The Walls of the Stomach and Duodenum #

At upper abdominal operations, with the organs in situ, a slight groove may be seen in the glistening serosal surface at the gastroduodenal junction. In the groove a small, superficial subserosal vein, lying vertically across the front of the gut, may be evident; this is the prepyloric vein, draining into the right gastric vein (Chap. 7). The groove and the prepyloric vein indicate the position of the pyloric ring. At palpation a sudden transition is felt at the ring between the thick walls of the pyloric region and the thin walls of the duodenum.

The wall of the pyloric part of the stomach and of the proximal 2.0 to 3.0 cm of the duodenum is composed of four coats. From without inwards these are the serous, muscular, submucous and mucous coats. The mucous coat is separated from the luminal contents by a layer of gastric mucus.

Serous Coat #

The serous coat or adventitia is formed by the peritoneum; it is a thin layer of loose connective tissue, covered on its outer aspect with mesothelium. It is closely attached to the subjacent muscular coat, except at the greater and lesser curvatures, where the connection is more lax and where it is continuous with the greater and lesser omentum respectively.

The lesser or gastro-hepatic omentum extends from the inferior and posterior surfaces of the liver to the stomach and proximal 2.0 to 3.0 cm of the duodenum. The portion of the lesser omentum between the porta hepatis and the duodenum contains the hepatic artery, the portal vein, the common bile duct, lymph glands, lymph vessels and nerves. On the right it ends in a rounded margin; immediately posterior to the free right edge is the opening into the lesser sac or epiploic foramen. The remainder of the lesser omentum, extending from the left end of the porta hepatis to the lesser curvature, contains the right and left gastric arteries and the accompanying veins, as well as lymph glands, lymph vessels and branches of the anterior and posterior vagus nerves.

The greater omentum is formed along the greater curvature of the stomach by the meeting of the peritoneal coats of the anterior and posterior gastric surfaces. On its left it shortens into the gastro-splenic omentum, containing the short gastric branches of the splenic artery between its two layers. On the right it is continued for 2.0 to 3.0 cm along the lower border of the first part of the duodenum. From its origin the greater omentum hangs down in front of the intestines as a loose apron, extending as far as the transverse colon, where its two layers separate to enclose that part of the colon. The upper part of the greater omentum contains the greater part of the right and left gastroepiploic arteries and their accompanying veins, lymph vessels, lymph glands, nerve filaments, fat and areolar tissue. On its surface it may have loose accumulations of histiocytes.

There is some doubt whether the subserous lymphatics are continuous with those of the duodenum (Chap. 7); the question is of importance in the spread of pyloric carcinoma (Chap. 33). Owing to its peritoneal attachments the proximal 2.0 to 3.0 cm of the duodenum, i.e. the proximal half of the first part of the duodenum, known as the duodenal bulb, is mobile. It shares the peritoneal covering of the pyloric region of the stomach, unlike the remainder of the duodenum, which is fixed to the posterior abdominal wall in a retroperitoneal position; consequently only the ventral and right lateral surfaces of the remainder of the duodenum are clothed by peritoneum; the duodenal curve abuts against the head of the pancreas.

Muscular Coat or Muscularis Externa #

There has long been disagreement among anatomists about the muscular build of the stomach. Some of the uncertainties can be traced back to the time of Willis (1682). On the one hand there were those who believed the stomach to be a simple, undifferentiated muscular chamber. On the other hand several authorities found that the stomach consisted of a number of different muscular divisions or regions. A huge literature has accumulated on this and related questions, the historical perspectives being fully dealt with by Cunningham (l906), Forssell (l913), Horton (l928) and Torgersen (l942).

The gastric muscular coat or muscularis externa is composed of smooth, unstriped or involuntary fibres. It is made up of three layers, viz. an external longitudinal, a middle circular and an inner oblique layer. The oblique fibres are arranged in inverted U-shaped bundles which loop over the fornix and extend downwards in the anterior and posterior gastric walls as far as the incisura angularis. Consequently the oblique fibres take no part in the muscular build of the distal part of the stomach, including the pyloric region; the muscularis externa here is composed solely of outer longitudinal and inner circular layers.

