The stomach [Latin: ventriculus ; gaster; French: estomac] is a dilation of the alimentary canal succeeding the esophagus. In the stomach the food is mixed with the gastric juice and reduced to a viscid, pulpy liquid, the chyme [chymus], which undergoes a certain amount of digestion and absorption before passing into the duodenum.
The stomach is a somewhat pear-shaped organ located in the upper, left side of the abdominal cavity. It presents a body [corpus ventriculi], with an enlarged upper end or fundus, on the right side of which is the cardia, the aperture communicating with the esophagus. The body of the stomach is extremely variable in form, as will be explained later, but is in general divisible into a more expanded upper two-thirds, the cardiac portion [pars cardiaca], which is nearly vertical, and a more constricted lower third, the pyloric portion [pars pylorica], which tm-ns horizontally toward the right. The pyloric portion often presents toward its lower end a slight, variable dilation, the antrum pylori, succeeded by a short constricted pyloric canal (Jonnesco). At the lower end of this canal the pylorus forms the aperture leading into the duodenum, and contains a thick sphincter derived from the circular fibers of the muscular layer. The stomach has two borders and two surfaces. The medial (or upper) border forms the lesser curvature [curvatura ventriculi minor], which is concave (except near the pylorus) and gives attachment to the lesser omentum. The lateral (or lower) border forms the greater curvature [curvatura ventriculi major], which is convex, and gives attachment to the great omentum. The curvatures separate the anterior surface [paries anterior], which faces forward and upward, from the posterior surface [paries posterior], which is placed backward and downward.
Dimensions of the stomach
The dimensions of the stomach are subject to great variation and therefore only a gross approximation can be given. The length of the lesser curvature averages about 10 cm. (7.5 cm. to 15 cm.), and that of the greater curvature is three or four times as great. The diameter varies exceedingly according to the amount of contents. When nearly empty, it presents, especially in the pyloric portion, a narrow tubular form, with a diameter of about 4 cm. or 5 cm. The diameter of the pylorus, which is the narrowest point in the alimentary canal when constricted is only about 1.5 cm. It is distensible, however, as hard bodies with diameters of 2 cm. or more may readily pass through.
The average capacity of the stomach is between one and two liters, being subject to extreme individual variations. In the newborn, it averages about 30 cc. (25 to 35 cc), increasing very rapidly in the early postnatal months and reaching an average of 270 cc. at one year (Lissenko). The average weight of the adult stomach is about 135 gm.
Position and relations of the stomach
The position and relations of the stomach, like its form and structure, are subject to many variations in different individuals, and in the same individual according to changes in physiological condition, posture, etc. It is therefore difficult to give a concise and accurate description.
The normal position of the stomach has long been disputed. It is generally recognized that the long axis is oblique, extending from above downward, forward and to the right. Some, however, especially among the older anatomists, have maintained that the gastric axis normally approaches more nearly to the horizontal type, with the pylorus but little below the cardia. Others, especially among the more recent anatomists, have maintained that the axis of the stomach is normally more nearly vertical in position. The results of an extended and careful study, both in formalin-hardened bodies and by means of the Roentgen-rays in the living body, demonstrate that there is much variability in the position of the stomach. Both the horizontal and the vertical types may occur as the extremes of normal variation, but the more usual type is the intermediate oblique position. The gastric axis, however, is not straight, but somewhat curved and bent in a reverse L-shape. The larger cardiac portion is approximately vertical (especially when the trunk is in the upright posture) the smaller pyloric portion more nearly horizontal. In the empty stomach, the pylorus opens into the duodenum from left to right. In distention, however, the pylorus is carried in front of the duodenum. In extreme distention, it is carried to the right and downward so as to open upward and to the left.
