sixth week, these two rudiments meet and unite with one another, forming a long slender glandular mass which passes backwards within the dorsal mesogastrium (meso-duodenum), between the vertebral column and the greater curvature of the stomach. The pancreas, so formed, follows the changes which occur in the position of the stomach and of the dorsal mesogastrium. Consequently its free dorsal extremity comes to be directed to the left, while the right extremity or head is included within the hollow of the curve formed by the duodenum. At first, it possesses a dorsal mesentery, a part of the dorsal mesogastrium, but from the fifth month this disappears, coincidently with the rotation of the gland into the transverse axis of the body.
The lower part of the head, the body, and tail of the gland arise from the ventral element, and the upper part of the head arises from the dorsal bud.
The primary diverticula give off buds, lined with cylindrical epithelium, and these in turn give off other buds, and the process goes on until the mass of the gland is formed. The islets of Langerhans are formed at a very early stage, from the entodermal lining cells of the branching diverticula which form the gland acini.
THE URO-GENITAL SYSTEM.
BY A. FRANCIS DIXON.
THE URINARY ORGANS.
THE kidneys, or glands which secrete the urine, are a pair of almost symmetrically placed organs, situated in the posterior part of the abdominal cavity, one on each side of the lower movable portion of the vertebral column. The fluid, or urine, secreted by the kidneys is received into the upper expanded portions of a pair of long tubes, the ureters, and by them it is conducted to the bladder, which is placed within the pelvic cavity. From the bladder the urine is passed, during micturition, along a passage called the urethra to the exterior. In the male the urethra is a relatively long passage, and traverses the prostate gland and the whole length of the penis; in the female it is a short tube, and opens on the surface just above the vaginal orifice.
The kidney (ren), when removed from a fresh subject, presents a bean-shaped contour. It is of a dark brown-red colour, and is surrounded by a thin glistening capsule, the tunica fibrosa, which gives to the whole organ a uniformly smooth surface. The kidney is not a solid body, but contains a cavity called the sinus renalis, the opening into which, termed the hilum renale, is situated on the medial and anterior part of the organ. Each kidney measures about 4 inches in length, 2 inches in width, and about 11 inches in thickness, and is placed so that its long axis is nearly vertical. The weight of the adult kidney is about 4 In the freshly removed kidney the superior and inferior,ends are smoothly rounded, and the extremitas superior or superior end is usually a little more bulky than the extremitas inferior or inferior end. The margo lateralis or lateral border, which is opposite the hilum, is rounded and convex, while the margo medialis or medial border, on which the hilum is placed, is concave from above downwards. These two borders separate the facies anterior or anterior surface from the facies posterior or posterior surface of the kidney.
The capsule, which envelops the whole organ, divides in the region of the hilum into two layers, one of which is continued over the lips of the hilum into the interior of the kidney, and lines the walls of the renal sinus. The other layer is prolonged to form a tubular sheath for the vessels and nerves of the kidney before they pass through the hilum to enter the sinus, within which they break up into branches. These branches, piercing the wall of the sinus, enter the substance of the kidney. The upper expanded portion of the ureter leaves the sinus, through the hilum, in company with the blood-vessels and
Position of the Kidneys.-The precise level of the kidney in the abdominal cavity is subject to a considerable amount of variation, and, further, it is usual to find a difference in the level of the right and left kidney in the same individual. Most frequently the left kidney is on a somewhat higher level than the right, but in many cases the kidneys are found to occupy the same level, or, the more usual condition being reversed, the right kidney is a little higher than the left.
If a line is drawn round the body at the level of the lowest part of the thoracic wall, the whole, or almost the whole, of the left kidney will be found to lie above the level of the subcostal plane so determined. It is, therefore, situated in the subcostal zone of the abdominal cavity. The right kidney, however, although it lies for the most part in the subcostal zone, usually projects at its inferior part somewhat below the subcostal plane, and hence lies to some extent in the umbilical zone. It is often stated that the kidneys are placed on a somewhat lower level in the female than in the male subject.