Cunningham (l906) studied the muscular anatomy of the stomach in man and the anthropoid ape in great detail. He pointed out that normally post-mortem changes set in rapidly, causing the muscular walls to relax, which hampered investigations in fresh specimens. However, by fixing a stomach in "its natural form and natural bed", it was possible to obtain hardened specimens suitable for dissection. According to Cunningham (l906) the demarcation of the cardiac and pyloric portions of the stomach is indicated on the lesser curvature by a notch or elbow-like band, the incisura angularis. The position of the incisura is not constant and is influenced by the degree of filling of the stomach; at some stages it may disappear altogether. The pyloric portion is subdivided into two parts, viz. the pyloric vestibule (the oral division) and the pyloric canal (the aboral division) (Fig. 3.1A). The vestibule and canal meet at the sulcus intermedius, a faint but very constant furrow on the exterior of the greater curvature, 2.5 to 3.0 cm proximal to the pyloric aperture. In other words, the pyloric canal extends from the sulcus intermedius to the pyloric aperture, while the vestibule is located on the oral side of the sulcus. On the exterior of the lesser curvature no demarcation between the canal and vestibule is evident in adults, but the subdivision is clear in the interior of the stomach.

No part of the stomach is more definite or more distinct than the pyloric canal, according to Cunningham (l906). It consists of a tubular or cylindrical thickening of the muscularis externa, approximately 3.0 cm in length; it is also called the pyloric cylinder. When the cylinder is contracted, it forms the pyloric canal (Fig. 3.1B), which is best demonstrated in the foetus or the child, and in adult specimens hardened in formalin. The canal is usually contracted along its whole length; when contracted, the lumen is obliterated by closely packed longitudinal mucosal folds.

AB

Fig. 3.1. A. Divisions of pyloric region according to Cunningham. P.S.C., pyloric sphincteric cylinder; P.V., pyloric vestibule; P.A., pyloric aperture; S.I., sulcus intermedius. B Contracted pyloric sphincteric cylinder according to Cunningham. P.C., pyloric canal; P.V., pyloric vestibule; P.A., pyloric aperture; S.I., sulcus intermedius

At the pyloro-duodenal junction the aboral margin of the muscular cylinder is increased in thickness, thereby forming the massive muscular ring which encircles the pyloric aperture. Cunningham (l906) called this ring the pyloric sphincteric ring (Fig. 3.2). The ring protrudes into the commencement of the duodenum; when viewed from the duodenal side, it presents as a smooth, rounded knob with a small puckered opening, the pyloric aperture, in its centre.

The pyloric sphincteric ring is not a separate anatomical structure, but constitutes a localized thickening of the cylinder, according to Cunningham (1906). On the gastric or oral side, the circular fibres of the ring merge imperceptibly into those of the cylinder, without any demonstrable anatomical boundary between the ring and cylinder. On the aboral or duodenal side conditions are completely different. Here the circular fibres of the ring are sharply demarcated from those of the duodenum by a fibrous septum; this ensures a complete break between the circular musculature of the pylorus and that of the duodenum (Fig. 3.2).

Fig. 3.2. Diagram of pyloric musculature according to Cunningham. P.S.C., pyloric sphincteric cylinder; P.S.R., pyloric sphincteric ring; F.S., fibrous septum; C.D.M., circular duodenal musculature; L.M., longitudinal musculature; C.M., circular gastric musculature; S.I., sulcus intermedius

Not only the circular, but also the longitudinal fibres are present in greater mass in the cylinder than in any other part of the stomach. In contrast to the circular fibres, a certain percentage of gastric longitudinal fibres is continuous with those of the duodenum. The more superficial fibres of the gastric longitudinal coat extend across the pyloro-duodenal junction to merge with those of the duodenum. The deeper longitudinal fibres, as they approach the pyloric aperture, dip into the sphincteric ring; some of these become interwoven with the circular fibres of the ring, while others extend through the circular coat to reach the submucosa.

