SECRETION (Part 1)

CHAPTER XIV SECRETION Secretion is a term applied to a process by which a portion of the constituents of the blood are separ- ated from the blood-stream, by the activities of the endothelial cells of the capillary walls, as the blood flows through the capillaries. In this process the endothelial cell is aided by the physical forces — diffusion, osmosis, and filtration. The materials thus separated are collectively termed lymph (Brubaker). These secretions are utilized and adapted to many and complex functions, dependent upon the secre- tory organ which secretes the fluid and the membrane it is poured out upon. They enable the tissues of the body to repair, grow, and produce heat and energy. Others are to promote digestion, etc., remove foreign bodies (dust, etc.) from membranes, as the conjunctiva, to prevent friction between the serous membranes, as the pericardium, pleura, and peri- toneum; and to prevent friction between the ex- tremities of the bones entering into the formation of the joints, as the fluid in synovial membranes. Secretions are divided into internal and external secretions. Internal secretions are fluids secreted by the epithelial cells of certain organs of the body which do not possess any ducts by which their secretion is poured into any cavity or organ, but is reabsorbed into the blood, and the contained specific constituents aid in the nutrition of the body. These organs are: the thyroid, thymus, adrenal, spleen, pituitary glands, hypophysis, etc. (See description of the Ductless Glands, page 303.) THE SECRETING MEMBRANES 291 External secretions are fluids of a definite consist- ency and known function which, when secreted by the epithelial cells and poured from the organ by means of a duct or ducts on to the membrane they are to bathe, etc., perform this given activity. Such secretions are: the saliva, mucus, milk, gastric juice, sebaceous matter, etc. The epithelium lining the secretory organs have a general similar histologic arrangement, and resem- blance; however, the difference in the constituents of the secretion is supposed to be based upon their ulti- mate chemic structure. The epithelial secretory organ consists of a thin, delicate membrane lined on its functionating surface by a layer of epithelial cells and on the outer side by a net-work of capillary bloodvessels, lymph vessels, and nerves. The epithelial secretor organs are subdivided into: (1) secreting membranes; (2) secreting glands. THE SECRETING MEMBRANES These are the membranes lining the pulmonary and gastro-intestinal tracts, the genito-urinary tracts, and the serous membranes lining closed cavities, such as the pleural, pericardial, peritoneal, and synovial membranes. The secretion from the various epithelial cells lining mucous membranes possesses different com- position, according to the locality. It is called mucus, a pale, semitransparent, alkaline fluid containing white cells and epithelial cells. Chemically it consists of water, mineral salts, and a protein mucin. Most of the mucus is secreted by the goblet cells. Mucus is classified according to where secreted, as nasal, bronchial, vaginal, urinary, and gastro-intestinal. The serous membranes are practically large lymph 292 SECRETION spaces and the contained fluid is practically lymph. It diminishes friction when the organs they enclose rub against one another. Synovial membranes secrete a fluid resembling lymph, but it also possesses a protein — a mucin-Iike sub- stance, which renders it viscid. Synovial membranes prevent friction between adjacent surfaces of bone entering into the formation of joints. The other secretions of the body, as the aqueous humor of the eye, gastric secretions, etc., will be described under the physiology of the parts. THE SECRETING GLANDS These are a group of cells given ofl' as a pouch from the mucous membrane or skin, and communicating with the lining membrane or surface the secretion is to act upon by means of an open passageway, called a duct. Their epithelial cells vary in their structure and function dependent on their location. The epithelial cells of the secretory glands are sur- rounded by a net-work of blood capillaries, lymph vessels, and nerves; the nerves are in direct connec- tion with the epithelial cells and bloodvessels. How these epithelial cells absorb from the lymph and blood plasma their essential constituents of the secretions and change them into their difl'erent chemic and physiologic fluids is not