tracheal extension in - bmj

Thorax (1959), 14, 201.

TRACHEAL EXTENSION IN RESPIRATIONBY

R. S. HARRISFrom the Department of Anatomy, University of Bristol*

(RECEIVED FOR PUBLICATION JANUARY 19. 1959)

It has long been known that the lower end andbifurcation of the trachea are displaced down-wards during inspiration. Macklin (1925)described a vertical respiratory displacement of21 mm. in a healthy young adult female.Bruckner (1952), in a radiological study,demonstrated an inspiratory descent of the carinaof about one thoracic vertebral body in youngadults of 19 to 25 years of age. Holden andArdran (1957), in a meticulous examination ofbronchograms from subjects aged 12 to 65 years,concluded that "in deep inspiration the tracheaincreased in length up to one fifth and in diameterby about one tenth." In a personal observation,on inspiration the carina descended 30 mm. in ayoung adult male with a full inspiratory move-ment. Thus, in young adults, the carina descendsby about 1 in. (2.5 cm.) with deep inspiration, i.e.,about 20% of the unextended length of thetrachea.There are no references in the literature to

similar observations in infants and young childrenin whom the technical difficulty of securing a fullinspiratory and expiratory movement for radio-graphic investigation is obvious. Also, mostobservers have examined only the movement ofthe carina and have not considered changes in thewhole length of the trachea.

Keith (1909) drew attention to the fact thatcertain parts of the lung are expanded morereadily than others. The bases and sterno-costalsurfaces related to the more mobile parts of thethoracic wall expand more readily than the apices,mediastinal surfaces, and posterior margins,related to less mobile parts. These areas hedescribed as areas of direct or of indirectexpansion.The thoracic wall does not move uniformly

during inspiration; the parts which move mostare the diaphragmatic and sterno-costal surfaces.Thus the lung tends to expand in a downward andforward direction. Now the lung root mustobviously descend to facilitate this movement. In

*Present address: Department of Child Health, Bristol RoyalHospital for Sick Children.

addition it moves anteriorly: for example, wehave found that in a healthy young adult malethere was an anterior respiratory displacement ofthe tracheal bifurcation of 0.75 in. (19 mm.).What is the significance of the respiratory descentof the tracheal bifurcation ? Does it facilitate theexpansion of the more apical parts of the lungallowing them to expand indirectly, i.e., aftermore mobile parts of the lung have movedinferiorly and anteriorly ?Now respiratory movements in infancy are

different from those in older children and adults.A conspicuous anatomical difference existsbetween the ribs of these groups. The ribs of aninfant are much more nearly horizontal thanthose of an adult (Mehnert, 1901), and it is statedby Engel (1947) that the adult obliquity is notassumed until about the seventh year. Observa-tion of the abdomen of an infant shows theimportance of diaphragmatic breathing in infancyand in this respect it was thought that the verticaldisplacement of the bifurcation might berelatively greater in infants. As a preliminaryinvestigation it was decided to measure theextensibility of the trachea in infancy and to makecomparisons with the adult.

METHODSpecimens were taken at necropsy. The larynx,

trachea, and the roots of the primary bronchi weredissected free from the surrounding tissues as they layin situ and then removed en bloc, taking care not tostretch the specimen. The arch of the cricoid wascarefully cleaned and defined. A fine pin was thenpushed through the point of bifurcation so that thecarina was transfixed. A measurement was then madebetween the lower border of the cricoid arch and thepin. The trachea was extended by gradually increasedweights applied to the carina.

It was found that increased loading of the tracheaproduced a gradually decreasing amount of extensionuntil a stage was reached at which further loadingproduced much less extension, and this was taken asthe point of full extension. It was found to occurwhen a weight of approximately 20 g, was appliedto an infant trachea, or about 80 g. to that of an

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R. S. HARRIS

adult in middle life, and about 140 g. to that of asenile adult. The increase in length was expressed asa percentage of the initial measurement.