On the oral side of the cylinder both its circular and longitudinal fibres merge imperceptibly into those of the remainder of the gastric wall. Except for the palpable thickening of the cylinder, and the shallow sulcus intermedius, no anatomical division can be demonstrated on the oral side of the cylinder between its musculature and that of the vestibule.

Cunningham (1906) called the muscular cylinder the pyloric sphincteric cylinder. The aboral thickening, but integral part of the cylinder, was the pyloric sphincteric ring (Fig. 3.2). Contraction of the cylinder caused formation of the pyloric canal, which had to be distinguished from the pyloric aperture.

He found minor variations in the arrangement of both circular and longitudinal muscle fibres in different specimens. In one specimen for instance, all the longitudinal fibres dipped into the sphincteric ring, while some superficial circular fibres of the ring were carried on to the duodenum for a short distance. Sometimes the deeper longitudinal fibres interlaced with the superficial circular fibres of the ring, forming a feltwork of mixed fibres which was carried on to the duodenum. These variations soon gave way to the proper coats of the duodenum, and in most specimens the arrangement was as indicated above. Cunningham (l906) inferred that contraction of the circular fibres of the sphincteric ring would close the aperture. However, it was much more common to find the cylinder as a whole to be contracted. Under such circumstances the entire cylinder acted as a sphincter, closing the whole length of the pyloric canal against the entrance of material from the proximal stomach (Fig. 3.1A and B). This gave rise to the concept of a sphincteric cylinder.

Owing to the direction of their insertion into the sphincteric ring, it was thought that the longitudinal fibres acted as a dilator of the ring, i.e. of the pyloric aperture. In this sense the circular and longitudinal fibres seemed to be antagonistic.

Cunningham (1906) also inferred that the powerful musculature of the sphincteric cylinder had an important function, which was probably under control of a special innervation (Chap. 8). He noted that physiologists up to that time had not recognized the muscular sphincteric cylinder as a specialized region of the stomach.

In his classic anatomical studies, Forssell (l913) dealt almost exclusively with the human stomach, though some descriptions are found of the corresponding conditions in animals; the muscular structure of the stomach in relation to its forms of movement as seen during radiographic examinations, was studied extensively. Forssell (l913) found that the longitudinal (or vertical) part of the stomach consisted of two muscular regions, viz. the fornix and the corpus, and the transverse (or horizontal) part of two more, namely the sinus and canalis egestorius (Fig. 3.3). The muscular regions depended solely on the arrangement and thickness of the fibres of the various layers; the different regions were not separated from each other by sphincters or similar anatomical structures. (Comment: It is clear that Forssell's sinus and canalis egestorius correspond to Cunningham's vestibule and pyloric canal respectively).

Fig. 3.3. Muscular regions of stomach according to Forssell, F, fornix; C, corpus; S, sinus; C.E., canalis egestorius; M.A., membrana angularis; S.I., sulcus intermedius

On the duodenal side, however, a fibrous septum separated the circular muscle of the pylorus from the corresponding coat of the duodenum. Some of the pyloric longitudinal fibres were continuous with those of the duodenum. The longitudinal fibres on the lesser curvature, a short distance orally to the pyloric aperture, were weak or scanty, leading to an intermittent outpouching of the lumen called the membrana angularis (Fig 3.3).

Forssell (l913) looked upon the canalis egestorius as an anatomically preformed structure. On the greater curvature it extended from the sulcus intermedius to the pyloric aperture (Fig. 3.3). The sulcus intermedius, present in anatomical specimens, was due to tonic contraction at the entrance to the canalis, and not to an independent muscular structure. In anatomical specimens the distance from the sulcus intermedius to the pylorus was found to be 3.5 to 5.0 cm in non-contracted stomachs, and 3.0 to 4.5 cm in contracted ones. On the lesser curvature Forssell's canalis included part of the membrana angularis; he called the area between the latter and the pyloric aperture the "end piece".

