Paper - Evolutionary factors in the production of pharyngeal diverticula

From Embryology
Embryology - 23 Oct 2019    Facebook link Pinterest link Twitter link  Expand to Translate  
Google Translate - select your language from the list shown below (this will open a new external page)

العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt    These external translations are automated and may not be accurate. (More? About Translations)

Negus VE. Evolutionary factors in the production of pharyngeal diverticula. (1925) J. Laryng. and Otology. 40(11): 702 - 224.

Online Editor 
Mark Hill.jpg

See also by this author - Negus VE. (1925). EVOLUTIONARY FACTORS IN THE PRODUCTION OF PHARYNGEAL DIVERTICULA. Br Med J , 2, 699-700. PMID: 20772203
Negus VE. (1942). The Mechanism of Swallowing: President's Address. Proc. R. Soc. Med. , 36, 85-92. PMID: 19992586
NEGUS VE & KILNER TP. (1949). Stenosis of the larynx. Proc. R. Soc. Med. , 42, 981-6, illust. PMID: 15399916
NEGUS VE. (1956). The organ of Jacobson. J. Anat. , 90, 515-9. PMID: 13366864

Obituary - Negus VE, Fish W & Campbell JM. (1974). In memoriam. (Sir Victor Ewings Negus MS DSC FRCS FRCSE (HON) FRCSI (HON) (1887-1974). J R Coll Surg Edinb , 19, 394. PMID: 4613828

Modern Notes: pharyngeal arch

Template:Gastrointestinal Tract links

Head Links: Introduction | Medicine Lecture | Medicine Lab | Science Lecture | Lecture Movie | Science Lab | pharyngeal arch | Craniofacial Seminar | mouth | palate | tongue | placode | skull | neural crest | Head and Face Movies | head abnormalities | Category:Head
Historic Head Embryology  
1910 Skull | 1910 Skull Images | 1912 Nasolacrimal Duct | 1921 Human Brain Vascular | 1923 Head Subcutaneous Plexus | 1919 21mm Embryo Skull | 1920 Human Embryo Head Size | 1921 43 mm Fetal Skull | Historic Disclaimer
Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Negus VE. Evolutionary factors in the production of pharyngeal diverticula. (1925) J. Laryng. and Otology. 40(11): 702 - 224.

Evolutionary Factors in the Production of Pharyngeal Diverticula

By V. E. Negus, M.S., F.R.C.S.

  • Paper read at the Sections of Laryngology and Otology, British Medical Association, Bath, July 1925. Published by kind permission of Sir Dawson Williams, Editor, British Medical Journal.

Introduction

The remarks I am about to make will be extremely vague and unpractical, because I do not propose to discuss the symptoms, signs, and treatment of a diverticulum of the pharynx after it has been produced, but rather to call attention to some of the wider issues which have made it possible for these pouches to occur.


I have not noticed any reference in literature to pathological pouches in the pharynx of animals. It is interesting, however, to inquire into the general causes which make it possible for man to be especially picked out as the subject of this complaint.


The condition now under consideration is that of pressure pouch, situated at the lower end of the pharynx. Because the symptoms and signs of the mature condition are in most cases referred to the oesophagus, the pouch is generally known as the oesophageal pouch. This, however, is not a correct term, for the simple reason that the dilatation does not arise from, or communicate with, the gullet. Traction pouches near the bifurcation of the trachea are oesophageal in origin. The pathological formations, which are the subject of the present review, arise in an area between the lowest circular and the adjacent Oblique fibres of the inferior constrictor muscle of the pharynx; hence they should be designated as pharyngeal (fig. 1).


These diverticula may attain to a great size when they have existed for many years; they cause considerable inconvenience because of their action as a kind of “jack the Giant Killer’s” pouch, and in many instances they cause death.


The symptoms and signs are well known; the treatment follows well-defined lines. I have nothing to add to the observations and suggestions of those who are infinitely better fitted than myself to deal with the fully-developed condition.


In the course of anatomical and physiological investigations on certain animals, various points have struck me which have a bearing on this subject; they are concerned with the general anatomical arrangement which makes it possible for these pathological conditions to arise. The observations now put forward are intended as matters of general interest and not as factors of direct use in the amelioration of the fully-formed pouch.

Reference Letters to Illustrations

Ar = Arytenoid Cartilage. As = Air Sac. C = Constrictor Muscle. Cr = Cricoid Cartilage. D = Dilator Muscle. Ep = Epiglottis. IF = Inferior Fold or True Vocal Cord. LV = Left Ventricle. SF = Superior Fold or Ventricular Band. TF = Thyro-arytenoid Fold. Th = Thyroid Cartilage. TM = Thyro-arytenoid Muscle.

In the sections of the larynx cut in the coronal plane, Ven = Ventral Section. D = Dorsal Section.



Fig. 1. Evolution of a pouch in the region of the inferior constrictor muscle.

Top left hand — Normal pharynx and oesophagus. Top right hand - Potential Pouch (Professor Parson’s Specimen in Museum, R.C.S., Eng.) Bottom left hand — Sma1l Pouch (Mr L. Co1Iedge’s Specimen in Museum, R.C.S., Eng.). Bottom right hand—Large Pouch (in Museum, R.C.S., Eng.)

The Age Incidence

The age incidence is an advanced one, and men are more often affected than women. The condition is not congenital, and whatever may be the factors at work they take many years to exert their influence. I do not know why one sex should be specially selected; but the fact that the deterioration of the teeth seems to be more or less constant in these cases would indicate that the more careless male suffers because of the repeated swallowing of unmasticated boluses of food. If soft substances alone were used by the aged and ill-toothed, there would no doubt be a diminution in the number of cases of the condition under review. All the factors involved appear to depend to a great extent on the misdirection of a mass of solid food over a prolonged period; such a condition is met with most often in men of advanced years.

