Some Uncertain Concepts #

1. The Pyloric Antrum #

Willis (l682) introduced the term "antrum pylori" (Greek antron = cave) to indicate the part of the stomach adjoining the pylorus; no further demarcation was given. Cunningham (1906) pointed out that the term was ambiguous and that it was rarely possible to obtain a clear anatomical conception of what it meant; it was largely responsible for much of the obscurity associated with the pyloric part of the stomach. Lewis (1912) also drew attention to the confusion caused by the term.

Forssell (1913) was totally opposed to the designation "antrum," as, in his view, it was vague and had no foundation in anatomical fact. Müller (l921) pointed out that the word was used in different senses; some authors used it to denote a small area close to the pylorus while others considered it to be the entire "transverse" stomach.

Cole et al. (l932) defined "antrum" as the portion of the stomach between the sulcus angularis and the pyloric valve; the fan-shaped muscle constituted a small portion of its distal end (Fig. 3.4). Golden (l937), on the other hand, used the term as a synonym for Forssell's canalis egestorius (i.e., the fan-shaped muscle) (Fig. 3.3).

While discussing a series of cases of gastric carcinoma, Coller et al. (l941) stated that in the majority the lesion was located either in the prepyloric region or in the pyloric antrum, from which it is concluded that these are different entities. In view of the ambiguities and lack of anatomical definition, Torgersen (l942) advocated abolition of the term "antrum."

Clarity is not forthcoming from the descriptions of Jenkinson (l955), who stated that the pyloric canal, approximately 3.0 cm in length, was located on the oral side of the "pyloric sphincter"; the antrum was said to be the region extending from the pyloric canal to the incisura angularis on the lesser curvature (Fig. 2.1). In another sentence the antrum was defined as the part of the stomach between the sulcus intermedius and the duodenum.

Fig. 2.1. Pyloric antrum according to Jenkinson. P.S., pyloric sphincter; P.C., pyloric canal; S.I., sulcus intermedius; I.A., incisura angularis. (In another sentence the antrum was said to be the region between the sulcus intermedius and the duodenum.)

Not only do anatomists disagree among themselves about the concept "antrum", but Foulk et al (l957) pointed out that gastric landmarks and boundaries differed in their details for the pathologist, the surgeon, the endoscopist and the radiologist. Grossman (l958) stated that many writers had commented on the lack of uniformity of terms and the ambiguity of some of them; according to him the pyloric portion was usually divided into the pyloric canal adjacent to the sphincter, and the pyloric vestibule or antrum adjacent to the corpus. He preferred to name functional divisions of the stomach in terms of their secretory characteristics; "pyloric gland area" was suggested for the mucus secreting, gastrin producing zone and "oxyntic gland area" for the acid producing region.

According to Edwards (l96l) it was widely accepted that the antrum represented the portion of the stomach distal to the incisura, which approximately defined the boundaries of the pyloric mucosal zone. Although some anatomists and clinicians adhered to this definition, du Plessis (l963), as a surgeon, was concerned with the fact that the pyloric mucosal zone was often larger than the anatomical antrum. Tanner (l964) pointed out that there was extreme variability in the extent of antral mucosa; if it were equated with the antrum, the boundaries of the latter would also be variable.

Moe et al. (l965) looked upon "antrum" as the pyloric mucosal zone. According to Capper et al (l966) the antrum was more of a physiologic concept than an anatomical one, and was best defined as the distal part of the stomach which contained and released gastrin; for antrectomy to be adequate, this zone had to be defined accurately and all of the gastrin-secreting mucosa had to be removed.

Carlson et al. (l966) and Code and Carlson (l968) defined the antrum as the region caudad to the incisura angularis. In terms of motor function it was divided into two segments of varying length (Fig. 2.2). The more caudad portion, called the terminal segment, participated in a simultaneous, segmental contraction called terminal antral contraction (TAC); the length of the segment was not fixed and its dimensions might change. The cephalad segment of the antrum was not involved in TAC but the two segments constituted a functional motor unit and contracted in a coordinated way.

Edwards and Rowlands (l968) again drew attention to the confusion in nomenclature; in their view the term "antrum" denoted the proximal part of the thickened muscle mass adjacent to the pylorus, and not the part of the stomach distal to the incisura angularis.