Specimens were examined from 31 adults and 20newborn infants, together with specimens from twochildren and two adolescents. The ages ranged frombirth to 88 years. Of the infants, some were stillbornand others had survived for periods from 15 minutesto three days. Eight infants were premature.Specimens were selected from cases without overtpulmonary disease.

RESULTSThe findings in mature infants and adults are

shown in Fig. 1. In the full-term infant thetrachea may be extended, on the average, by46% of its original length (range 32.5-64.5) andthis may be compared with the extensibility of 15to 20% in the aged. There is a significant inverserelationship between age and extensibility(coefficient of relation, P=<0.001). It was feltthat post-mortem changes might influence theextensibility. The coefficient of relation betweenextensibility and time elapsing between death andexamination was calculated (P=>0.10). Itwould therefore appear that there is no relation-ship between the amount of extensibility and thetime elapsing between death and examination ofthe material, under the conditions of theseexperiments (the maximum time elapsing was63 hours and the minimum three hours, with amean of 38 hours).

Thus, the trachea may be extended further ininfancy than in later adult life. A regression line(Fig. 1) was calculated for the data from thespecimens from subjects of the age group 16 to85 years. There appears to be a linear relation-ship between age and extensibility of the trachea

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FIG. 1.-The maximum extensibility, expressed as percentage

elongation under maximal tension, of the trachea in relation to

age.

from about 15 years of age onwards. Over eachdecade the extensibility decreases by 3%. Moredata would be desirable between the period ofbirth to 15 years before the relationship at thisperiod is established, but preliminary observationssuggest little change in extensibility over thisperiod.

DISCUSSIONWhat is the significance of the great extensibility

of the trachea at birth and in infancy and thegradual decline in the extensibility of the tracheafrom the age of 15 years onwards ? This mightbe due to gradually increasing senile changes inthe elastic tissue in the trachea (Figs. 2, 3, 4)occurring pari passu with changes in elastic tissuethroughout the body as a whole. Lansing,Roberts, Ramasarma, Rosenthal, and Alex (1951)described changes in the chemistry of arterialelastic tissue with age. There was an increasein the dicarboxylic amino-acid content withincreasing age and this was associated with anincreased affinity for calcium by aortic elastin, theincrease beginning between the ages of 15 and 25years. It may be, therefore, that the greatertracheal extensibility is merely an expression ofthe juvenile state of elastic tissue in infancy inthe body as a whole and that it does not haveany specific functional significance. However,Lansing and others (1951) also examinedpulmonary arterial elastin from subjects in theage range 15-92 years and no significant changeswere found in the amino-acid composition.Generalizations upon the properties of elastin invarious tissues at various ages must therefore beguarded.

If, however, there is a special functionalsignificance of the greater extensibility of thetrachea in infancy, in what mechanisms may itbe concerned ? There are three in which it mayplay a part. First, it may be related to themechanism of lung expansion in the infant.Secondly, it may be related to the mechanicalexpression of mucus from the abundant mucousglands in the submucosa of the infant trachea.Thirdly, it may be associated with a free range ofmovement of the head and neck (Testut andLatarjet, 1949). However, as the neck of aninfant is comparatively short this does not explainthe greater extensibility during infancy. Also,according to Mehnert (1901) the bifurcation ofthe trachea lies at a higher level in infants and isat the level of the third thoracic vertebral body.