The Normal Mechanism of Deg1utition

The normal mechanism of the deglutition of solids can be described, in brief, as the propulsion of a bolus by a contracting band of muscle in the pharynx or oesophagus, preceded at a lower level by a band of muscular relaxation; this indicates a squeezing onwards of the mass. What happens in the mouth does not concern us; it is only after the bolus has left the tongue and has been received into the pharynx that it becomes of interest in this review.


Similarly, the bolus is no longer of any significance as soon as it has safely entered the oesophagus. To the usual description I would add one word, namely, that the part played by the longitudinal muscle fibres of the pharynx and oesophagus is neglected in most of the published accounts.


If one considers the case of a fish, destitute of a larynx, the subject is simpler. There is no sharp division of the parts of the alimentary canal into buccal cavity, pharynx, and oesophagus. All these regions have circular and longitudinal fibres, and the method of propulsion of solid food seems to be simple.


If a band of circular muscle contracts round the object and prevents it from moving backwards towards the mouth, and if the longitudinal muscle fibres then contract, the latter will draw up a segment of the gut over the bolus. If the band of circular fibres at the upper part of the next segment of gut then contracts, it will hold the bolus at a slightly lower level than before. A repetition of the circular fixation and longitudinal progression will result in transference of the bolus down the food passage. The process, however, will not be of the nature of a squeezing on, but rather of a creeping of the gut over the bolus.


If a snake swallows a rabbit it appears to creep forward over its prey; the rabbit is not squeezed down the gullet, but the gullet appears to creep forward over the rabbit. Much in the same way a leucocyte engulfs a bacterium by creeping over and round it, or an amoeba takes in a morsel of nutriment.

The point arrived at~——if this suggestion be true—is, that the longitudinal muscle fibres possess great importance in the mechanism of deglutition.

Before proceeding to study their action in further detail, it is necessary to consider, at greater length, the band of muscular relaxation.

Function of Crico-Pharyngeus Muscle

There is no necessity, in ordinary circumstances, for the greater part of the pharynx to relax in front of the progressing bolus of food, because the circular muscle is not in active contraction. It has a definite tonus, but not one of sufficient power to close the pharynx.


Normally the pharynx is open in order to allow respiration to proceed, and there is no active contraction of the circular muscular fibres. A different state of affairs, however, exists in the region of the lowest circular fibres of the pharynx, namely those arising from the back of the cricoid cartilage, and known as the crico-pharyngeus muscle. This collection of fibres is habitually in contraction, and_ is onlypin relaxation when the bolus of food is passing into the oesophagus immediately subjacent. The fact that it is contracted when the other circular fibres of the pharynx and oesophagus are relaxed, and that it relaxes when the others contract, entitles ‘that muscle to bear an independent name. It is a definite muscular band‘ of considerable interest and one offering great difficulty to the endoscopist.


The bundle of fibres, known as the crico-pharyngeus, arises from the lower part of the cricoid ring on its lateral surfaces; the fibres fuse with one another behind.


In reptiles the band does not exist as a separate entity; its function is assumed by the ordinary circular fibres of the oesophagus. In birds the circular fibres just below the level of the larynx are evident and more powerful than at other parts above and below ; only a few fibres, however, are attached to the crico-thyroid ring. In marsupials the band arises from the cricoid element of the fused crico—thyroid ring; it is broad and somewhat fanned out. In other mammals it is narrower and is very easily seen. Its general direction is seldom truly circular ; in dissected specimens it appears to run, in most cases, rather obliquely upwards but in a few, downwards.


The fibres, however, seem to have an actual anatomical inclination—apart from any effect of dissection —- in an upward direction, in those species in which the oesophagus is suspended mainly from the tips of the cartilages of Santorini. Of such a type are most of the ruminants and also members of the dog tribe.


In men, the attachment of the oesophagus at a lower point— namely to the back of the cricoid plate-— appears to have caused alteration in the direction of the fibres, some of which run slightly downwards.

In man the, muscle is very much fanned out. At its posterior part it covers an area almost double the size of that at its origin from the cricoid ring. Consequently the fibres are somewhat separated.


The point of interest at the present moment is, why does this band remain contracted except when food is passing? It is not to prevent regurgitation from the oesophagus, because that function is provided for at the gastric end of the gullet. Nor is it contracted in order to allow for a collection of food above it, as appears to happen, for instance, in the food pouch situated at the base of the tongue of the great ant eater. As soon as saliva or food reaches the region of the hypopharynx behind the larynx, there is set in action a stimulus which causes relaxation of the muscular sphincter. "There is no known advantage in food being delayed at the level of the crico-pharyngeus. Nor is it delayed; for, on observation with a fluorescent screen, an opaque meal is seen to shoot with great speed from the pharynx into the oesophagus. Such rapid mechanism is desirable in order to obviate the necessity of prolonged closure of the glottic aperture.

Some other reason must be sought for; it seems to be supplied by the requirements of respiration.


When an inspiration is taken, the negative pressure in the pleural cavity causes enlargement of the lungs and temporary diminution of intra-pulmonary pressure. There is also diminished pressure in the mediastinum during this period. Air is drawn into the pharynx through the nose or mouth, or through both channels. When it reaches the level of the laryngeal aperture, it arrives at a junction where the air and food passages diverge. There is no doubt, however, as to the further course of the air, because the laryngeal orifice is open while the oesophageal aperture is closed. Consequently, the current of air passes down the trachea into the lungs.


If the crico-pharyngeus muscle, however, were not in a condition of tonic contraction, a considerable portion of the volume of tidal air would pass into the oesophagus and dilate it. The passage into the oesophagus, when its mouth is relaxed, is wider and freer than that into the trachea, owing to the narrowed glottic aperture. The forces at work would balloon the oesophagus at each inspiration, and would not draw sufficient air into the lungs, were it not for the tonic contraction of the crico-pharyngeus. The evidence furnished by comparative anatomy is considerable, but only one or two points will be alluded to.