Fig. 2.2. Pyloric antrum according to Carlson et al. I.A., incisura angularis; T.A., terminal antrum

Fig. 2.3. Pyloric antrum according to Gray's Anatomy (American Edition). P.A., pyloric aperture; S.I., sulcus intermedius; P.V., pyloric vestibule; I.A., incisura angularis

Different definitions of the pyloric antrum were encountered in the American and British editions of an anatomical textbook, both published in the same year. In the 29th American edition of Gray's Anatomy of the Human Body (l973) the antrum was said to be located on the caudal side of the sulcus intermedius, the latter being 2.5cm from the aperture (Fig 2.3); the pyloric vestibule was said to be on the oral side of the sulcus, extending to the incisura angularis. It was also stated categorically that the antrum was much smaller than the pyloric mucosal zone. In the 35th British edition of Gray's Anatomy (l973) the antrum was said to be on the oral side of the sulcus intermedius (Fig 2.4).

Fig. 2.4. Pyloric antrum according to Gray's Anatomy (British Edition). P.A., pyloric aperture; S.I., sulcus intermedius

Strickland and Mackay (l973) again equated "antrum" with the pyloric mucosal zone. According to Spiro (l977) the antrum was too loosely defined, but was usually considered to be the portion of the stomach beyond the angulus. It differed from the remainder of the stomach in that its motor activity was greater, its mucosa was different (being the source of gastrin production) and it had an absence of mucosal folds (Comment: The angulus, seen endoscopically, corresponds to the incisura angularis. Radiology shows that mucosal folds are by no means absent in the normal living, distal stomach.)

Stave and Brandtzaeg (l978) considered the "distal" antrum to be the region extending proximally from the pylorus for a distance of 3.0 cm. Polak et al. (l977), Royston et al (l978) and Dockray (l978) equated "antrum" with the pyloric mucosal zone. According to Ito (l98l) the antrum is proximal to the pyloric canal. (Comment: Ito's pyloric canal would have been considered to be the terminal antrum by Stave and Brandtzaeg.) Szurszewski (l98l) defined the antrum by drawing an imaginary line from the incisura angularis to a point opposite it on the greater curvature; the region between the line and the gastroduodenal junction was looked upon as the antrum (Fig. 2.5). The muscular zone between the intermediate sphincter (sulcus intermedius) and the gastroduodenal junction formed the terminal antrum. Bolondi et al (l984) considered the antrum to be the region extending from the angulus to the pyloric aperture. The terminal antrum was described as the immediate prepyloric region by King et al (l984). Ganong (l985) again equated "antrum" with the gastrin secreting mucosal zone.

Fig. 2.5. Pyloric antrum according to Szurszewski. P.A., pyloric aperture; S.I., sulcus intermedius; I.A., incisura angularis; T.A., terminal antrum

According to K.L. Moore (l985) the pyloric part of the stomach consists of a narrow portion 1.0 to 2.0 cm long, which is continuous with the pylorus and called the pyloric canal, and a wider pyloric antrum on the oral side of the canal.

J.G. Moore et al (l986) described a midgastric transverse band in humans, which, in their view, represented an anatomic separation between the gastric reservoir (the fornix and corpus) and the antrum; the entire region between the transverse band and pylorus was considered to be the antrum. Collins et al. (1991) ascribed an important role to the transverse band in differential emptying of solids and liquids; it appeared to be responsible for the initial retention of solids. (It appears if the transverse band may correspond to the musculus sphincter antri, i.e. a loop at the termination of the oblique fibres of the muscularis externa, described by Torgersen in l942. It was also called the lower segmental loop and formed the oral boundary of the sinus.)

Discussion #

One has to agree with Cunningham (1906), Forssell (1913) and Torgersen (1942) that the term "antrum" has been used in many different senses, that it is ambiguous and responsible for much of the obscurity associated with the pyloric part of the stomach. For instance, a number of authors equated "antrum" with the pyloric mucosal zone (Moe et al 1965; Strickland and Mackay 1973; Polak et al. 1977; Ryston et al. 1978; Dockray 1978; Ganong 1985), yet it has been shown that this zone is not constant and may migrate up and down the stomach in cases of gastric and duodenal ulceration (Chap. 5). Tanner (1964) mentioned the "extreme variability" in the extent of pyloric mucosa. In anatomy again, it has been stated categorically that the antrum is much smaller than the pyloric mucosal zone (Gray 1973).