definitely known, except that they are the result of metabolic processes going on within the cells. All secretory glands are controlled by nerve centres situated in the central nerve system. Normal secretions of glands are brought about by a reflex action. In all reflexes there must be a sensitive surface to receive the impression (skin, mucous mem- brane, etc.), an afferent nerve (one which transmits the impression to the centres in the brain), an efferent nerve (one which transmits the return stimulus to THE SECRETING GLANDS 293 a responsive organ — in this case the cells of the secre- tory organ or gland). The active discharge of the secretion from the cells is interrupted by periods of rest, during which time, if they be examined under the microscope, after the absorption of lymph, they will show accumula- tions within themselves of their characteristic prod- ucts as globules of mucin — granules which are the basic formation of the digestive ferments or enzymes, granules of glycogen, globules of fat, sugar, and protein, as in the case of the mammary gland. Excretion is a process similiar to secretion, the only difference being that the fluids removed are the waste products from the cells formed as a result of metabolism. The Mammary Glands. — These are accessory to the reproductive system and secrete the milk. They are two rounded eminences, one on each side of the thorax, between the sternum and axilla and the third and seventh ribs. Just below the centre is a conical eminence, the nipple, which is dark, and is surrounded by a pinkish areola which darkens in pregnancy. It presents the orifices of the lactiferous ducts, and consists of vessels mixed in with plain muscular fibers,, and by friction may be made to undergo erection. The mamma consists of a number of lobes separated by fibrous tissue and some adipose tissue. The lobes are divided and subdivided into smaller lobules, which are in turn made up of alveoli. Each lobe has an excretory (galactophorous) duct, and these, about sixteen in number, converge to the areola, there dilating into ampullae or sinuses. TJiey then become smaller again, and surrounded by areolar tissue and vessels, pass through the nipple to empty on the surface by separate orifices. Milk. — Milk as obtained from the breast during active secretion or lactation is an opaque, bluish- 294 SECRETION white fluid, without any oder, sweetish in taste, alkaHne in reaction, and has a specific gravity of from 1.025 to 1.040. Examined microscopically it presents a clear fluid called the plasma, which holds in suspension great numbers of oil globules. The Fig. 108 Lactiferous duct. Lobule. Ampulla. Loculi i)) connective tissue. Dissection of the lower half of the female breast during the period of lactation. (From Luschka.) amount of milk secreted each day by a healthy woman averages about 1200 c.c. Milk is the most important of the animal foods, containing all the elements necessary to properly nourish and develop the body, and is used as a food. THE SECRETING GLANDS 295 Differences in chemical composition of human and cows' milk (Holt): Woman's milk Cows' milk average average per cent per cent. Fat 4.00 4.00 Sugar 7.00 4.50 Proteins 1.50 3.50 Salts 0.20 0.75 Water 87.30 87.25 100.00 100.00 By the above it will be seen that cows' milk has an excess of proteins and salts, and is deficient in sugar. The secretion of milk is influenced by emotional states, both as to quantity and quality, due to some connection between the nerve centres and the gland cells. Colostrum. — This is the first fluid secreted by the breasts after the birth of the infant. It is a liquid which resembles milk, and contains epithelial cells, fat globules, colostrum corpuscles. Colostrum is richer than the milk, containing more lactose and inorganic salts. According to some authors it contains compounds which act as a laxative to the newborn. The Liver (Hepar). — Description of the Liver. — The liver is the largest gland of the body, and fills the entire hypochondrium, the greater portion of the epigastrium, sometimes extending into the left hypo- chondrium. It weighs from 50 to 60 ounces in the male; 40 to 50 ounces in the female. Constitutes one-eighteenth of the body weight in the adult, and one-thirty-sixth of the body weight in the fetus. It measures, transversely, from 8 to 9 inches; antero- posterior, 4 to 5 inches, and vertically, near its right surface, about 6 or 7 inches. Its specific gravity is 1.05. The liver presents a superior