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vestibule is continued beyond a fold termed the plica gastropancreatica into the true bursa omentalis, which presents two main parts, the recessus superior and recessus inferior. The recessus superior passes from the vestibule upwards behind the porta hepatis to the dorsal surface of the lobus caudatus. Posteriorly it is in contact with the diaphragm and aorta. The recessus inferior extends medially in front of the pancreas, and behind the stomach, and passes to the left towards the spleen as the recessus lienalis.
The recessus inferior omentalis and the vestibulum bursæ omentalis communicate with one another by a rounded orifice, which is constricted by the sickle-shaped forward projecting fold, the plica gastropancreatica. This fold is an elevation of the peritoneum of the posterior wall of the omental bursa, raised up by the arteries of the stomach as they pass forwards to that organ from the posterior wall.
As the peritoneal wall of the omental bursa is described in two main parts, an anterior and a posterior, it will be necessary to follow each of these separately. Above, the peritoneum forming the anterior wall clothes the caudate lobe; it then passes down (from the posterior margin of the porta hepatis, and the fossa of the ductus venosus) to the lesser curvature of the stomach and the duodenum
Inferior vena cava
FIG. 968.-TRANSVERSE SECTION OF ABDOMEN THROUGH THE FOURTH LUMBAR VERTEBRÆ
as the posterior layer of the lesser omentum. Continued on from this, it clothes the posterior (or visceral) surface of the stomach as far as the greater curvature, with the exception of the small "uncovered area" below and to the left of the cardia (Fig. 969), but it does not actually come in contact with the oesophagus itself, the back and right side of which are uncovered. On the left, it is reflected from the back of the stomach to the spleen as the deeper layer of the gastro-splenic ligament.
From the greater curvature of the stomach it is continued down, forming the posterior layer of the gastro-colic ligament as far as to the transverse colon. In some cases it is not attached to the colon, but is continued onwards as the posterior of the anterior two layers of the omentum, and in such cases at the inferior part of the omentum it meets and becomes continuous with the posterior layer of the omental bursa.
The peritoneum forming the posterior wall of the bursa omentalis, in passing through the epiploic foramen, clothes the front of the inferior vena cava (Fig. 969); beyond this, it covers the coeliac artery, and passes upwards to line the slight depression on the posterior abdominal wall (diaphragm), against which the caudate lobe rests. Then, passing over to the left, it covers the superior surface of the pancreas, the top of the left kidney and suprarenal gland, and the medial part of the gastric surface of the spleen (Fig. 969). From the anterior border of the
pancreas it is prolonged anteriorly and downwards-as the anterior or upper layer of the transverse mesocolon-to the transverse colon (Fig. 969). Here it usually joins the gastro-colic ligament, but in English text-books it is described, and here it is figured, as being continued down as the anterior layer of the posterior fold of the greater omentum, almost to its inferior border, where it becomes continuous with the anterior layer of the omental bursa already described.
The transverse tracings at the level of the epiploic foramen and pylorus are shown in Figs. 966 and 967, and can be easily followed without any further description than is there given.
Top of omental bursa
Corresponds to uncovered area' of stomach
Left end of transverse mesocolon
Transverse mesocolon (cut)
FIG. 969.-THE PERITONEAL RELATIONS OF THE DUODENUM, PANCREAS, SPLEEN, KIDNEYS, ETC.
The lesser omentum is described at p. 1162; it need only be pointed out now that it is composed of two layers, the anterior belonging to the general peritoneal cavity, and the posterior belonging to the omental bursa; and both layers are extremely thin-sometimes even cribriform.
The greater omentum is a large apron-like fold of peritoneum, usually more or less loaded with fat, which is suspended from the transverse colon, and hangs down in front of the intestines to a variable extent. When the abdomen is carefully opened without disturbing the viscera, it is rare to find the greater omentum evenly spread over the front of the intestines. More commonly it is folded in between some of the coils of intestine, or tucked into the left hypochondrium; or perhaps it is carried upwards in front of the stomach by a distended transverse colon. The gastro-colic ligament continues the anterior layers of the greater omentum upwards to the stomach, and the gastro-lienal ligament continues them
between the spleen and stomach. The greater omentum is thus commonly described as extending between the greater curvature of the stomach above and the transverse colon below, not taking the shortest course from one of these to the other, but hanging down as a loose fold between the two, and containing between the anterior and posterior folds a cavity continuous with the omental bursa (Fig. 968).
This is the condition in the embryo, but in the adult the anterior fold usually becomes adherent to the colon, and, below it, also to the posterior fold, and hence the cavity becomes largely obliterated. If the gastro-colic ligament be included as part of the greater omentum, and the embryonic condition is retained, as is usually described in English text-books, the greater omentum may be said to consist of two folds, each formed of two layers, one derived from each sac of the peritoneum. The anterior or descending fold begins at the greater curvature of the stomach, where it is formed by the meeting of the two layers from the anterior and posterior surfaces of that organ respectively; from there it descends to the front of the transverse colon. Leaving the colon, or passing anterior to it, see Fig. 964, the two layers proceed to the lower border of the omentum, where, turning back (Fig. 964), they pass up as the posterior or ascending fold. That runs upwards until it meets the transverse colon; there its two layers separate to enclose and cover that colon-and the greater omentum, properly so called, ceases. Its two layers, however, unite at the superior margin of the colon (Fig. 964) to form the transverse mesocolon, which is continued upwards and posteriorly to the anterior border of the pancreas. There the layers of the transverse mesocolon again separate-the superior running upwards over the anterior surface of the pancreas to the posterior abdominal wall and lining the omental bursa; the lower passing downwards on the posterior abdominal wall, as already explained.