The pyloric ring at the aboral end of the canalis was not a separate anatomical structure but formed part of the musculature of the canalis. However, Forssell stated that "this does not diminish, even in a small way, its anatomic or physiologic character as a sphincter" and that, on account of its anatomic structure, the pyloric ring could be looked upon as a proper sphincter, with both constrictor and dilator mechanisms.

During life the enire canalis contracted concentrically, an event which Forssell called the "annular wave". The contraction commenced more or less in the region of the sulcus intermedius, at a distance of 2.5 to 4.0 cm from the pylorus.

Horton (l928) studied the distribution and arrangement of the circular and longitudinal musculature in 90 normal, fresh post-mortem stomachs; this included the study of 5171 microscopic sections. An attempt was made to determine the percentage of circular and longitudinal fibres in the pyloric region which were continuous with the corresponding fibres of the duodenum. The circular musculature of the pyloric canal was found to be 4 to 5 times as thick as the longitudinal; it was broken up into incomplete bundles by connective tissue septa which entered the muscle from the submucosa and ran at right angles to the long axis of the lumen. The septa usually extended through the circular as far as the longitudinal layer; circular bundles formed in this way were not separate rings, but anastomosed freely with one another.

Of 84 specimens examined, 81 showed a complete break between the circular muscle of the pyloric region and that of the duodenum; this was confirmed in 97 percent of 1210 microscopic sections. The break between the two circular coats, caused by a septum of connective tissue and blood vessels, was up to 1.5mm wide, but as a rule somewhat narrower than this. A few minor variations were encountered; in 3 cases, for instance, a small quantity (less than 2 percent) of pyloric circular fibres were carried over into the duodenum for a short distance. The circular musculature of the duodenum, which was much thinner than that of the pylorus, began on the distal aspect of the connective tissue septum. The longitudinal muscle formed a continuous layer over the pyloric aperture in all of the specimens from subjects aged more than one year. In the average subject from 21 to 24 percent of the longitudinal fibres of the pyloric region, consisting of the more superficial bundles, were continuous with those of the duodenum. Most of the deeper longitudinal fibres, as they approached the pyloric "sphincter", dipped into the circular coat to take part in the formation of the "sphincter", some reaching the submucosa. From the anatomical arrangement it was concluded that these longitudinal fibres constituted the dilator muscle of the pylorus. (Comment: The term "sphincter" apparently indicated the pyloric ring).

In the microscopic sections some variations were found, depending mainly on the point on the circumference at which biopsies were taken. These variations did not appear to follow a definite pattern; in one specimen for instance, there were small areas on the anterior surface of the duodenum where both circular and longitudinal fibres were absent. In l9 percent of the sections, not only the circular, but also the longitudinal coat showed a break at the pylorus. Occasionally a few circular fibres from the pyloric ring were seen to be continuous with the circular fibres of the duodenum.

Cole (l928) did not describe his anatomical dissections in detail but came to the conclusion that the distal part of the gastric "antrum" was surrounded by a dense, thick, fan- or harp-shaped muscle. It seemed to fan out from a narrow area on the lesser curvature to a relatively wide area on the greater curvature (Fig. 3.4). It was apparently a continuation of the circular muscle coat, but its size and density suggested that it was a special division with a specialized function. When contracting, it did so in a segmental or concentric, rather than peristaltic way; full contraction of this muscular structure caused the formation of the pyloric canal (not to be confused with the pyloric aperture). Normally the fan-shaped muscle contracted during a short stage of each gastric peristaltic cycle; it was also contracted during rigor mortis. (Comment: The "fan-shaped" muscle described by Cole corresponds to Cunningham's sphincteric cylinder and Forssell's canalis egestorius).

Fig. 3.4. F.M., fan-shaped muscle according to Cole; its concentric contraction causes formation of the pyloric canal; P.A., pyloric aperture

Torgersen (l942) studied the muscular build and movements of the stomach and duodenal bulb from the point of view of comparative anatomy and embryology. Although his methodology was quite different from that of Forssell (1913), his results verified the latter's conception of the canalis egestorius in all important respects and he accepted Forssell's terminology. He differed from Forssell in a few details; for instance, whereas Forssell included part of the membrana angularis on the lesser curvature in the canalis egestorius, Torgersen regarded these as two separate regions.