Numerous other examples of conflicting views have been quoted above. It seems that the antrum may be errant or wandering in more ways than one, and that for this reason it is a designation which is best avoided.

References #

1. Bolondi L, Bortolotti M, Santi V, et al. Measurement of gastric emptying time by real-time ultrasonography. Gastroenterology l985, 90, 752-759.
2. Capper WM, Butler TJ, Buckler KG, et al. Variation in size of the gastric antrum: measurement of alkaline area associated with ulceration and pyloric stenosis. Ann Surg l966, 163, 281-290.
3. Carlson HC, Code CF, Nelson RA. Motor action of the canine gastroduodenal junction: a cineradiographic, pressure and electrical study. Amer J Dig Dis l966, ll, 155- 172.
4. Code CF, Carlson HC. Motor activity of the stomach. In: Handbook of Physiology Sect 6: Alimentary Canal, Vol 4: Motility. Amer Physiol Soc, Washington DC l968, pp 1903-1916.
5. Cole LG and Collaborators. Important anatomical data of the digestive tract. Radiology l932, l8, 471-520.
6. Coller FA, Kay EB, McIntyre RS. Regional lymphatic metastases of carcinoma of the stomach. Arch Surg l941, 43, 748-761.
7. Collins PJ, Houghton LA, Read NW, et al. Role of the proximal and distal stomach in mixed solid and liquid meal emptying. Gut 1991, 32, 615-619.
8. Cunningham DJ. The varying form of the stomach in man and the anthropoid ape. Trans Roy Soc Edin l906, 45, 9-47.
9. Dockray GJ. Gastrin overview. In: Gut Hormones, edit Bloom SR, Grossman MI, Churchill Livingstone, London l978, pp 129-139.
10. Du Plessis DJ. The importance of the pyloric antrum in peptic ulceration. South Afr J Surg l963, 1, 3-11.
11. Edwards D. Some radiological aspects of pyloric disease. Proc Roy Soc Med l96l, 54, 933-937.
12. Edwards DAW, Rowlands EN. Physiology of the gastroduodenal junction. In: Handbook of Physiology Sect 6: Alimentary Canal Vol 4: Motility. Amer Physiol Soc, Washington DC l968, l985-2000.
13. Forssell G. Über die Beziehung der Röntgenbilder des menschlichen Magens zu seinem anatomischen Bau. Fortschr Geb Röntgenstr l9l3, Suppl 30, 1-265.
14. Foulk WT, Comfort MW, Butt HR, et al. Peptic ulcer near the pylorus. Gastroenterology l957, 32, 395-403.
15. Ganong WF. Review of Medical Physiology. Lange Medical Publications, Los Altos l985, 12th Edit, p 401.
16. Golden R. Antral gastritis and spasm. J Amer Med Assoc l937, 109, 1497-1500.
17. Gray H. Anatomy of the Human Body. 29th American Edition, ed Goss CM. Lea and Febiger, Philadelphia l973, p 1220.
18. Gray H. Gray's Anatomy. 35th British Edition, ed Warwick R, Williams PH. Longman Co, Edinburgh l973, p 1273.
19. Grossman MI. The names of the parts of the stomach. Gastroenterology l958, 34, 1159-1162.
20. Ham AW. Histology. JB Lippincott Co, Philadelphia l969, 7th edit, p 655.
21. Ito S. Functional gastric morphology. In: Physiology of the Gastrointestinal Tract. Ed Johnson LR, Raven Press, New York l98l, Vol 1, pp 517-550.
22. Jenkinson EL. The pyloric antrum of the stomach. Amer J Roentgen Rad Ther l955, 73, 905-937.
23. King PM, Adam RD, Pryde A, et al. Relationships of human antroduodenal motility and transpyloric fluid movement: non-invasive observations with real-time ultrasound. Gut l984, 25, 1384-1391.
24. Lewis FT. The form of the stomach in human embryos with notes upon the nomenclature of the stomach. Amer J Anat l9l2, 13, 477-503.
25. Moe RE, Klopper PJ, Nyhus LM. Demonstration of the functional anatomy of the canine gastric antrum. Amer J Surg l965, 110, 277-285.
26. Moore KL. Clinically Oriented Anatomy. Williams and Wilkins Publ, Baltimore 2nd ed, 1985.
27. Moore JG, Dubois A, Christian PE, et al. Evidence for a midgastric transverse band in humans. Gastroenterology 1986, 91, 540-545.
28. Müller E. Anatomischen und röntgenologischen Untersuchungen über Form, Bau und Lage des Magens. Erg Anat l921, 23, 310. Quoted by Torgersen (l942).
29. Polak JM, Sullivan SN, Bloom SR, et al. Enkephalin-like immunoreactivity in the human gastrointestinal tract. Lancet l977, l, 972-974.
30. Royston CMS, Polak J, Bloom SR, et al. G cell population of the gastric antrum, plasma gastrin, and gastric acid secretion in patients with and without duodenal ulcer. Gut l978, l9, 689-698.
31. Spiro HM. Clinical Gastroenterology. Macmillan Publ, New York l977, 2nd ed, p 148.
32. Stave R, Brandtzaeg P. Immunohistochemical investigation of gastrin producing cells (G cells). Scand J Gastroent l978, 13, 199-203.
33. Strickland RG, Mackay IR. A reappraisal of the nature and significance of chronic atrophic gastritis. Amer J Dig Dis l973, 18, 426-440.
34. Szurszewski JH. Electrical basis for gastrointestinal motility. In: Physiology of the Gastrointestinal Tract. Ed Johnson LR, Raven Press, New York l98l, Vol 2, pp 1435- 1466.
35. Tanner NC. The surgical treatment of peptic ulcer. Brit J Surg l964, 51, 5-23.
36. Torgersen J. The muscular build and movements of the stomach and duodenal bulb. Acta Rad l942, Suppl 45, l-191.
37. Willis T. Opera omnia. Amstelaedami l682. Quoted by Cunningham (1906).