In surface relation, the stomach lies within the left hypochondriac and the epigastric regions. Often, however, especially when distended, it extends into the umbilical and even the right hypochondriac region. When empty, it usually lies almost entirely in the left half of the body, with the pylorus not more than 1 cm. or 2 cm. to the right of the mid-sagittal plane. When distended, the long axis of the stomach is lengthened and the pylorus is displaced 5 cm. or more to the right and downward. In distention, the stomach expands in all directions (except posteriorly), and does not appear to rotate as is sometimes stated. The position of the stomach, especially when distended, also varies appreciably according to the posture of the body. It sags downward when the body is in the upright position, and to the right or left when the body is placed on the corresponding side. The cardia lies on the left side of the 10th or 11th thoracic vertebra, and corresponds to a surface point behind the left 7th costal cartilage about 2.5 cm. from its sternal end. The pylorus usually lies opposite the right side of the 1st lumbar vertebra, about midway between ensiform cartilage and umbilicus, or in Addison's 'transpyloric line', midway between the suprasternal notch and the symphysis pubis, when the body is recumbent; but descends to the 2d or lower in upright posture. The fundus corresponds to the left dome of the diaphragm (which separates it from the lung and heart) , opposite the sixth sterno- costal junction. The fundus of course rises and falls with respiratory movements of the diaphragm, the excursion being from 2 to 6 cm.
The anterior surface is in contact on the right with the left lobe of the liver, the pylorus reaching the quadrate lobe; on the left it is in contact with the diaphragm (separating it from the heart and left lung) ; and below with the anterior body wall by a triangular area of variable size. The posterior surface is in relation (separated by the lesser sac) with the pancreas, above which are areas of contact with the diaphragm, spleen, left kidney and suprarenal body; below the pancreas, the stomach is in contact with the transverse mesocolon, and through this with the transverse colon and coils of small intestine. Further details concerning topography of the stomach are given in Clinical and Topographical Anatomy.
Peritoneal relations of the stomach
The stomach is covered by peritoneum in its whole extent, except immediately along the curvatures and upon a small triangular space at the back of the cardiac orifice, where the viscus lies in direct contact with the diaphragm and possibly with the upper part of the left suprarenal gland. It is enclosed between two layers. These two layers at its lesser curvature come together to form the gastro-hepatic portion of the lesser omentum, and at the greater curvature extend downward to form the great omentum. At the left of the esophagus the two layers pass to the diaphragm, forming the gastro-phrenic ligament; and at the fundus they pass on to the spleen, forming the gastro-splenic ligament.
The posterior surface of the stomach is in relation with the lesser sac (bursa omentalis), forming part of its anterior wall. The anterior surface of the stomach is in relation with the greater sac of the peritoneal cavity.
The stomach is composed of the four typical layers of the alimentary canal - mucosa, submucosa, muscularis and serosa. The mucosa is thrown into a series of coarse folds (plicae mucosae), chiefly longitudinal, which disappear when the stomach is distended. Along the lesser curvature, the ridges are more regular (corresponding to Waldeyer's 'Magenstrasse') and form a longitudinal grooved channel from cardia to pylorus. Upon closer examination the inner surface of the mucosa presents a somewhat warty ('mammilated') appearance, due to numerous small elevated areas [areae gastricae], varying from 1 to 6 mm. in diameter. When examined with a lens, it is seen that each area is beset with numerous small pits [foveolae gastricae], separated by partitions which sometimes (especially in the pyloric region) bear villus-hke prolongations [plicae villosae]. The average number of foveolae is estimated at 87 per sq. mm., or more than 6 millions for the entire stomach (Toldt). Into each pit or foveola open 3 to 5 gastric glands. The entire surface is covered with a simple columnar mucigenous epithelium.
The thickness of the mucosa varies, being greatest (about 2 mm.) in the pyloric region, decreasing to less than .5 mm. in the cardiac region (Kolliker). The lamina propria is crowded with glands, of which three varieties are distinguished. The cardiac glands are tubulo-racemose (chiefly mucous) glands occupying a narrow zone a few millimeters in width adjacent to the cardiac orifice. The fundic glands [gl. gastricae propriae] occupy the greater part of the stomach, and are simple (partly branched) tubular glands. They contain three varieties of cells- mucous cells, peptic cells, and parietal cells. The parietal cells may secrete an organic chloride com- pound, but the HCl of the gastric juice is formed not in the gland tubules but at the surface of the mucosa (Harvey and Bensley). The pyloric glands [gl. pyloricae] are branched tubular glands occupying the pyloric region. Whether they are merely mucous or also secrete pepsin is still in dispute.