Closure of the aperture of the oesophagus is required by all lung-breathing animals. In those which breathe by blowing air into their lungs——such as fish, amphibians, and some reptiles-——the food passage must be kept shut to avoid diversion of the air current into the lower gut. In animals which breathe by the suction method-—-through the medium of the diaphragm and rigid thoracic Walls—there is greater necessity for closure of the oesophagus at its upper end in those in which the laryngeal airway is narrow, than in those in which it is wide. For instance, in fast-running animals such as deer and antelopes, the aperture through the larynx is very big in comparison with that in slower running species like monkeys and men. Consequently, if theoesophagus were left open in each case, a greater volume of air would pass into the lungs in the fast-running species than in the slow.


Fig. 2. Valvular nature of the larynx (coronal sections of larynx).

Lung fish (lep2'dosz‘2'en); feeble inlet valve (photographed from a microscopical Section). The larynx is seen as a fissure in the floor of the transversely placed pharynx. Lace Monitor (Varmzus z*arius); powerful inlet valve. Lion (Felis Zea); powerful inlet valve. There is no division of thyro-arytenoid fold into vocal cord and ventricular band. Ox (Bos taurus); no valve ; flat undivided thyro-arytenoid folds. Man ; powerful inlet and exit valves, the former formed by the vocal cords and the latter by the ventricular bands. Lemur (lemur flavgfroazs) ; very powerful double inlet valve, consisting of the Vocal cords and ventricular bands.



In certain animals the epiglottis, or at least its tip,1ies above and behind the soft palate. In these animals the larynx is intra-narial. There is less necessity for closure of the oesophagus in those species. The gullet is thereby more or less shut off ‘from the external atmosphere; lateral food channels, z'.e. the space bounded medially by the aryepiglottic folds and laterally by the thyro—hyoid membrane, ag. the pyriform sinus in man, even if present in animals with an intra-narial larynx, are potential passages only, and do not readily allow the entrance of inspired air.


In every case, however, constant closure of the oesophageal aperture, while inspiration proceeds, is essential. The result is that a tightly contracted band of muscle is placed on the food passage, and if it does not relax when the bolus of food reaches its neighbourhood, obstruction will result.


I would suggest that there is one particular time at which the muscle must be in contraction, in order to prevent the passage of air into the oesophagus; namely, during the period of fixation of the ribs for purposes of fore-limb effort.


I have attempted to show elsewhere1 that the vocal cords are in reality inlet valves placed at the orifice of the air tract to the lungs (fig. 2). Their function is to close the airway when the individual desires to hang on to branches, to adduct the fore limbs, or to steady them for prehension. Thus, air cannot enter the lungs as they tend to expand and fill the enlarging thoracic cavity. A potential reduction of pressure is therefore produced which helps to bring the ribs to a position of rest.


It is obvious that if this mechanism is as described here, firm closure of the crico-pharyngeal fold will be necessary during fore—limb effort; and furthermore, the necessity will arise in animals which have the power of independent use of the fore limbs. Of such a type is man; hence, we have still another reason why man has a tonically contracted cricopharyngeal muscle. It is a factor which specially predisposes him to the acquisition of a pouch.


Contraction of the crico—pharyngeus must be very powerful in order to prevent the passage of air. In many other regions of the body a valvular mechanism is used for somewhat similar purposes, as, for instance, in the heart, the veins, the larynx, the ileo-caecal region, and anal canal. When reliance is placed on muscular action alone, it is necessary that the muscle should be very powerful. At the entrance to the oesophagus it would not be possible to have a valve, because food would be entrapped and held up; therefore a muscular ba11d is present to serve the function. Its presence conduces, in some cases, to the development of pathological conditions.


A breakdown in the mechanism whereby a wave of contraction of circular and longitudinal muscle fibres is unaccompanied by relaxation of the crico-pharyngeus, will predispose to the production of a pharyngeal diverticulum. In man the lowest circular fibres of the inferior constrictor remain in tonic contraction, and just above the level of these fibres diverticula arise. Consequently, it is obvious that lack of relaxation of this muscle band plays an important part in the production of pouches.


Attachment of the (Esophagus to the La.rynx.—As the bolus of food passes down the pharynx the oblique fibres of the constrictors draw up the tube; the elevators of the larynx act similarly in the anterior region, and the palato-pharyngeus and stylo-pharyngeus muscles in addition, help in drawing the pharynx over the bolus. In this way the creeping mechanism already referred to is carried out, and the mass of food passes into the oesophagus.


In those fish which have no larynx the longitudinal fibres are arranged regularly, and the food tube is pulled up equally on all sides. In the dipnoi or lung-breathing fish, the larynx is so diminutive and so mobile that the same mechanism applies, and movement of the pharynx upwards over the bolus is carried out as easily as if no larynx were present. A similar arrangement is found in amphibians, reptiles, and birds, and no important modification needs to be brought in. The larynx in all these species has no epiglottis or aryepiglottic folds, and, in consequence, it has to be closed completely when food is passing.


In mammals the larynx is large in proportion to the size of the pharynx; the opening has become inclined in its axis so as to straighten out the air tract. The larynx, too, is less mobile than in the lower species, in which there is a diminutive organ; consequently, there is an alteration in the mechanism of deglutition.


Fig. 3. Suspension of oesophagus from cartilages of Santorini.


In each case.the larynx has been dissected, and the right sagittal section is illustrated. The process ofSant01‘ini is seen projecting backwards from the arytenoid cartilage and arching over the inter-arytenoid muscle.


In many mammals there is a peculiarity in the attachment of the oesophagus anteriorly, which compensates to a great extent for the comparative lack of mobility of the larynx, and enables the creeping mechanism to be carried out. This consists in the attachment of the aponeurotic fibres of the oesophagus to the tips of the cartilages of Santorini, as described in a previous paper 1 (fig. 3). These cartilages are prolongations of the arytenoids in a posterior direction; they are hook-like processes turning. down towards the oesophagus, and arching over the inter-arytenoid muscle (fig. 4). When the latter contracts in conjunction with the rest of the sphincteric girdle of the larynx, it tilts the arytenoids and the processes of Santorini forwards towards the thyroid cartilage, and thereby produces a funnel-shaped opening of the oesophagus at the moment when the air tract is closed. In this way the upper end of the oesophagus is opened and also pulled up over the bolus of food. The anterior wall of the oesophagus specially is raised, but the main effect of the arrangement is to provide a wide aperture through which food may be propelled into the gullet. If the crico-pharyngeus muscle be relaxed at the moment when the funnel-like opening of the oesophagus is formed, and if the circularly arranged fibres of the inferior constrictor form a band of contraction above the bolus, there will necessarily be a movement of the mass of food downwards into the oesophagus.