The greater omentum is continued to the right for a short distance (25 mm.) along the inferior border of the duodenum. At the left end it shortens very much, and is directly continued into the gastro-lienal ligament; the spleen, as it were, being introduced between the two layers instead of the colon.
Functions of the Greater Omentum.-Numerous uses have been assigned to the great omentum; the chief seem to be: (1) To act as a movable and easily adjustable packing material, capable of filling all temporarily-produced spaces in the abdomen. In this respect it may be compared with the Haversian fatty pads in joints. (2) It probably, to some extent, prevents the passage of the small intestines up into the stomach chamber, and helps to keep them from getting entangled there. (3) It is a storehouse of fat. (4) It is said to be "the great protector against peritoneal infectious invasions." Being freely movable, it can pass to almost any part of the abdomen, and there "build up barriers of exudations to check infection."
Mr. Lockwood has made the interesting observation (in connexion with the contents of hernia) that, in bodies under forty-five years of age, the omentum can rarely be drawn down below the level of the pubic tubercle; in older bodies the reverse is the rule.
The gastro-lienal ligament is a short fold composed of two layers-an anterior, lining the general peritoneal sac, and a posterior, lining the omental bursa (Fig. 966). It is attached by one margin to the fundus and greater curvature of the stomach, and by the other to the gastric surface of the spleen just in front of the hilum. Between its two layers the vasa brevia of the splenic artery pass from the spleen to the stomach. Below and in front, its layers are continued into the corresponding layers of the gastro-colic ligament; above and behind, they separate at the "uncovered area" of the stomach (Fig. 969).
Minor Folds of Peritoneum.-The phrenico-colic ligament, passing from the left flexure of the colon to the diaphragm opposite the 10th or 11th ribs, has been described, and also the mesentery of the vermiform process, the lieno-renal and hepato-renal ligaments.
DEVELOPMENT OF THE DIGESTIVE SYSTEM.
In the section on General Embryology it has been pointed out that the alimentary canal is formed from three separate parts. (1) A middle large entodermal portion, and (2) two smaller ectodermal parts, one anterior, the oral sinus or stomodæum, and one posterior, the proctodæum.
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It has been further shown that the three portions are at first separated from one another by septa, the pharyngeal membrane and the proctodeal membrane, respectively, but the septa disappear at an early date, and the three parts are thrown into continuity. There is thus formed a tube, the primitive alimentary canal, extending through the body from the mouth aperture in front to the anal orifice posteriorly.
The entodermal segment of the primitive alimentary canal is divided into three portions as follows:
(1) That portion which is enclosed within the head fold is termed the foregut; the portion within the tail fold is termed the hindgut; and the intermediate portion is termed the midgut. The midgut at first lies opposite the communication with the yolk sac, and the other portions cephalic to and caudal to this level.
From the foregut are formed the posterior part of the mouth, the pharynx, œsophagus, stomach, and the greater part of the duodenum. From the mid and hindgut are formed the rest of the small intestine, and the whole of the large intestine, as far as to the "white line " of the anal orifice. There is no sharp limit between the mid and hindgut or between the portions of the intestinal canal formed from them (see p. 47).
The different parts of the tube become modified in their structure, so as to form special organs, and in many regions outgrowths occur in the form of hollow diverticula, from which the accessory glands are formed, which lie outside the wall of the original tube. Organs such as the liver, pancreas, and salivary glands arise in this way.
It should be stated here, also, that this primitive intestinal tube forms the basis, not only of the alimentary canal and its associated organs, but it also is the source from which many other organs, not ultimately connected with digestion, are formed.
Thus, the respiratory tract below the level of the orifice of the larynx is formed as an outgrowth from the ventral wall of the primitive foregut, and remains permanently connected with it at that point, though in structure and function it becomes very different from the tube from which it is derived.
Other structures also, namely, the thyreoid gland, the parathyreoid glands, and the thymus, are formed from the alimentary canal as diverticula, but they eventually loose their connexion with the wall of the tube, and become specialised in function and in
Furthermore, the allantois, a diverticulum from the hindgut, is an important rudimentary organ, and a part of the primitive hindgut is cut off from the primitive cloaca to form the urinary bladder and a portion of the urethra.
Accounts of the development of the organs mentioned which are not connected with the alimentary canal in the adult will be found in the sections dealing with them.
Development of Mouth and Pharynx.-The development of the mouth and of the pharynx from the oral sinus and anterior part of the foregut are intimately associated with one another, and with the formation of the mouth and nose.
The stomodæum or oral sinus first appears as a depression situated between the primitive forebrain above and the pericardial region below.