The interstitial tissue of the lamina propria contains diffuse lymphoid tissue and a few small lymph nodules, especially in the pyloric region. The muscularis mucosae is a thin sheet of smooth muscle lying just below the fundus of the glands and is composed of an inner circular and an outer longitudinal layer.
The tela submucosa is a very loose areolar, vascular layer which permits the wrinkling of the mucosa according to the degree of distention.
The tunica muscularis contains three layers of smooth muscle. The outer or longitudinal layer [stratum longitudinale] is thickest along the lesser curvature, and is continuous with the longitudinal fibers of the esophagus and the duodenum. On the anterior and posterior walls of the antrum pylori, the longitudinal fibers form thickened bands, the ligamenta pylori. The middle or circular layer [stratum circulare] is continuous with the circular fibers of esophagus and duodenum and surrounds the entire stomach. It is especially thickened in the region of the pyloric canal, at the lower end of which it forms a thickened ring-like band, the pyloric sphincter [m. sphincter pylori]. The inner or oblique layer [fibrae obliquae] is composed of fibers continuous with the deepest circular fibers of the esophagus. They form an incomplete layer which encircles the fundus and passes obliquely downward around the body of the stomach toward the greater curvature.
The external tunica serosa is formed by the peritoneum, and has the smooth shiny appearance and the structure typical for a serous membrane.
Blood-vessels of the stomach
The stomach receives its blood-supply from many branches. From the coeliac axis there is the left gastric artery, which runs along the lesser curve from left to right, anastomosing with the right gastric branch of the hepatic. Along the greater curve run the right and left gastro-epiploic arteries, anastomosing at the middle of the border, the left being a branch of the splenic, the right a branch of the hepatic, through the gastro-duodenal artery. The stomach also receives branches from the splenic (vasa brevia) at the fundus. The vascular arches along the curvatures of the stomach are comparable to those in the intestinal mesentery (Mall).
The blood of the stomach is returned into the portal vein. The coronary vein and pyloric vein open separately into the portal vein; the right gastro-epiploic vein opens into the superior mesenteric, the left into the splenic.
The rich capillary plexus in the mucosa supplies the glands and also serves for absorption.
Lymphatics of the stomach
There is a set of nodes lying along the lesser and the pyloric portion of the greater curvature, and others at the pyloric and cardiac ends. These are entered by lymphatic vessels which, beginning in the mucous membrane, accompany all the gastric veins, but chiefly those of the lesser curvature. Vessels also accompany the left gastro-epiploic veins to terminate in the splenic nodes. On its way to the receptaculum chyli, the gastric lymph passes through groups of nodes [lymphoglandulae pancreaticolienales] situated above and behind the head and neck of the pancreas.
Nerves of the stomach
The nerves of the stomach are derived in part from the vagi (which form the motor fibers of the stomach), the right vagus descending on the posterior wall, and the left on the anterior wall. The stomach also receives sympathetic branches from the coeliac plexus, following the arteries. Small ganglia occur along both vagus and sympathetic branches (Remak). The nerves join the gangliated plexuses, myenteric and submucous, in the wall of the stomach, from which branches are distributed to the muscularis and the mucosa as for the intestine in general.
Development of the stomach
The stomach at first lies in the mid-sagittal plane in the cervical region. It participates in the general descent of the viscera (the esophagus becoming correspondingly lengthened) and reaches its permanent vertebral level in the 17 mm. embryo (Jackson). In the meantime, beginning in the 7.5 mm. embryo (F. T. Lewis), a rotation of the stomach has occurred. The rotation is around the long axis, so that the anterior border (lesser curvature) is turned to the right, and the posterior border (greater curvature) to the left. The right surface therefore becomes posterior and the left anterior. During the process of descent, the pyloric end is the first to become fixed (at about 12 mm.). As the cardiac end continues to descend, it is displaced to the left, so the oblique position of the stomach is established early. The stomach is at first spindle-shaped, but the upper end begins to enlarge at about 10 mm. The fundus develops somewhat later as a localized outgrowth (Keith and Jones).