This mode of attachment of the oesophagus to the processes of Santorini is well seen in almost all species of mammals, with the exception of man. In the cat tribe, in marsupials, ruminants, the dog tribe, and bears it is well represented, and a similar state of things is found in most monkeys. In all these animals there is practically no attachment of the oesophagus to the back of the cricoid cartilage; but in man there‘ is a fairly extensive and firm fixation to the cartilage, and, in consequence, a partial loss of the important mechanism of synchronous closure of the larynx and opening of the oesophagus exists. Change of diet, the softer character of the food, and increased reliance on mastication appear to furnish the explanation.


Man is descended from a stock which is not adapted for swallowing large boluses of solid food. \/Vith soft food, or with hard food well masticated, he gets on fairly well. If he becomes edentulous and then attempts to swallow solid masses of food, he wil1—if he keeps up the habit for several years—become liable to develop a pharyngeal diverticulum. In the production of this pathological condition the fact that the oesophagus does not dilate widely at its mouth in a funnel-shaped manner, as in most animals, must play an additional important part.

Fig. 4. Processes oz: santorini

Fig. 5. Cnt1nuity of long1in a kangaroo (mddropus tudi.\tal fibres or oesophagus gigamfeus) viewed from behind with thyr0 - aryt en0113 (from a specimen in the museum, muscle in a dog (c‘”"3' r_c_s_ eng), familiaris).

The long processes are arching The dissected larynx is viewed over the inter-aIYteI10id 111115016, in from the left. The cartilage of which a sesamoid cartilage is de- Wrisberg helps to support the atyveloped. The whole of the oeso- epiglottic fold, and below and behind phagus has been removed. it a long process of Santorini gives support to the oesophagus, which is held back by a hook.

Disposition of the Longitudinal fibres of the (E30pha.gus.—In close relation to the attachment of the oesophagus to the larynx is the arrangement of its muscle fibres. In the lower species there is an orderly disposition of the circular and longitudinal fibres round the Walls of the gullet. In a snake the longitudinal are the more evident of the two, but in a bird the circular fibres are well seen, arranged in a truly circular manner.


In some mammals the longitudinal fibres are more or less distributed over the lateral and posterior walls of the pharynx, but at the upper end of the oesophagus these fibres tend to collect into bundles, lying on the lateral and posterior aspects of the food passage. In a tapir, for instance, there are powerful bands running from the pharynx on to the oesophagus, and it is evident that when they contract they will elevate all parts of the tube and draw it upwards over the bolus.


In other species the fibres lie only at the lateral borders of the oesophagus, and not on the posterior surface. Even when thus placed they will draw the oesophagus towards the pharynx in a regular manner ; of this type is the reindeer.


The arrangement of the fibres varies, but in no type is it exactly the same as in man. In him, the longitudinal fibres are practically absent from the posterior surface of the pharynx at its lowest part, and from the oesophagus posteriorly. All the longitudinal fibres of the oesophagus collect at a point situated about one and a quarter inches from the upper end of the tube, and are directed towards the anterior surface. Here some‘ of them extend upwards into the pharynx, but the majority end in an attachment to an aponeurotic sheet, which is suspended partly from the back of the cricoid cartilage and partly from the cartilages of Santorini. In the dog some of the longitudinal fibres become continuous with fibres of the thyro-arytenoid muscle (fig. 5). Consequently, in man during deglutition the contraction of the longitudinal fibres draws up the anterior part of the oesophagus more than the posterior. The latter remains more or less stationary, and therefore the drawing up of the oesophagus over the bolus of food is less easily affected in man than in the lower animals and in most mammals.


A special collection of longitudinal fibres is provided by the palato-pharyngeus muscle, which is enclosed in the posterior pillar of the palate. It is well developed in all animals with broad and strong posterior palatine folds, and is therefore especially prominent in ruminants. From its disposition, it is evident that it will elevate the lower part of the pharynx in the posterior region, and will help to raise the posterior wall of the oesophagus when the tube is drawn up. In those animals provided with an arcus palato-pharyngeus, the position of these muscular fibres is in close relation to the upper end of the oesophagus on its posterior wall; hence the mechanism of elevation of the gullet over the bolus of food will be particularly efficient.


In man the palato-pharyngeus is relatively inefficient, because it fades away in the upper part of the pharynx. Consequently, there is found in him a relative deficiency of longitudinal fibres in the lower part of the pharynx and upper part of the oesophagus, as compared with most mammals. This constitutes another factor in the chain predisposing to the formation of diverticula.

Posterior Palatine Pillars

(a) In Relation to 0{factz'o7z.— Animals which rely to a great extent on their powers of scent, for discriminating prospective food, for detecting enemies or intended prey, or for both purposes, must inspire through the nose with as little interruption as possible. If the mouth'be kept shut, all air must of necessity pass over the olfactory region; if the mouth be open, then a great part, if not most of the inspired air, will enter through it, because the buccal cavity is Wide and provides a more easy passage than does the narrow nose.

Consequently, in macrosmatic animals which use their mouths for seizing prey or for cropping herbage, some provision must be made to prevent air from passing through the mouth when open. The mechanism provided takes the form of an epiglottis, in association with palatine folds; the two together are able to shut off’ the mouth, and provide a barrier against the passage of air (fig. 6).