2 What is a Sphincter? #

The definition of sphincter (Greek sphinkter = that which binds tight, binder) remains elusive. Williams (l962) pointed out that early anatomists and surgeons saw and felt a narrow powerful band of muscle enclosing the distal end of the stomach and concluded that it was a sphincter controlling the width of the lumen; later observers thought the sphincter could be part of the circular musculature of the pyloric canal, constituting an inherent part of the gastroduodenal pump. He called the pyloric sphincter a muscularis propria ring, consisting mainly of circular but also containing some longitudinal fibres, which surrounds the pyloric aperture; it narrows the distal end of the stomach to the point at which the lumen can be occluded by a mucosal plug. The sphincter also takes part in pump-like contractions of the fan-shaped muscle to close the pyloric canal.

Edwards and Rowlands (l968) define sphincter as a muscle surrounding and serving to close an orifice; the cricopharyngeus is mentioned as an example of a tonically closed sphincter, detectable by manometry as a zone of sustained elevation of pressure, undergoing precisely timed and brief openings. However, the pyloric ring in man does not appear to be a tonically contracted sphincter which opens occasionally; it behaves as the distal end of the thickened muscle mass surrounding the "antrum," and forms part of this muscular unit.

According to Didio and Anderson (l968) a sphincter is a specialized accumulation of smooth muscle in the bowel wall which serves to slow down transit of contents and to prevent retrograde flow. It is characterized by a thickening of the circular muscle layer creating an observable constriction both on gross inspection and on radiographs. In some cases it causes a recordable zone of increased intraluminal pressure; at appropriate times it closes to prevent flow, while at other times it opens to facilitate emptying. According to these authors, the emphasis laid on the closing properties of sphincters is anatomically and physiologically inaccurate and clouds the understanding of the functional significance of sphincters; the mechanism for opening is equally important as the one for closing, and it would be preferable to regard all sphincters as gatekeepers.

Black's Medical Dictionary (l97l) defines sphincter as a circular muscle which surrounds the opening from an organ; by maintaining constantly a state of moderate contraction it prevents escape of contents.