surface which includes the right and left lobes; an inferior surface, including the right, left, caudate, spigelian, and quadrate lobes; 296 SECRETION |i 1 1 ■Sj » «0 a. o 1 S'l 1 1 <.6 •e S ^ « ^ THE SECRETING GLANDS 297 anterior and posterior surfaces ('()ini)rising the right and left lol^es; a lateral surface of the right lobe, only. It has an inferior border or margin which is thin and sharp, and notched opposite the falciform ligament, for the round ligament (umbilical notch), and opposite the cartilage of the ninth rib by a second notch for the fundus of the gall-bladder. The left extremity of the inferior margin of liver is thin and flattened from above downward. The ligaments of the liver are all peritoneal folds, except the round ligament, which is a fetal remnant of the umbilical vein. The ligaments hold the liver in position, and are as follows: Falciform or suspensory. Left lateral. Coronary. Round. Right lateral. The lobes of the liver are also five in number. The right is the largest, being six times as large as the left. The left lobe is flattened, lies in the epigastrium, and is in relation below with the stomach. The lobus quadratus is on the under surface of the right lobe. The Spigelian lobe lies behind and above the preceding. The caudate lobe, or tubercidum caudatum, runs outward from the base of the Spigelian lobe to the under surface of the right lobe. The fissures of the liver are five. The longitudinal separates the right and left lobes. The fissure of the ductus venosus is the part of the longitudinal fissure behind the transverse. The transverse is the point of exit (hepatic ducts) and entrance of the portal vein, hepatic arteries, nerves, and lymphatic vessels. The fissure for the gall-bladder is on the under surface of the right lobe, parallel to the longitudinal fissure, sepa- rated from it by the quadrate lobe. The fissure for the inferior vena cava, sometimes a complete canal, lies to the right of the Spigelian lobule. The Structure of the Liver. — It is covered by a serous layer derived from the peritoneum, except the posterior 298 SECRETION surface, which is in relation with the diaphragm for about 3 inches, included between the reflections of the coronary ligaments. Beneath this serous covering is a fibrous or areolar capsule (capsule of Glisson), which passes into the transverse fissure around the vessels and blends with the areolar tissue which holds the liver lobules together. Fig. 110 Triinlc of infralobn vein. .ntralobnlar rein Horizontal section of injected liver (dog). The lobules compose the main mass of the liver substance, and consist of irregular chains of hepatic cells, which secrete the bile, and are surrounded by a capillary net-work of intralobular veins, which are the minute terminations of the portal vein; they course toward the centre of the lobule, opening into a central intralobular vein; also small arteries, branches of the hepatic artery, lie between the cells. In addition, within the chain of cells are the minute biliary ducts, or capillaries, which are the commence- ment of the hepatic duct that conveys the bile formed THE SECRETING GLANDS 299 by the liver cells to the intestinal canal and gall- bladder. The Functions of the Liver. — (1) The liver secretes the bile; (2) produces and stores glycogen until needed to aid in the nutrition of the tissues; (3) aids in the formation and excretion of urea. The production of bile and its physiologic actions have been described. (See Part Played by the Bile in Digestion, page 281.) The Formation and Function of Glycogen. — Glycogen is derived from the dextrose resulting from the action of the intestinal juices upon the food. It represents the products of the carbohydrates absorbed as dextrose and carried in the blood by the branches of the portal vein to the liver, when it undergoes chemical changes, due to the action of the liver cells, and is deposited as a non-diffusible body. Glycogen is stored in the liver until needed by the body tissues. The Formation of Urea. — Urea is believed to be produced and excreted by the liver cells. It is formed from ammonium salts, as carbonate, lactates, which are formed as a result of tissue metabolism upon the proteins contained in the food we eat, and these salts are absorbed from the tissues or from the intes- tines, and conveyed by the blood to the liver cells, where they are converted into urea and eliminated as a waste product. It is excreted by the kidneys and found in the urine. The