This mechanism is present in all macrosmatic mammals. In the dog tribe the epiglottis lies below the soft palate in most instances. It acts as a valve in conjunction with the short but broad posterior palatine folds, and prevents air from entering by the mouth, while permitting it to leave by this route. The result is that all the inspired air passes over the extensive olfactory mucous membrane of the nose, while some of the expired air leaves by the mouth and causes evaporation from the tongue when the animal pants.

The cat tribe has an anatomical arrangement of a similar type; but in these animals the epiglottis lies normally above the soft palate, in such a position that both inspired and expired air currents pass through the nose. Most animals which eat grass or other herbage, rely to a considerable extent on the olfactory sense to protect them against poisonous foods, and to warn them against the proximity of enemies. In almost every instance these animals have an intra-narial‘ epiglottis. Examples are seen in marsupials, deer, antelopes, oxen, horses, rhinoceroses, tapirs, elephants, rabbits, hares, porcupines, and cavies, to mention only a few.


Fig. 6. Relation of epiglottis to soft palate for purposes of olfaction (mesial sagittal sections of the head).

Dog (Cam's famz°/2'a1'z's); the epiglottis lies below and apposed to the palate. Capuchin Monkey (Cebus fatm//us); the epiglottis lies above the palate. Indian Muntjac or Barking Deer (Ce;-wuius mumjac); the cpiglottis lies above the palate in an intra-narial position. Human foetus at seventh month ; the epiglottis is widely separated from the short soft palate.


In all these animals the larynx is placed high in the neck and is in close relation to the folds of the soft palate. The result is that food is directed by the palatine folds, as by an inclined plane, in its passage through the pharynx. The food is directed downwards and backwards towards the opening of the oesophagus, and is prevented from impinging on that part of the pharynx which lies above the level of the soft palate, and which is functionally part of the naso-pharynx. Man has discarded almost entirely the use of olfaction, and has adopted the sense of sight in the selection of his food, and in locating his enemies. Thus he has lost the necessity for the constant passage of inspired air over the olfactory mucous membrane, and is at little disadvantage if the air current enters by the mouth.


Consequently the need for close relation between the larynx and soft palate has disappeared, and, as a result, a wide interval separates the two structures. Therefore the posterior palatine pillars give less assistance in directing the bolus of food towards the oesophagus, by reason of the poorer development of the soft palate and its pillars, and because of the distant position of the mouth of the gullet in relation to the fauces. There is nothing to prevent the bolus impinging on the part of the pharynx consisting of the lower fibres of the inferior constrictor muscle, an area which in many animals is part of the nasopharynx. This provides a further reason for the production. of a diverticulum.


(b) In ReZczz‘z'o7z to DegZutz'z‘z'01z.-——Animals which feed on herbage have to consume enormous quantities if they are to sustain life. The chewed-up grass or leaves form a semiliquid mass which accumulates in the mouth, little by little, in the process of nibbling or cropping. If respiration had to be interrupted whenever deglutition took place, the animal would be hampered by the frequent pauses required for respiration.


Hence in animals frequently swallowing small masses of semiliquid food, a mechanism is provided to allow of simultaneous respiration and deglutition. This is provided by a modification of the posterior folds of the soft palate. The folds in this type of animal are long and broad, and provide, in association with lateral epiglottic or aryteno-epiglottic folds, two channels for food, one at either side of the larynx (fig. 7). These are the lateral food channels described by Goppertz and others.


They direct the passage of the food into the oesophagus and prevent its entrance into the open larynx, through which respiration is being carried on without cessation; it is only necessary to observe a cow or a horse cropping grass to realise the truth of this description.


In many animals the palatine folds meet in the midline behind the larynx; in this case they form a ledge which encircles the spout-like larynx (fig. 8). The complete structure is then known as the arcus-palato-pharyngeus ; it is seen in most rnarsupials, in almost all ruminants except llamas, and in whales, porpoises, and dolphins. In the latter class of animals the arcus reaches its most perfect development, because the cetacea require to inspire as soon as possible after reaching the surface, even when the mouth and pharynx are full of water containing a supply of fish. A dolphin is able to keep ahead of a torpedo-boat destroyer steaming at full speed, and can take in a sufficient quantity of tidal air in a remarkably short time, even though the pharynx is full of water.


The mechanism provided by the palatine folds in this case is one designed to allow of simultaneous deglutition and respiration, and has no relation to the olfactory sense; cetaceans are microsmatic. 111 many instances, however, the anatomical arrangement is directed both to the maintenance of the olfactory sense when the mouth is open, and to the function of breathing, While food is passing into the oesophagus.


The arcus—palato-pharyngeus is situated posteriorly just between the circular and oblique fibres of the inferior constrictor. It has the crico-pharyngeus muscle below it, while the region of the pharynx lying above it is functionally part of the naso-pharynx.


Man does not live on grass ; therefore he has no arcus. He is thereby deprived of an anatomical barrier capable of directing food into the oesophagus, and preventing it from striking on the part of the pharynx between the circular and oblique fibres of the inferior constrictor, which in him is a weak spot.

It is interesting to observe that in domestic pigs a protrusion of mucous membrane is to be found in some instances in this region; it is a hernial sac similar to the pharyngeal pouch of man (fig. 9). The sac is uncovered by muscle fibres, but is not of pathological interest, because it lies above the arcuspalato-pharyngeus, and is therefore in the naso-pharynx. On account of the arcus, it is not possible for food to enter the hernial sac, which consequently does not enlarge. It is well known that only by the frequent entrance of food and the gradual distension of a hernial sac, a pouch may become of pathological interest.


Fig. 7. Extent of posterior palatine foms in relation to deglutition (mesial sagittal sections of the larynx).

Chimpanzee (A72!/'zropop2.'t/zecus tmgfodytes); very short palatine folds not in relation with the epiglottis. Almost no lateral food channel. Horse (Egan: caéalfus); well-formed arcus palato-pharyngeus with an efficient lateral food channel below it. Dolphin (D349/2522255 deézfi/2775); permanent intr21—narial larynx, with extremely efficient lateral food channels. (From a specimen in the Museum, R.C.S., Eng.).


fiG. 8.—RELATION OF POSTERIOR PALATINE PILLARS T0 LARYNX IN VARIOUS ANIMALS.