Stedman’s Medical Dictionary (1972) describes a sphincter as an accumulation of circular muscle fibres or specially arranged oblique fibres, acting to reduce or occlude the lumen of a tube, the orifice of an organ or the cavity of a viscus; it is the closing component of a gatekeeper. Still another medical dictionary, Gould (1972), defines it as a muscle surrounding and closing an orifice. Ruch and Patton (l973) state that the pyloric sphincter is a true anatomic sphincter, formed by a distinct thickening of the circular fibres of the muscularis externa. It is open or its mucosa is in weak apposition during most of the gastric emptying cycle, both when the "antrum" is inactive and between "antral" peristaltic waves. However, it seems not to play an essential role in controlling gastric emptying. As far as the pyloric sphincter is concerned, there is a definite lack of agreement between anatomic predictions and physiologic thinking. The inferior oesophageal sphincter in man, on the other hand, cannot be recognized anatomically as a sphincter although it functions as such.

According to Gray’s Anatomy (l973) the pyloric sphincter is a muscular ring composed of a thickened portion of the circular muscle layer, but also containing some longitudinal fibres which dip inwards to interlace with the circular fibres of the ring. Shepro et al. (l974) call the pyloric sphincter the circular muscle band that controls the opening of the stomach into the small intestine. Mehta et al. (l974) looked upon a sphincter as a zone of tonically elevated pressure; they failed to demonstrate such a zone at the pylorus.

In the gastro-intestinal tract, according to Alumets et al (l978), a particularly rich innervation of vasoactive intestinal peptide (VIP) nerves is seen in the region of sphincters; an evaluation of the density of these nerves may assist in anatomically defining a sphincter.

Wheater et al. (l979) state that the pyloric sphincter consists of an extreme thickening of the circular layer of the muscularis externa at the gastro-duodenal junction; in an accompanying illustration the sphincter is equated with the pyloric ring. Reeve (l98l) points out that the traditional definition of a sphincter is a ring-like muscle which controls the opening of a body orifice or constricts the lumen of a natural body passage, one of the essential functions being to delay the passage of intestinal contents. However, many other functions may be attributable to sphincters and anatomical studies of certain sphincters run into difficulty if the traditional definition is adhered to. The pyloric sphincter, for instance, is a ring of muscle consisting of an aggregation of the circular fibres of the muscularis externa at the terminal aspect of the stomach; it is not independent of the preceding part of the gastric musculature.

According to Thomas and Mann (l98l) it used to be thought that the function of a sphincter was to delay the onward passage of luminal contents, but it is now realised that this concept was too facile. Sphincteric zones act in a very complex regulatory fashion, and the organisation of a particular sphincter is not necessarily an exact replica of the others. A sphincter can be defined in different ways. Physiologically, it is an area which is tonically closed and which has the ability to relax and contract; some sphincters however, are not permanently closed and have an “open” mechanism. Anatomically it is a thickening of circular muscle fibres in a hollow viscus; the exception here is the intrinsic lower oesophageal sphincter in man. Pharmacologically a sphincteric region can be identified by the theory of “reciprocal innervation”, whereby sympathetic stimulation produces sphincteric contraction and parasympathetic stimulation induces relaxation;
these effects are opposite to those produced in neighbouring smooth muscle. This definition is considered to be sound for all smooth muscle sphincters (but is not applicable to the crico-pharyngeal and external anal sphincters, which contain striated muscle). Thomas and Mann (l98l) admit that none of these definitions is ideal.

Schulze-Delrieux and Shirazi (l983) state that the role of the pylorus as a sphincter remains controversial because the pylorus does not consistently close

off the gastroduodenal junction or affect the rate of gastric emptying. According to Wingate (l987) it is uncertain whether the pylorus is indeed a sphincter; it depends on what is meant by the term “sphincter”. The limitation of language has not yet allowed a comprehensive definition of the concept.

An illustrated medical dictionary (Dorland l988) again defines sphincter as a ringlike band of muscle fibres that constricts a passage or closes a natural orifice; the pyloric sphincter is said to be a thickening of the circular muscle of the stomach around its opening into the duodenum.