RELATIONSHIP TO LUNG EXPANSION IN THEINFANT.-The more horizontal position of theribs in infants implies a lesser role of the thoracic

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TRACHEAL EXTENSION IN RESPIRATION

movement in respiration, compared with thatof the diaphragm. Dally (1908) described the 8 8 q ;relatively greater proportion of the diaphrag- it.matic surface of the lung to that of the remaining ' 8 iit..pleural surfaces of the lung in the infant comparedwith the adult. Thus, if the total pleural surfaceof the lung is represented as 100 units, then in theinfant the diaphragmatic surface constitutes about25 % of the total. By the age of 35 years this has -'fallen to about 15%. If diaphragmatic movementis more important in infants than in adults, then 4

lung expansion is mainly in a supero-inferiordirection and the vertical movement of the carinawould be greater. Increased extensibility of the --cZ-trachea would obviously facilitate this movement. - ';' ;- hBut inspiratory descent of the carina could result '_from either of two mechanisms or a combination * ; -of these, viz., actual elongation of the trachea andinferior displacement of the trachea as a whole, >which in its turn requires inferior displacement .2 - -of the larynx.

Respiratory displacement of the larynx has been FIG. 3studied by a number of workers. Weingaertner(1920) investigated changes in the level of the > 6.larynx and of the carina, also changes in thelength of the trachea with respiratory movements . -in 20 subjects, aged 16 to 56 years. He foundthat in ordinary respiration there was aninspiratory depression of the larynx by 1 to 10mm. in eight cases, whereas in three there wasan inspiratory elevation by 1 to 5 mm. In 11cases he measured respiratory changes in the

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FIG. 2.-Transverse section of trachea, infant aged 5 months, showinginternal elastic lamina. Orcein, haematoxylin, acid fuchsin. x 60.

* , - . .-., * FIG. 3.-Longitudinal section of trachea, adult aged 46 years, showinginternal elastic lamina. Orcein, haematoxylin, acid fuchsin. x 60.

FicI. 4.-Longitudinal section of trachea, adult aged 85 years, showinginternal elastic lamina. Orcein, haematoxylin, acid fuchsin. x 60.

FIG. 2

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length of the larynx: in five cases there wasinspiratory lengthening by 2 to 7 mm., in threecases no change, and in three an expiratoryelongation by 1 to 3 mm. Measurements weremade by endoscopic techniques in anaesthetizedsubjects with the head extended. The inhibitoryeffect of head extension upon movement at thecarina has been noted elsewhere (Harris, 1959).Mitchinson and Yoffey (1947) examined 23normal subjects aged 15 to 45 years, and radio-graphs were taken at the extremes of inspirationand expiration. In five cases there was slightinspiratory elevation, in four cases slightinspiratory descent (the amount of movement,whether elevation or depression, did not exceed- in. (6 mm.) in any of the cases), and in theremaining 14 cases no respiratory displacementoccurred. Bruckner (1952), using the position ofthe vocal folds as an index of the laryngealposition, noted a definite elevation with expira-

tion, the change in level being, on the average, theheight of one cervical vertebral body and in somecases even of two vertebrae. Holden and Ardran(1957) noted that although there was little move-ment of the larynx with quiet breathing it moveddownwards with inspiration. These authors alsonoted that on deep inspiration the tracheaincreased in length up to one-fifth.

Thus, two factors facilitate descent of thecarina. First, actual extension of the trachea,accounting for most of the movement, and,secondly, in the terminal part of inspiration,inferior displacement of the trachea and larynxas a whole. Displacement of the trachea as a

whole accounts for about 16% of the totalmovement of the carina in young adult males(Harris, 1959).

It is interesting to note that in old age thelarynx has descended to a lower level (Mehnert,1901; Negus, 1930). The descent of the larynxwith advancing age possibly compensates for thediminished extensibility of the trachea and wouldfacilitate the inferior respiratory displacement ofthe carina.

ROLE OF TRACHEAL EXTENSION IN PROMOTINGLUNG EXPANSION.-Keith (1909) noted that theincrease in thoracic capacity due to an inspira-tory movement occurs mainly inferiorly due tomovement of the diaphragm. Numerous investi-gations have shown that the bronchi elongate withinspiration. Holden and Ardran (1957) noted thatthe bronchi elongate by 25% on deep inspiration.Since the apices cannot expand superiorly due tothe resistance of the suprapleural membrane andscalene muscles and but little antero-posteriorly,

any elongation of the bronchi in the upper lobesmust be mainly in an inferior direction, i.e., bya downward movement of their points of origin(Macklin, 1929) which move with the lung root.Macklin (1925) examined bronchograms forevidence of greater elongation of the upper thanthe lower lobe bronchi, but, due to technicallimitations, the bronchial outlines at expirationcould not be compared with those at inspiration.Tracheal extension would thus facilitate thevertical elongation of the bronchi above the levelof the lung root.