The larynx is represented diagrammatically as seen from above. In each case the central black area is the laryngeal aperture leading to the trachea; the stippled area is the glottic margin made up of thyro-arytenoid folds; the striped area is the epiglottis——with its lateral processes in some cases; the small striped areas are the cartilages of Wrisberg; and the heavy interrupted line is the free margin of the soft palate and posterior palatine folds, which in some instances meet in the midline and encircle the larynx. In the case of Echidna the upper interrupted line represents a mouth flap.


Traction pouches of the oesophagus in man, which occur usually opposite the bifurcation of the trachea, never become large because no food gets into them, by reason of the direction in which the mouth of the sac opens. Similarly, the pharyngeal sac of a pig does not enlarge, because no food can enter it.

Position of the Larynx

The position of the larynx has a great influence on the anatomical arrangement of the lower part of the pharynx and The larynx, lower part of pharynx, and upper end of oesophagus, have been dissected the upper part of the and are seen from the right. The hernia of oesophagus.

fiG. 9.—PO'I‘EN'I‘IAL PHARYNGEAL. DIVER TICULUM IN A PIG (Sus domestzcus).

mucous membrane is projecting between the

In fishes, amphibians, owest C1I'Cl1.':l.I' and the ob1que bres of the reptiles, and birds the are reversed. The larynx is a very important structure, instead of being a diminutive and insignificant organ. It is large, and many of the muscle fibres of the lower pharynx take origin from it. Furthermore, instead of the larynx being suspended in the floor of the pharynx, the oesophagus is suspended from the larynx. In consequence of these changes considerable modifications may be observed.

inferior constrictor muscle. larynx IS diminutive in comparison with the size of the pharynx. It is suspended in the floor of the capacious pharynx, and is subservient to the latter in the movements of deglutition. The larynx is placed almost in the mouth, and does not interfere to any appreciable extent with the mechanism of swallowing, nor has it exerted any marked effect on the anatomical arrangement of the lower pharynx.

In mammals, however, and especially in man, the conditions


The larynx is placed further away from the mouth, for various reasons. This is due in the first place to the size and importance of the tongue, which has reached large dimensions because of the necessities of mastication. Secondly, the aperture of the larynx has become inclined in relation to the axis of the pharynx in order that air may more easily enter it. Thirdly, the larynx is elevated above the floor of the pharynx by virtue of the upward growth of its margins, in association with the presence of an epiglottis, and in many cases also with aryepiglottic folds. The naso-pharynx is long and brings the nasal airway into direct relation with the laryngeal aperture.

Because of these changes, the larynx lies further from the mouth than in the lower animals. There is, therefore, in most cases an obliquity in the direction of the fibres of the constrictor muscles which extend from the thyroid and cricoid cartilages to the median raphe. This obliquity is displayed in its most marked degree in the inferior constrictor.

Amphibians, reptiles, and birds have the larynx placed opposite the base of the skull; in them the circular muscles of the pharynx are arranged in a truly circular manner (fig. 10). In marsupials, and in the cat tribe, there is a slight obliquity of the inferior constrictor. In ruminants the obliquity is rather greater, and in the higher apes it is still more marked. In man the obliquity is most pronounced, because of the low position which the larynx occupies.

The reasons for the change of position of the larynx in man are several. In the first place he does not require to preserve a close relation between the laryngeal aperture and the nasal airway, because of the feeble development of the olfactory sense and the lessened needs of rapid air exchange as compared with fast-running animals, such as antelopes, horses, or dogs. The larynx therefore may occupy a position far away from the nasal air tract without great disadvantage; the soft palate, moreover, has attained less development and led to a wider gap than would exist if the naso-pharynx were of the same length as, for instance, in the deer.


Fig. 10. Arrangement or inferior constrictor muscle in various animals.

The larynx is on the left and the pharynx and oesophagus on the right. In each case the dissection is viewed from the left. The epiglottis, thyroid, and cricoid cartilages, and the process of Santorini are indicated in continuous lines. The posterior palatine folds, or arcus-palato—pharyngeus, in the Wombat (Pkascofomys mitc/Eelli), Tapir (Tapirus indicus), and Pig (Sm domesticus), are shown by a heavy interrupted line. The inferior constrictor muscle and longitudinal and circular fibres of the oesophagus are indicated by fine interrupted lines. The naso-pharynx has been divided and its cut edge is shown by double circular lines.


In the second place the angle at which the human head is carried has undergone great modifications, and is very different from that of pronograde mammals or even of orthograde apes. Assumption of the erect posture has led to a downward inclination of the head in order that the eyes may look straight forward; the alteration has taken place at the cranio-spinal and pituitary angles. The bending down of the head causes depression of the lower jaw and consequent lowering of the position of the larynx.


Thirdly, the growth of the face takes a forward and downward direction, as described by Keith and Campion.3 As age advances, this change causes further depression of the lower jaw and is accompanied by increased descent of the larynx.


All these factors combine in causing displacement of the larynx from its foetal position opposite the base of the skull, to one opposite the fourth, fifth, and sixth cervical vertebrae. The median raphe of the inferior constrictor reaches a point in relation to the skull and vertebral column which is almost as high as that in the lower mammals and in apes. There is therefore great obliquity of the inferior constrictor, especially of its upper fibres. As the oesophagus commences opposite the lower border of the cricoid cartilage, at a point rather lower than that found in most mammals, the inferior constrictor covers a very wide area, and consequently has become fanned out. Its lowest fibres take the function of the sphincteric band known as the crico-pharyngeus muscle; they are almost horizontal or at right angles to the axis of the pharynx. Hence, from a narrow origin on the thyroid and cricoid cartilages—-one not more extensive than that of most mammals —-the muscle fans out considerably.