Discussion #

It is clear that consensus has not been reached on the definition of “sphincter” in general terms. It would appear, however, that the definition as stated in most medical dictionaries is totally inadequate.

A time-honoured view is that a sphincter is a band of circular musculature which is tonically contracted most of the time, creating a zone of sustained elevation of pressure preventing flow of intestinal contents in either direction; it relaxes intermittently, usually in reciprocity with an oncoming peristaltic wave, to allow flow. In this view a sphincter functions more or less independently of the surrounding musculature and probably has a separate innervation.

As far as the pylorus is concerned, the question arises whether the pyloric ring should be looked upon as a sphincter in the sense implied above. Is it a ring consisting totally or almost totally of muscular fibres, or is it merely a constriction which impedes flow by its narrowness and poor distensibility, as suggested by Stadaas and Aune (l970) and by Schulze-Delrieu (l983)?

Another view of sphincters is that proposed by Thomas and Mann (l98l), who mention the possibility of sphincteric zones acting in complex regulatory fashion. However, on anatomical grounds it is difficult to accept the theory of reciprocal innervation, in which a particular nerve stimulus is purported to produce opposite effects in the “sphincteric” and the adjacent smooth musculature.

A third possibility is that a rather intricate sphincteric mechanism may exist at junctional zones, e.g. the pyloroduodenal junction. In this instance localized rings of circular musculature are inherent components of a muscular region or cylinder consisting of both circular and longitudinal fibres, the entire structure functioning as a unit.

It is proposed to discuss these and related questions in more detail in subsequent chapters.

References #

1. Alumets J, Hakanson R, Sundler F, et al. VIP innervation of sphincters. Scand J Gastroent l978, 13 (Suppl 49), 6 (abstract).
2. Black's Medical Dictionary, edit Thomas WAR, A & C Black, London 29th ed, l97l.
3. Didio LJA, Anderson MC. The "Sphincters" of the Digestive System. Williams and Wilkins, Baltimore l968.
4. Dorland's Illustrated Medical Dictionary, edit Taylor EJ, WB Saunders Co, Philadelphia, 27th ed, l988.
5. Edwards DAW, Rowlands EN. Physiology of the gastroduodenal junction. In: Handbook of Physiology, Sect 6, Vol 4, Motility. Edit Code CF. The American Physiological Society, Washington DC, l968, l985-2000.
6. Gould Medical Dictionary. McGraw Hill, 3rd edit, New York l972, p 1444.
7. Gray's Anatomy. 35th British Edition, ed Warwick R, Williams PL. Longman Co, Edinburgh l973, p 1273.
8. Mehta SJ, Kaye MD, Showalter JP. Is there a pyloric sphincter? Gastroenterology l974, 66, 746 (abstract).
9. Reeve DRE. Anatomy of the sphincters of the alimentary canal. In: Alimentary Sphincters and their Disorders. Edit Thomas PA, Mann CV. MacMillan Publ, London l98l, pp 1-26.
10. Ruch TC, Patton HD. Physiology and Biophysics. WB Saunders Co, London l973, pp 4, 18.
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12. Schulze-Delrieu K. Volume accomodation by distension of gastric fundus (rabbit) and gastric corpus (cat). Dig Dis Sci l983, 28, 625-632.
13. Shepro D, Belamarich F, Levy C. Human Anatomy and Physiology. Holt Rinehart Winston Inc, New York l974, p 598.
14. Stadaas J, Aune S. Intragastric pressure-volume relationship before and after vagotomy. Acta Chir Scand l970, 136, 611-615.
15. Stedman's Medical Dictionary. Williams and Wilkins, 22nd edit, Baltimore l972, p 1172.
16. Thomas PA, Mann CV. Alimentary Sphincters and their Disorders. MacMillan Publ, London l98l, pp xi, 227-232.
17. Wheater PR, Burkitt HG, Daniels VG. Functional Histology. Churchill Livingstone, London l979, pp 184, 188.
18. Williams I. Closure of the pylorus. Brit J Rad l962, 35, 653-670.
19. Wingate DL. Functional disorders of the stomach and small bowel. Scand J Gastroenterol l987, 22 (Suppl 128), 62-68.