EFFECT OF HEAD POSTURE UPON AIR FLOWIN RESPIRATION

To investigate the importance of the inferiorinspiratory displacement of the carina inpulmonary ventilation, the forced inspiratoryvolume in normal subjects was compared withthe head in the flexed and then in the fullyextended position (the terminology used in thissection i3 that recommended by Gandevia andHugh-Jones, 1957). With full head extension thelarynx might be held more fixed by the variousattachments of the thyroid cartilage to the hyoidbone, which, in its turn, is attached to the base ofthe skull (Mitchinson and Yoffey, 1947). Also,with head extension, the trachea is extended fromits upper end (Harris, 1959) and consequentlyinspiratory descent of the carina is impaired (v.i.).

Recordings were made using a vital-capacityspirometer with a very light-weight spirometerbell. The bell was connected to a wide-boremetal mouthpiece (4.5 cm. internal diameter) bya rubber connexion of the same internal diameterto minimize resistance to air flow. Its movementswere recorded on an electrically driven kymo-graph moving at 1.7 cm./sec. The drum speedwas checked by an electric time-marker.Care was taken that the position of the subject

was constant during each examination and thatthe only variation in this respect was that of thesubject's head. Peltier and Visscher (1952)emphasized the effect upon tidal volume ofresistance to air flow in the upper respiratorypassages by comparing nasal and mouth breath-ing. Tidal volumes were greater when breathingwith the mouth widely open. Accordingly, toeliminate the variability of nasal breathing andto ensure minimal buccal resistance to air flow,the nose was clipped and a large-aperture mouth-piece used. The head was supported in theextended position.With the subject sitting upright, two successive

pairs of inspiratory movements were recorded

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(with a rest period between each pair) with thehead in flexion and then two pairs with the headin full extension. Thus the mean values for theforced inspiratory volume and the forcedventilatory capacity (inspiratory) both in flexionand extension are based upon the mean of fourmeasurements made upon each of 15 individuals.The values obtained in 15 healthy young adults

(2 female and 13 male, average age 32 years) forthe forced inspiratory volumes (F.I.V.) duringeach 0.25 second of the first second of inspiratorymovements with the head in flexion and then infull extension were as follows:

During the first 0.25 sec. (F.I.V.0.25) the meanflow in flexion was 1,510 ml. (S.E. 250), whereasin extension it was 1,160 ml. (S.E. 180). Thusthe F.I.V.0.25 in flexion exceeded that inextension by 350 ml. During the second 0.25sec. (F.I.V.0.25..0.5), the mean flow in flexionwas 1,450 ml. (S.E. 170) and in extension 1,320ml. (S.E. 120), the rate in flexion thus exceedingthat in extension by 130 ml. Therefore over thefirst 0.5 sec. of inspiration the air flow with thehead in the flexed position exceeded by almost500 ml. that in head and neck extension, thedifference being highly significant statistically(P=<0.001). However, in the second 0.5 sec.of the inspiratory movement there was nosignificant difference between the two sets ofdata (P= >0.05). Thus the F.I.V.0.,50.75 inflexion was 770 ml. (S.E. 110) whereas in exten-sion it was 840 ml. (S.E. 90). The F.I.V.0.75_1.rin flexion was 370 ml. (S.E. 80) and in extensionwas 430 ml. (S.E. 90). The forced vital capacity(inspiratory) in the position of flexion was 4,340ml. and in that of extension was 4,010 ml.