The fetal stomach is somewhat crowded to the left by the relatively large liver, and its relations to surrounding organs undergo considerable change. Even in the fetus it is quite variable, but its general form and position do not differ essentially from the adult condition.
Glands of the stomach
According to Johnson, in an embryo of 16 mm., the lining epithelium shows the primitive foveolae as pit-like depressions which become elongated, forming irregular anastomosing grooves, separated by villus-like projections. The pits multiply and deepen, and from their bottoms the gastric glands bud off (at 120 mm.). The parietal cells appear very early in the gland fundus, but the differentiation of gland cells is still incomplete at birth.
The circular layer of muscle is indicated at 16 mm.; the longitudinal much later, about 90 mm., and not completed before 240 mm. (F. T. Lewis).
Variations of the stomach
The great variability of the stomach in form, position and relations has already been repeatedly emphasized. These variations have been most carefully studied recently by various observers in the living body by means of the Roentgen-rays.
It would appear that most of the variations in the form of the stomach that have been described are merely various phases in the series of changes undergone by the stomach during the normal process of physiological digestion. The following account of these changes is based largely upon the radiographic observations of Cole. Earlier observations by various investigators upon the living stomach of man and lower animals (and especially the radiographic study of the cat by Cannon) have shown that the cardiac portion of the stomach is the first to become distended with food (and gas). Until a considerable degree of distention is reached, the pyloric portion usually remains a somewhat narrow contracted canal, along which distinct peristaltic contractions pass pylorusward.
Under favorable conditions, however, the peristaltic contractions may be observed to begin in the cardiac portion, although they are usually most distinct in the pyloric portion. Each individual contraction travels at the rate of about 2,5 cm. (1 inch) per second, so that it requires several seconds for a contraction to travel from fundus to pylorus. The number of simultaneous contractions present in the stomach varies from 1 to 6 or 7, 3 or 4 being the most common. The peristaltic movements are further complicated by the appearance (simultaneously in all) of successive periods of 'systole,' during which the peristaltic contractions become stronger and deeper, and ' diastole,' in which the contractions relax and become less distinct (Cole).' A 'systole' and a 'diastole' together make up a ' gastric cycle.' During the entire progress of an individual peristaltic contraction from fundus to pylorus, the number of 'cycles' appears to correspond to the number of peristaltic contractions present. The time required for a ' cycle ' varies widely, the average (in the 3- or 4-cycle type) being about 2 or 3 seconds.
In the earlier stages of gastric digestion the pylorus usually remains closed, but after a variable time it relaxes slightly (lumen about 3 mm. in diameter) at intervals, allowing the chyme to be spurted into the duodenum.
Thus the various constrictions often found in the formalin-hardened stomachs, and the pyloric antrum, appear to be merely transient phases of the digestive process. The 'hour- glass' stomach is in many cases to be explained in this way; in others, however, the constriction is pathological and permanent. Various forms of abnormal lobulations and dilations also rarely occur.
Gastroptosis is a very common abnormality in which the body of the stomach extends vertically downward to the umbilicus, or lower, forming a sharp bend beyond which the pyloric portion turns upward to reach its termination. This form is especially common in women, due to tight lacing.
The primitive stomach is perhaps merely a receptacle for food, true digestive glands being absent in many of the fishes. The vertebrate stomach is a dilated sac of variable form, but is typically somewhat looped, with cardiac and pyloric segments. In birds, there is a peculiar arrangement, correlated with the absence of teeth. The stomach is divided into an anterior glandular proventriculus, and a posterior muscular gizzard with a horny lining serving to grind the food. The mammalian stomach is the most variable in form and structure which are correlated with the method and character of alimentation. The cardiac end of the stomach is often lined to a variable extent with a prolongation of the esophageal stratified squamous epithelium. The three kinds of glands, cardiac, fundic and pyloric, are typically present. In general, the stomach is larger and more complicated in herbivora than in carnivora. Instead of being a single sac, the stomach may be more or less divided into chambers. An incomplete division into cardiac and pyloric portions is so common that it may be considered typical. The most extreme specialization is found in the ruminants. In these the stomach has four chambers, the first two of which, however, are expansions of the esophagus.
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