It is this fanning out—foremost among many factors—-that leads to the production of the weak point through which pharyngeal diverticula make their way.


Summary of the Conditions in Different Species.--(oz) In gz'Z[-z’2_reczz‘/zz'¢zg fish the pharynx is a region differing little in arrangement from the buccal cavity on the one hand and the oesophagus on the other. Circular and longitudinal muscle fibres are regularly arranged and are not disturbed by the presence of a larynx. A sphincteric band is not required to prevent air from passing down the food tract in the majority of fish, but a constrictor has to be provided in order to keep out water used in respiration, and to ensure that it shall travel out through the gills. This constrictor, however, is not a powerful one, and it has no relation to a structure such as the larynx; on deglutition it relaxes. Therefore in fish there is no tendency to the production of pharyngeal diverticula. .

(b) Lzmg fis/z, amp/zz'&z'a7zs, cma.’ repz‘z'Zes, which breathe by blowing air into the lungs, have a small larynx in a wide pharynx. The proportion of size is such that, in most species, the presence of a larynx scarcely alters the mechanism of deglutition from that present in gill-breathing fish. When air is being blown down the trachea, either by the movements of swallowing or by the bellows-like working of the floor of the mouth, it is necessary for the oesophagus to be closed in order to prevent the food passage from being distended by air.

The constricting band of muscle does not appear to be strong; it is situated below the level of the larynx, and, therefore, does not come in direct relation with the latter. The needs of respiration are slight as compared with those of mammals, and, therefore, there is no necessity for rapid descent of the bolus of food.

The food passage gradually diminishes in calibre and has no sharp point of constriction ; its fibres are regularly arranged, and as the contracting band can be relaxed for a prolonged period, there is no tendency to herniation of any part of the walls.

(c) Repzz'Zes and bzkrds, which breathe by suction——induced by movements of the thoracic walls, aided in some cases by a feeble diaphragm—are in exactly the same position as pressurebreathing reptiles. In the case of flying birds, however, it is probable that the oesophagus has to be kept shut for purposes of fixation of the thorax during use of the Wings, in order to prevent the passage of air down it, when diminished intrathoracic pressure is produced.

Birds breathe to a considerable extent by means of an internal system of air sacs, and it is probable that respiration through the larynx can be delayed for a considerable length of time. Deglutition, therefore, can proceed through a relaxed and gradually tapering pharynx when the bird is not flying and is only using its internal respiratory mechanism.

(d) Cczrizivorozts Jllammczls.-—Most animals of this type have short posterior palatine folds, and have therefore no efficient lateral food channel to direct food into the mouth of the oesophagus.

The larynx lies high in the neck, however, and in close relation to poorly developed palatine folds; because of its position there is no fanning out of the inferior constrictor. The oesophagus, too, is suspended almost entirely from the cartilages of Santorini, and on deglutition opens in a funnel-like manner.

Because of these factors, there is no tendency to the production of pouches.

(e) Iferézrzorozas MammaZs.—In most animals of this type there is a well-formed arcus-palato-pharyngeus. This fact, in conjunction with the high position of the larynx in the neck and the absence of fanning out of the inferior constrictor, makes the possibility of the production of a pouch very remote.

The lack of necessity for fixation of the thorax is another safeguard against unduly prolonged contraction of the cricopharyngeus muscle.

(f) H23“/zer Apes. —- If the chimpanzee be taken as an example, it will be seen that the larynx has not descended in the neck as it has in man. The inferior constrictor is not fanned out, and consequently a pouch is very unlikely to occur.

The oesophagus also has more extensive insertion into the cartilages of Santorini and less into the back of the cricoid cartilage than in man ;' consequently it opens widely during deglutition.

(g) _/lfcm.---The factors which combine to the production of a diverticulum are several, and may be recapitulated.

(I) Descent of the Larynx 2'72 #23 Neal? and famzing out of #23 Inferior Ccmsz‘rz'cz‘or (fig. 11).

When the inferior and middle constrictors contract they raise the larynx, in association with the action of the thyro-hyoid and other muscles. The larynx is free to move upwards, but the oesophagus is not. The longitudinal fibres of the oesophagus pull the top of the gullet down, and the constrictors, palatopharyngeus, and stylo—pharyngeus pull the lower part of the pharynx up. The absence of longitudinal fibres at the lower end of the pharynx and upper end of the oesophagus leads to stretching at this weak point, with partial separation of the encircling fibres, whereby an area is produced unprotected by muscle.

(2) Lack of Re/axatz'o¢z Q)‘ the Crz'co-pkczryngeus M’z¢.rc[e.«—--This muscle relaxes at the moment when the bolus is passing, but is in a state of tonic contraction at all other times.

If the bolus is big and misdirected, and fails to pass into the oesophagus at the first attempt, when the crico-pharyngeus is relaxed, the latter again contracts to prevent the inward passage of air.


Fig. 11. Disposition of Muscles of Pharynx And Oesophagus In Man. '


The pharynx, larynx and upper end of the oesophagus have been dissected and are viewed from the left. Attached to the hyoid bone above are the mylohyoid, digastric, genio-hyoid, geniehyo—glossus and hyo-glossus muscles and the middle constrictor. Attached to the hyoid below are the sterno—hyoid, omo-hyoid, and thyro-hyoid muscles. Attached to the larynx are the thyrohyoid, sterno-thyroid and inferior constrictor muscles. The diagram illustrates the lines of pull of the various muscles and the reason why separation of the fibres of the inferior constrictor occurs in its lower region.


If a contracting band above the bolus is still causing pressure, the bolus will press against the walls of the pharynx in all directions. It cannot move upwards, because of contraction of the oblique fibres of the inferior constrictor, nor downwards because of contraction of the crico-pharyngeus. Its forward passage is prevented by the obstruction offered by the larynx and the longitudinal fibres of the oesophagus, which go up under cover of the inferior constrictor to the aponeurosis attached to the back of the cricoid and the tips of the cartilages of Santorini.