Torgersen (l942) was able to add important new findings which further elucidated the muscular anatomy of the "transverse" part of the stomach. His monumental work commenced with an historical review of the anatomy of the stomach from the time of Willis (1682) to the era following Forssell (1913). He showed in detail how previous anatomists such as Retzius, Luschka, von Aufschnaiter, Jonnesco and E. Müller opened the way for Cunningham (l906) and Forssell (l913). On the other hand a few anatomists, the most notable being Pernkopf (l922, l924), differed from the latter; while Forssell held that the musculature of the stomach was highly differentiated into separate but contiguous regions, Pernkopf maintained that there was no differentiation in the musculature at all. According to Pernkopf the regions lacked anatomical foundation, and the forms of movement were entirely of a functional nature; nevertheless he agreed that the movements were not devoid of comparative anatomical interest, as they imitated the more complex stomachs of other vertebrates.

According to Torgersen (l942) the circular musculature of the canalis egestorius in man and other vertebrates contains two annular thickenings or loops. The aboral loop is called the right canalis loop (Fig. 3.5). (Comment: At times he also referred to this loop as the pyloric sphincter; the word "sphincter" was an unfortunate choice, as it will become clear that Torgersen regarded the "sphincter" as a complex structure consisting of various loops, of which the right canalis loop constituted but one component. In a personal communication to the present author in 1962, Torgersen confirmed that the right canalis loop was the muscular component of the pyloric ring).

Fig. 3.5. Diagram of circular musculature of sphincteric cylinder (canalis egestorius) according to Torgersen. R.P.L., right pyloric (canalis) loop; L.P.L., left pyloric (canalis) loop; P.M.K., pyloric muscle knot (torus); S, stomach; D.B., duodenal bulb. (Ring of circular musculature surrounding commencement of duodenum not shown.)

The left canalis loop is located on the oral side of the right loop. The two loops, each being placed obliquely, meet on the lesser curvature in a muscle torus (Fig. 3.5). From the torus the loops diverge to encircle the greater curvature, where they are 3.0 to 5.0 cm apart. It is evident from the course of the fibres that the two loops are not independent anatomical structures; their musculature is intimately interlaced in the muscle torus on the lesser curvature, and also with the intervening circular muscle fibres in the anterior and posterior gastric walls.

The left canalis loop corresponds to the sulcus intermedius on the greater curvature. The circular fibres on the oral side of the left loop merge imperceptibly into the circular fibres of the adjacent sinus. The circular musculature of the canalis is thicker than that of the sinus, but in other respects no boundary can be demonstrated between these two regions. On the lesser curvature the concentrated circular musculature in the muscle torus is continuous with the thin musculature of the membrana angularis.

Torgersen (l942) found that the two loops were distinctly visible in some of the illustrations presented by previous anatomists such as Cunningham (1906), Wernstedt (who named the left loop the sphincter intermedius), and even Pernkopf (l921, l924).

On the duodenal side a connective tissue septum separates the main mass of the right canalis loop from the circular fibres of the duodenum. On the aboral side of the septum a strong loop of circular musculature surrounds the base of the duodenal bulb. A few of the circular fibres of the muscle torus on the lesser curvature are continuous with those of the duodenal loop. In the anterior and posterior gastric walls the right canalis loop and circular duodenal loop are loosely connected by the intervening connective tissue septum and a few muscular anastomoses. On the greater curvature the right canalis loop is connected more intimately to the circular duodenal loop.

Torgersen (l942) regarded the circular muscle loop at the base of the duodenal bulb as part of the pyloric sphincteric mechanism. In his view the pyloric sphincter, as far as the circular musculature was concerned, consisted of gastric and duodenal parts, viz. the right and left canalis loops on the gastric side, and the loop surrounding the commencement of the duodenum on the aboral side of the fibrous tissue septum.