Gall-bladder. — This is a pear-shaped sac lying in the impression of the right lobe of the liver. It is the reservoir for the bile. It is 4 inches long and 1| inches broad, holding 8 to 12 drams, and is held in place by areolar tissue and the peritoneum. Its relations are as follows: Above, liver; below, ascending duodenum, pyloric end of stomach, hepatic flexure of colon; in front, abdominal, wall (ninth or tenth costal cartilages) . The hepatic duct is formed by the junction at an obtuse angle of a branch from each lobe of the 300 SECRETION liver and runs downward and to the right for nearly 2 inches and joins the cystic duct to form the common bile duct. The cystic duct of the gall-bladder is 1 1 inches Fig. Ill COMMON DUCT GALL- BLADDER The gall-bladder and bile ducts, opened up. (Spalteholz.) long, and descends tow^ard the left and joins the above as described. The common bile duct is nearly 3 inches long and 3 lines in diameter. It runs along the right border of the lesser omentum, behind the first part THE SECRETING GLANDS 301 of the duodenum, and between the pancreas and descending duodenum, then to the right of the pan- creatic duct, with which it communicates by a common orifice, at the summit of a papilla situated just below the middle of the inner wall of the second portion of the duodenum. The cystic artery and veins comprise the blood-supply of the gall-bladder and its duct. Fig. 112 The pancreas and its relations. (Gray.) The Pancreas. — The pancreas is a compound race- mose gland, of a pinkish-white color. Situated at the back of the epigastrium and left hypochondrium ; connected to the posterior abdominal wall by connected tissue, and lies posterior to the stomach and behind the peritoneum. It is 5 or 6 inches long; its breadth is 1| inches; its thickness J to 1 inch, being greater at its right extremity and upper border. The pancreas is divided into a head, a neck, a body, and a tail. The duct of the pancreas is called the pancreatic duct or canal of Wirsung. It extends transversely 302 SECRETION through the substance of the gland to drain the lobules by means of small ducts which open into it. Increasing in size it reaches the neck, passes down- ward, backward, and obliquely to the right, piercing the muscular and mucous coat of the second portion of the duodenum where it opens into the ampulla of Vater, common to it and the bile duct; the latter opens into the canal of the duodenum. The Structure of the Pancreas. — It is similar in structure to the salivary glands, consisting of numbers of lobules, forming lobes, and all held together by connective tissue. Each lobule contains one of the branches of the main duct, which terminates in the grape-like alveoli. The alveoli are lined by cylindric cells, which differ in their appearance. They are divided into a central set, in the end of the alveoli, which are dark and granular, and a peripheral set, in the outside of the former, which are clear. During digestion the granular area becomes broader and the cells show an increase in granules; in the interval of rest following active digestion the clear zone increases in width, showing an absence of granules. The Areas or Islands of Langerhans are groups of globular cells arranged in columns situated between the alveoli; surrounded by connective tissue, which separates them from the alveoli and each other. The connective tissue contains large, twisted, capillary bloodvessels. These groups of goblet cells are supposed to secrete an internal secretion, which is absorbed by the blood and carried to the different tissues. Metab- olism of the carbohydrates is interfered with, if any diseased condition or removal of the pancreas takes place. The secretion from the cells of the alveoli, on the other hand, secretes the pancreatic juice. The pancreatic secretion^ leaves the pancreas by way of the duct of Wirsung; it is supposed to create ^ See page 279 for action of pancreatic secretion during digestion. THE SECRETING GLANDS 303 an internal secretion which regulates the production of glycogen by the liver, thus possessing both an internal and external secretory function. Fig. 113 Interlobular duct. Section of human pancreas, showing pancreatic islands. (Radasch.) Ductless Glands. — The ductless glands of the body are: Thyroid. Parathyroids. Spleen. Carotid. Thymus. Suprarenal capsules. Pituitary body. Coccygeal. The above glands aid in the nutrition of the whole body as well as in that of individual organs by means of an internal