The floor of this depression is formed by the pharyngeal membrane, which consists of ectoderm and entoderm only, mesoderm being absent; the membrane separates the oral sinus from the anterior end of the foregut, but at an early stage it ruptures and disappears.
The mouth cavity of the adult is formed in part from the oral sinus, and in part from the anterior end of the foregut, or primitive pharynx. The line of division between the portions of the mouth derived from these two parts is difficult to trace, on account of the very extensive changes which occur after the pharyngeal membrane has disappeared, and which are associated with the formation of the face and of the nose. The portion of the mouth cavity derived from the primitive pharynx is lined with entoderm, and that from the oral sinus with ectoderm. The position of the original pharyngeal membrane may be represented by an imaginary plane extending from the anterior part of the body of the sphenoid to the base of the alveolar process of the mandible on its lingual surface.
Thus, the cavity of the nose is derived from the upper part of the oral sinus, while the floor of the mouth is formed from the pharynx. The adamant (O.T. enamel) of the teeth and the secreting epithelium of the parotid gland are ectodermal structures, while the epithelium of the tongue and submaxillary and sublingual glands is entodermal in origin.
The ectodermal or oral sinus portion of the mouth, then, gives origin to the lips, teeth, and parotid glands; while in the pharyngeal portion are developed the tongue, submaxillary and sublingual glands.
The upper lip is formed from the tissues covering the frontal and maxillary processes (see development of face).
The lower lip similarly is formed from the tissues covering the mandibular arches.
By an ingrowth of epithelium from the surface of the frontal and maxillary processes into the subjacent mesoderm and by subsequent desquamation of the superficial layers, a groove is formed between the lips and cheeks, on the one hand, and the alveolar ridges on the other. This groove when deepened forms the vestibule of the mouth, and is termed the alveolo-labial sulcus, or labial groove.
The surface covered with mucous membrane becomes everted, to form the red portion of the lips, and at birth is divided into an outer smooth portion, and an inner portion whose surface is villous, termed pars villosa. The distinction between these two parts disappears shortly after birth.
Several explanations of the formation of the philtrum or groove on the front of the upper lip have been put forward; most probably it is produced by the union of the margins of the two processus globulares with one another, the floor of the groove being formed by their line of union, and the ridges bounding the groove at the sides corresponding to the medial portions of the globular processes.
The parotid glands are formed as outgrowths of the epithelium in the outer wall of the alveolo-labial sulcus.
The outgrowth in each side has been found in embryos 8 mm. long. It is at first a furrow. The posterior part of the furrow becomes closed off from the mouth cavity and forms a tube, which grows backwards for some distance on the surface of the first visceral arch.
The terminal portion of the tube formed in this way gives rise to a number of buds, which divide repeatedly, and form the lobules of the gland.
These are at first solid, and the alveoli do not become hollowed until about the twenty-second week.
The epithelium of the terminal buds forms the secreting glandular epithelium, while that of the stalk forms the lining epithelium of the duct.
DEVELOPMENT OF THE TEETH.
A tooth may be described as a calcified papilla of the mucous membrane, composed of two chief parts-namely, the substantia adamantina formed by the epithelial layer, and the substantia eburnea by the connective tissue layer of the mucous membrane. The details of the process by which such a tooth is developed from the two layers of the mucous membrane are both numerous and intricate, and can be but briefly described here.
In lower vertebrates (sharks, rays, etc.), teeth which correspond essentially, both in structure and development, to those of mammals are found on the surface of the body, and are known as dermal teeth. The following outline of the development of the dermal tooth of a shark may assist in rendering the development of the human teeth more intelligible :—
First, a papilla is formed from the corium or connective tissue layer of the skin (Fig. 970, B), and this papilla is covered over by the epithelial layer.
Next, the superficial (connective tissue) cells of the papilla begin to form a layer of ivory on the surface of the papilla (Fig. 970, C), which it soon encases, the remains of the papilla persisting in the interior as the future tooth pulp. At the same time the deepest cells of the epithelium deposit a layer of adamant outside the ivory over the summit of the papilla (Fig. 970, C), and subsequently the two-adamant and ivorybecome inseparably united, thus giving rise to the substance of the tooth.
At a later period the epithelium covering the summit disappears and the tooth comes to the surface; this constitutes its eruption (Fig. 970, D).
In the case of the mammalian tooth a similar process takes place, not, however, on the surface, but deep down in the substance of the gum, into which a downgrowth of epithelium has previously taken place. This epithelial downgrowth spreads out in the substance of the jaw, and into it the papilla grows up, and goes through the other changes described above, as if the whole process took place on the surface.
Development of Human Teeth.-The following is a brief summary of the chief events in the development of a human tooth. For convenience in expression and terms, the description refers to the development of a lower tooth. The upper teeth are, of course, developed in a manner exactly similar.
1.. The first distinct evidence of the development of the teeth is to be found in a thickening of the mouth epithelium, at the site of the future gum, and a resulting downgrowth of its deeper portion into the substance of the primitive jaw (Fig. 971, I.). This epithelial downgrowth is continued along the whole length of the gum, and is known as