Posteriorly, however, there is an unprotected area above the crico-pharyngeus muscle and below the oblique fibres of the inferior constrictor. Therefore the force exerted on the bolus is concentrated on this spot in such a way that herniation occurs. Once a protrusion of mucous membrane has commenced, passage of subsequent masses of food into it cause gradual enlargement.

(3) Aésevzce of Dz°rec!z'7zg' Posterzbr Palatme Folds.-——Because man relies much on sight and but little on his olfactory sense, he has no need of uninterrupted nasal respiration, as is necessary

in animals chewing herbage, which they discriminate by scent and not by vision. The larynx, therefore, does not occupy an intranarial position, with the advantage which the soft palate affords of directing the food into the oesophagus.

In the course of evolution the larynx has become separated from the palatine folds, and the latter are short and feeble and give no assistance in directing food into the oesophagus.

Owing to man’s type of diet, no arcus-palato—pharyngeus is required as is present in herbage-chewing animals.

(4) Attachment of the Esophagus to tire [,o:zrym:.—-Because in man the oesophagus is attached to the back of the cricoid, as well as to the cartilages of Santorini, its funnel-like opening is less than in the great majority of mammals. This, combined with the sudden narrowing of the food tract at the junction of the pharynx and oesophagus, is an important factor.

General Summary

If man, who is normally adapted to a diet of soft and finely divided material, commences when edentulous to swallow solid lumps of unmasticated food, he will render himself liable to herniation of the pharynx, if he continues the habit for a prolonged time.


The main factors at Work are misdirection of the bolus, and its impingement on a part of the pharynx weakened by opposing forces which stretch it and separate its muscular fibres; associated with these is the absence of a funnel-like opening of the oesophagus and lack of relaxation of the crico—pharyngeus muscle. ”


ZVote.——The illustrations are mainly from specimens in the author’s possession, obtained through the kindness of Dr Sonntag of the Zoological Society of London, to whom renewed thanks are given. For permission to reproduce two specimens illustrating the formation of diverticula, thanks are tendered to Professor Parsons of St Thomas’s Hospital, and to Mr L. Colledge of St George’s Hospital.

Illustrations of the larynx of a kangaroo and dolphin, and one of a large pharyngeal diverticulum are included by permission of Sir Arthur Keith, to whom the author is extremely grateful, not only for the above but also for frequent help and advice in the practical study of the anatomical specimens and the preparation of this paper. The drawings have been made by Mr S. A. Sewell and the photographs by Mr Henry George.


As a means to identification of species, the following list is appended :—

Aard WoIf—Proteles Cristatus. Agouti—Dasyprocta prymnolopha. Badger—Me1es meles.

Indian Fruit-Bat—Pteropus medius. Bear—-Ursus.

Bison-——Bison Americanus. Capybara——Hydroch-Jerus hydrochoerus. Capuchin——Cebus.

Brown Capuchin—Cebus fatuellus. Domestic Cat—Felis domesticus. Spotted Cavy-—C0elogenys paca. Cercopitheque—Cercopithecus. Chimpanzee — Anthropopithecus troglo dytes.

Common Crane——-Grus Communis. Indian Civet—-Paradoxurus niger. Cheetah——Cynoe1urus jubatus.

Water Cheviotain———Hyomoschus aquaticus.

Tasmanian Devi1——Sarcophilus Harrisi. Dog—Canis familiaris. Dolphin—Delphinus.

Common Dolphin—Delphinus delphis. Echidna——Echidna hystrix.

Lung fish—Lepidosiren. Girafi'e—Giraffa camelopardalis. Gnu—Connochoetes gnu. White-Thighed Guereza——Colobus vi1Ier osus.

Horse—Equus caballus. Hyaena—-—-Hyaena crocuta.

Hare—-Lepus timidus.

Hyrax—Hyrax capensis.

Long~ Eared Hedgehog — Erinaceus auritus.

Kangaroo—Macropus giganteus. Lemur—Lemur flavifrons.

Lion—Felis Leo.

Sea Lion—Otaria Californiana. Clouded Leopard——Felis nebulosa.

Black Leopard—Felis pardus.

Llama—Lama glama.

Slow Loris—Nycticebus tardigradus. Prairie Marmot-—Cynomys ludovicianus. Marmoset——Hapale. Mangabey—Cercocebus. Macaque——Macacus.

Howler Monkey——Mycetes.

Spider Monkey-—Ateles.

Woolly Monkey—Lagothrix Humboldti. Lace Monitor—-Varanus varius. Muntj ac—Cervulus muntj ac. Ox—-—Bos taurus.

Musk Ox-—Ovibus rnoschatus. 0kapi—0kipia johnstoni. Otter—Lutra lutra. Guinea—Pig—Cavia porcellus. Domestic Pig——Sus domesticus. Penguin-——Spheniscus demersus. Porpoise——Phocaena.

Brown Rat—Rattus norvegicus. Reindeer——Rangifer tarandus. Rhinoceros—Rhinoceros unicornis. Squirrel—Sciurus hudsonianus. Tapir—Tapirus indianus. Tiger—Felis tigris. Takin—-—Budor(ms taxicolor. Viscacha—Viscacia viscacia. Wal1aby—-—Macropus dorsalis. Walrus—Odobaenus rosmarus. White Whale——Delphinapterus leucas. Common Wolf-—Canis lupus. Wombat—Phasco1omys mitchelli.


References

1 Negus, V. E., Journ. of Laryngol. and 02221., Jan.

2 Goppert, E., “Der Kehlkopf der 1924 ; Lancet, 17th May 1924. Amphibien und Reptilien,” M07?/L. falzréucla, 1900, vol. xxviii.

3 Keith, Sir A., and Campion, G. G., “ Mechanism of Growth of the Human Face,”


Cite this page: Hill, M.A. (2019, October 23) Embryology Paper - Evolutionary factors in the production of pharyngeal diverticula. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Evolutionary_factors_in_the_production_of_pharyngeal_diverticula

What Links Here?
© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G