The longitudinal musculature of the sinus becomes abruptly thicker at the left canalis loop and forms a powerful band between the right and left loops on the greater curvature, according to Torgersen (l942). The majority of these longitudinal fibres, as well as those in the anterior and posterior gastric walls, dip into the musculature of the right canalis loop (i.e. the muscular component of the pyloric ring); only a few longitudinal fibres are carried across the connective tissue septum into the duodenum on the greater curvature side. On the lesser curvature side most longitudinal bundles proceed uninterruptedly across the septum into the duodenum.

According to Torgersen (l942) the canalis egestorius consists of the muscle torus, the left and right circular loops, and the circular and longitudinal muscle fibres between these structures. The sphincteric mechanism at the pylorus consists of the canalis egestorius, the circular musculature surrounding the commencement of the duodenum, and the intervening fibrous septum.

Torgersen (l942) found the muscular build of the stomach and duodenum in the dog, rabbit, horse, pig and ox to be essentially similar to that in man. In all these animals the right and left circular loops, the muscle torus on the lesser curvature and the longitudinal fibres between the circular loops were clearly discernible. The circular fibres at the commencement of the duodenum, the connective tissue septum between these and the right loop, and the arrangement of the longitudinal fibres across the septum were also similar to that in man. There were a few minor variations; for instance, the circular canalis loops in the dog appeared to be more powerful on the greater curvature side, while in the horse the loops were equally powerful throughout their circumference. The duodenal bulb was more prominent in man and the horse than in the other animals studied. In the cat the right loop differed in that it was only developed on the greater curvature side.

Torgersen (l942) concluded that there was a common principle of build of this part of the stomach in the higher vertebrates and in man. He found the canalis to be an anatomically preformed structure, an anatomical reality with a sound foundation in comparative anatomy.

McNaught (l957) confirmed the presence of the left canalis loop in fresh gastric resection specimens. Williams (l962) stated that the contracted fan-shaped muscle was occasionally seen in fresh partial gastrectomy specimens.

Morbid anatomical study #

Keet and Heydenrych (l982) studied the width of the gastric walls in the pyloric region in adults, in 5 morbid anatomical specimens fixed in formalin. Having identified the pyloric ring by means of a wire marker, the lumen of the stomach and duodenum was filled with barium. A narrow layer of barium paste was painted on the serosal surface of the lesser curvature, and another on the greater curvature. Radiographs of each specimen were taken in the anteroposterior position (Fig. 3.6). The space between the luminal barium and that on the external surface indicated the thickness of the wall, consisting of mucosa, submucosa, muscularis externa and serosa. As the mucosal, submucosal and serosal layers were more or less uniformly thick in all parts of the stomach, any variation in wall thickness would be due to thickening of the muscular coat.

In all specimens the following was seen: extending orally from the pyloric ring there was a cylindrical region approximately 3.0 cm in length in which the wall had a thickness of 6.0 to 7.0 mm; it was slightly shorter on the lesser than on the greater curvature. In the remainder of the stomach the wall thickness was 2.0 to 3.0 mm. The pyloric ring formed the aboral part of the muscular thickening.

It was concluded that there was a tube of thickened pyloric musculature, approximately 3.0 cm in length and incorporating the pyloric ring, in adult morbid anatomical specimens.

Fig. 3.6. Radiograph of morbid anatomical specimen. Barium fills the lumen and outlines the serosa of the lesser and greater curvatures. A short tube of thickened muscularis externa extends orally from the pyloric ring

Development of musculature #

According to Torgersen (l949) the developmental anatomy of the pyloric region is related to asymmetrical development of the viscera. The fact that the circular muscle fibres of the canalis egestorius radiate fan-like from the lesser curvature to embrace the greater curvature, is essentially an expression of structural and topographical asymmetry of the stomach.