By far the greater part, usually two-thirds or more, of the kidney lies to the medial side of a line drawn vertically upwards through the middle point of the inguinal ligament.
The posterior aspect of the kidney is closely applied against the muscles attached to the bodies of the last thoracic and upper three lumbar vertebræ, and is placed in front of the last rib and of the transverse processes of the
FIG. 979.-DISSECTION TO SHOW THE RELATIONSHIPS OF THE KIDNEYS. The greater part of the stomach has been removed by an incision made close to the pylorus. The transverse colon has been taken away and the small intestine has been cut across close to the duodeno-jejunal flexure.
A model prepared by the late Professor Birmingham has been made use of in this drawing. lumbar vertebræ. In some cases, more frequently on the left side of the body, the eleventh rib also lies behind the upper part of the kidney. The relationship of the kidney to the lower two ribs is, however, very inconstant, owing partly to the great variability in size and inclination of these bones.
The inferior end of the kidney is usually situated from 1 to 2 inches above the highest part of the crest of the ilium; the interval between the kidney and the ilium being usually greater on the left side of the body.
Sometimes the inferior end of the kidney lies on the same level as, or only a short distance above, the iliac crest; this condition is sometimes due to the crest rising to a higher level than usual, the kidney occupying its normal position in relation to the vertebral column. It is important to remember that during life the kidney moves upwards and downwards, following the respiratory movements of the part of the diaphragm against which it rests.
The long axis of each kidney is somewhat oblique, its superior end approaching nearer to the median plane than the inferior. The surface of the kidney which is applied against the muscles forming the posterior wall of the abdomen looks,
as a whole, backwards and medially, and that which projects into the abdominal cavity looks forwards and laterally. Hence it happens that the lateral border lies on a more posterior plane than the medial border. The kidney is rotated in this manner on its long axis to such a degree that the medial margin and hilum are scarcely visible from behind, and only a limited view of the lateral border can be obtained from the front (Figs. 979 and 980).
The kidneys are placed behind the peritoneum, and project into the posterior part of the abdominal cavity. Each is surrounded by a considerable amount of loose tissue, often loaded with fat; the fatty tissue, or capsula adiposa, being present in greater quantity round the margins of the kidney, and only to a less extent in front of and behind the organ. The renal vessels and nerves lie in this fat before they enter the kidney, and the adipose tissue is continued, along with the vessels, through the hilum into the renal sinus, where it fills up all the space unoccupied by the vessels and nerves.
Embedded in the soft fatty tissue surrounding the kidney is a layer of fibrous
FIG. 980.-TRANSVERSE SECTION THROUGH THE BODY OF A CHILD. The position and relationships of the kidneys are well seen, and the arrangement of the fascia renalis is indicated. The fascia is coloured green.
tissue to which the term fascia renalis is applied. This fascia surrounds the kidney and a considerable amount of its fatty capsule in the form of a loose sheath, in which may be distinguished anterior and posterior walls. The sheath is open inferiorly and medially, but closed above and to the lateral side of the kidney by the apposition of its walls. Laterally, the anterior and posterior walls of the sheath come into contact and are connected with the retro-peritoneal tissue. Medially they remain distinct, and the anterior wall is continued across the median plane, in front of the renal vessels and the aorta, to join the corresponding layer of the opposite side, while the posterior wall fuses with the fascia covering the psoas and quadratus lumborum muscles. Inferiorly, below the level of the kidney, the anterior and posterior layers of the renal fascia remain separate, and can be traced downwards into the iliac fossa. Above the level of the kidney and the suprarenal gland the layers of the renal fascia unite and join the fascia covering the diaphragm. It has been suggested that the terms "tunica adiposa" and "perinephric fat" should be restricted to the loose fatty tissue enclosed along with the kidney within the sheath of renal fascia, and that the term "paranephric body," or "fat," should be used to denote the tissue outside the sheath. The fibrous capsule of the kidney is joined to the loose sheath formed by