In 13 of the 15 subjects there was a consistentlygreater flow of air throughout the whole of thefirst second of the inspiratory movement in headflexion compared with head extension. Thus theamount by which the F.I.V. in flexion exceededthat in extension, during the first second ofinspiratory movements, was as follows: F-I.VN0.25350 ml. (S.E. 130); F.I.V.0.s, 480 ml. (S.E. 120);F.I.V.N.7, 370 ml. (S.E. 70); F.INV1 09 300 ml.(S.E. 70).

If the forced inspiratory volumes during eachquarter of the first second of inspiratory move-ments are expressed as a percentage of themaximum forced vital capacity achieved, then theflow rates in the second half of the inspiratorymovement are practically the same when thehead is in either the flexed or the extendedposition (30.4% and 31.9% respectively of theforced vital capacity). But during the first half

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FIG. 5.-Percentage of forced vital capacity achieved during firstsecond of inspiratory movements in head flexion and extension.(Mean values from 15 adults.)

of the movement the value in flexion exceeds thatin extension by about 10%, i.e., 67.4% comparedwith 56.9%. By the end of the first second ofinspiration, 97.8% of the air flow has taken placewhen the head is flexed, but only 88.8 % whenthe head is extended. Thus inspiration isprolonged in head extension (Fig. 5).

EFFECT OF HEAD EXTENSION UPON FORCEDINSPIRATORY VoLuMEs.-Head extension producesextension of the trachea and infrahyoid respiratorypassage (Harris, 1959) which may interfere withlung expansion in three ways:

(1) Mechanical Limitation of Inferior Displace-ment of the Carina.-Head extension stretches thetrachea by an amount equal to its inferiorinspiratory displacement, about 2.5 cm. (Harris,1959), and this would leave less extensibilityavailable for inferior respiratory displacement ofthe carina. If the lung root does not descend thenexpansion of the apical regions of the lung mightbe limited.

(2) "Sensitization" of Stretch Reflexes.-Sensorynerve endings, termed " smooth muscle nerve

spindles," have been described in the smoothmuscle of the human trachea (Jabonero, 1952).Similar nerve spindles, sensitive to mechanicalstimuli, have been described in the human mainbronchi (Larsell and Dow, 1933; Gaylor, 1934).Larsell and Dow also noted sensory fibres in theperichondrium on the inner surfaces of cartilagesin the main bronchi and considered these were

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FIG. 6.-Transverse section of trachea,full-term infant, showing ductsleading from acini to surface ofmucous membrane. Haematoxylin,Masson red, and light green. x 60.

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stimulated mechanically. Further experimentspresented in this paper (v.i.) show that trachealextension produces deformation of the cartilagesand perichondria. This supports the concept ofmechanical stimulation of the perichondrial nerve

endings with tracheal extension. Tracheal extensionand consequent stimulation of the mechano-receptors could produce reflex respiratory slowingand bronchoconstriction (Widdicombe, 1954). Ina trachea already stretched by head extension,further extension by inspiratory descent of thelung root will accentuate stimulation of thesereceptors so that the inhibitory respiratory effectof the reflexes which they mediate would occur

earlier in the course of the inspiratory movements.

(3) Narrowing of the Tracheal Lumen.-Headextension causes a significant narrowing of thetracheal lumen (Harris, 1959) and consequently an

increase in resistance to air flow.Extension of the head and neck might interfere

with the function of the muscles acting upon thethoracic inlet (scaleni and sternomastoids) andthis could occur in two ways: first, by a changein the resting length of the muscle fibres and,secondly, by a chang2 in their angle of insertionTo investigate the first of these two factors thedistance between the transverse processes of thefirst and fourth cervical vertebrae and the medialend of the clavicle (taken as an index of the medialend of the first rib) was measured on radiographsused in a study of head and neck extension upon

the trachea (Harris, 1959). On the average, theresting length of the upper fibres of the scaleniwas increased by 4.3% with head extension andthat of the lower fibres by 7.2%. Hill (1925)investigated the tension developed in muscle

whose resting length was varied. With a 5%increase in resting length, the tension developedwas about 90% of the maximum attainable. Thushead extension, by slightly stretching the scalenemuscles, may slightly diminish their potentialactivity. The effect of this upon a forcedinspiratory movement is difficult to predict.