Welch (l921) studied the development of the musculature of the stomach in the foetus and in newborn infants. The stomach first appeared as an expansion of the primitive gut at a foetal crown-rump length of 6.0 mm. The first evidence of stratification of the gastric wall was seen at this stage. At 11.0 mm differentiation of the layers had begun and three primitive layers, namely entodermal epithelium, mesenchyma including a myoblastic layer, and peritoneal epithelium were discernible; the myoblasts were seen to be arranged circularly. At l7.0 mm there was further condensation of the mesenchyma with actual transformation to circular fibres. Welch stated that the circular layer was complete at 24.0 mm and was definitely thickened over the pylorus. At 33.0 mm the gastric wall showed a general increase in circular fibres. The 41.0 mm embryo showed a high degree of organization with the circular fibres becoming continuous. At 65.0 mm there was a well formed circular coat with a thickness of approximately 0.01 mm; at the pyloric "sphincter" the thickness of the circular layer was 0.03 cm.

At birth the circular layer was well developed and constituted the major part of the gastric musculature. Its fibres were arranged in parallel rings approximately at right angles to the lumen, the rings anastomosing freely with each other. During the first year of life a marked thickening of the circular layer occurred.

Unlike the circular layer, which showed a simultaneous differentiation over the entire stomach, the longitudinal coat first appeared as discrete, scattered bundles at the l7.0 mm stage. Compared with the circular layer, its subsequent development was much delayed. At the 41.0 mm stage there was an increase in the number and distribution of the groups of muscle cells, this being particularly apparent at the pylorus, where an intermingling of cells of the longitudinal and circular layers was seen. At 65.0 mm a layer of muscle was evident.

Welch (l92l) stated that the development of the longitudinal coat was not complete until the first year of postnatal life; after birth there was an increase in these fibres. At the "pyloric antrum" the longitudinal fibres converged to form a complete cylinder.

Welch (l921) found that the oblique fibres became separated from the circular layer at the 24.0 mm crown-rump stage. At 33.0 mm they were seen to continue almost to the pylorus. In the 65.0 mm embryo they formed a band which terminated by joining the circular layer near the greater curvature. In the newborn the oblique fibres extended to the pylorus and in some cases reached the proximal portion of the "sphincter of the pylorus". On the lesser curvature some of the oblique fibres invariably joined the circular layer.

The muscularis mucosae was identified at the 65.0 mm stage, although it was still incomplete.

According to Welch, Forsell's description of the musculature of the adult stomach resembled the appearances seen in the foetus and newborn infant. The stomach of the newborn is characterized by a very thick circular layer; the longitudinal layer is more continuous than it is in the foetus, and invests the entire organ. The oblique fibres extend to the proximal part of the "sphincter of the pylorus". At birth, due to swallowing of air and feeds, a marked dilatation of the stomach, with a general thinning of the musculature, occurs.

Discussion #

The anatomists Cunningham (1906), Forsell (l913), Welch (l921), Cole (l928) and Torgersen (l942) showed that the muscularis externa in the distal 3.0 to 4.0 cm of the stomach is thicker than that in the remainder of the stomach. The region involved is longer on the greater than on the lesser curvature, i.e. it has a roughly triangular or fan- like shape. It was called the pyloric sphincteric cylinder by Cunningham (l906), the canalis egestorius by Forssell (l913) and Torgersen (l942), and the fan-shaped muscle by Cole (l928). At its aboral end an additional, ring-like thickening of this muscular cylinder forms the muscular part of the pyloric ring; the ring is not a separate anatomical structure, its musculature being an inherent part of the cylinder. At its oral end the musculature of the cylinder merges imperceptibly into that of the remainder of the stomach.

Normally the entire pyloric sphincteric cylinder contracts in a concentric or systolic way, with obliteration of the lumen, to form a tightly contracted canal approximately 2.0 to 3.0 cm in length. In this way the entire cylinder acts as a sphincter, closing the whole length of the pyloric canal against the entrance of luminal contents from the proximal stomach. Contraction of the cylinder results in formation of the pyloric canal, which is a temporary, physiological structure to be differentiated from the pyloric aperture.

Forssell (l913) and Torgersen (l942) showed that the circular musculature of the canalis egestorius (i.e. the pyloric sphincteric cylinder) is arranged into a system of rings or loops. The right canalis loop is the muscular part of the pyloric ring. The left canalis loop is located at the oral end of the cylinder; it is less well developed than the right and corresponds to the sulcus intermedius on the greater curvature. The two loops meet and interlace on the lesser curvature in a muscle torus or knot, from which they diverge to encircle the greater curvature. The loops are connected by intervening curcular as well as by overlying longitudinal fibres; many of the latter dip into the right canalis loop.

Cunningham (l906), Forssell (l913), Welch (l921) and Torgersen (l942) looked upon the sphincteric cylinder as an anatomically preformed structure. Torgersen (l942) termed it an anatomical reality with a sound foundation in comparative anatomy.

Sphincteric mechanism at pylorus #

In view of the muscular build of the distal stomach it appears unlikely that the pyloric ring as such, constitutes a sphincter in the usually accepted sense of the word. According to Torgersen (l942) the sphincteric mechanism at the pylorus is more intricate; it involves the various divisions of the musculature of the sphincteric cylinder as described above, the fibrous septum separating the circular musculature of the cylinder from that of the duodenum, and the circular musculature surrounding the commencement of the duodenum. In other words, it consists of gastric and duodenal divisions.

It seems to us that like the Roman God of the Gates, Janus (Latin janua: gates), the pyloric sphincteric cylinder acts as a keeper of entrances and exits. It is orientated backwards (i.e. orally) as well as forwards (i.e. caudally). It has to receive swallowed liquids and solids, accomodate and prepare them for as long as necessary, and deliver them into the duodenum. It appears that it may play an important role, not only in gastric emptying of solids and liquids, but also in various aspects of gastro-duodenal motility.

Terminology #

The pyloric sphincteric cylinder is the same muscular structure as the canalis egestorius and the fan-shaped muscle. It is suggested that the terms right and left pyloric loops be used for right and left canalis loops respectively. For muscle torus the term muscle knot is preferred as the two loops meet in that situation. Pyloric ring is the fold (consisting of muscular and mucosal/submucosal components) separating the lumen of the stomach from that of the duodenum. Pyloric aperture is the opening surrounded by the lips of the pyloric ring. Its diameter varies, depending mainly on the degree of contraction and relaxation of the cylinder. Pyloric canal is the contracted pyloric sphincteric cylinder; it is 2.0 to 3.0 cm in length in adults.

References #

1. Cole LG. The living stomach and its motor phenomenon. Acta Rad l928, 9, 533- 545.
2. Cunningham DJ. The varying form of the stomach in man and the anthropoid ape. Trans Roy Soc Edinb l906, 45, 9-47.
3. Forssell G. Über die Beziehung der Röntgenbilder des menschlichen Magens zu seinem anatomischen Bau. Fortschr Geb Röntgenstr l913, Suppl 30, 1-265.
4. Horton BT. Pyloric musculature with special reference to pyloric block. Amer J Anat l928, 41, l97-225.
5. Keet AD, Heydenrych JJ. The anatomy and movements of the pyloric sphincteric cylinder. South Afr Med J l982, 62, 15-18.
6. McNaught GHD. Simple pyloric hypertrophy in the adult. J Roy Coll Surg Edin l957, 3, 35-41.
7. Pernkopf E. Die Entwicklung der Form des Magendarmkanals beim Menschen. Zeitschr gesam Anat l921, 64, 96-275.
8. Pernkopf E. Idem. Ibid. l924, 73, 1-144.
9. Torgersen J. The muscular build and movements of the stomach and duodenal bulb. Acta Rad l942, Suppl 45, l-l9l.
10. Torgersen J. The developmental anatomy of the pyloric canal and the etiology of infantile pyloric stenosis. Acta Rad l949, 32, 435-438.
11. Welch EH. Development of the musculature of the stomach with special reference to its condition in the newborn child and the premature infant. Papers from Mayo Foundation and Medical School Univ Minnesota l921 - l922, 2, 3-23.
12. Williams I. Closure of the pylorus. Brit J Rad l962, 35, 653-670.