CLINICAL SIGNIFICANCE OF TRACHEAL EXTENSION

The retrosternal pain associated with acutetracheitis and bronchitis is accentuated by deepinspiration. Under these circumstances reflexrestriction of the descent of the lung root bypainful stimuli might be produced by extension ofthe inflamed trachea and bronchi. If this wereassociated with concomitant diseases, e.g.,nutritional anaemia and congestive cardiac failure,then the restriction of pulmonary ventilationmight become significant.

Disease of the tracheo-bronchial lymph nodescould restrict lung root movement in two ways.First, inflammatory disease of these nodes maycause inflammation of the adjacent mediastinalpleura which then, if pulled upon by forcestransmitted from the inferior displacement of thecarina, causes pain and consequent reflex inhibi-t.on of inspiration. Secondly, the lung root maybecome fixed mechanically. The evidence upon thisquestion is equivocal. Wilson, Edwards, and Liss(1924) studied children with tracheo-bronchialadenopathy and concluded that a significantreduction in vital capacity occurred with latent orhealed infection and was even greater in activeinfection. The decrease in vital capacity wasexplained by loss of lung elasticity, but patho-logical investigations were not made and they did

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FIG. 7.-Transverse section of trachea,infant aged 5 months, showing abun-dant mucous glands. Orcein, hae-matoxylin, acid fuchsin. x 60.

not consider such factors as pulmonary vascularengorgement associated with active infection ormechanical obstruction of pulmonary vessels byenlarged nodes. Friedman and Hawes (1929)investigated children with clinical and radiographicsigns of enlarged tracheo-bronchial nodes and didnot find any significant decrease in vital capacity.

MECHANISM OF SECRETION OF TRACHEAL MUCUS

Two groups of cells in the human tracheaproduce mucus: goblet cells in the surfaceepithelium, and tubulo-acinar glands in thesubmucosa, lying external to the internal elasticlamina. The secretion reaches the surface of themucous membrane through the tubular ductsleading through gaps in the elastic lamina (Fig. 6).In addition, there are numerous serous glands,situated with the mucous glands, whose secretionobviously varies the viscosity of the ultimatesecretion. Glands are particularly abundant in thesubmucosa of the newborn infant (Fig. 7).

Florey, Carleton, and Wells (1932), consideringthe mechanism of tracheal mucus secretion, notedthat vagal stimulation excites copious secretionfrom the glands in the submucosa, but that in theheavily atropinized cat the deep glands still secretemucus and that some other, possibly reflex,mechanism may be concerned. Secretion ofmucus from the stomach can be provoked bymechanical stimuli alone. Thus, some factor orfactors, other than direct neural control, may alsobe concerned in tracheal mucus secretion. Thetracheal muscle and elastic tissue might beinvolved.

What part may the tracheal muscle play in thesecretion of mucus ? Borelli (1949) found thatcertain of the transverse muscle fibres, lyingdeeper than those attached to the cartilages, wereattached to the submucosa and internal membraneof the trachea, and on contraction could trans-versely shorten the pars membranacea, raisingthe mucous membrane into longitudinal folds.This, he considered, might have a squeezing actionupon the glandular elements. But the glandularelements lie superficial to the muscle (Stirling,1883; Miller, 1913) and there would seem littleevidence, structurally, that the tracheal muscledirectly promotes mucus secretion by mechanicalcompression of the glandular acini.ROLE OF TRACHEAL ELASTIC TISSUE.-Grossly

the elastic tissue is arranged in two main strata:a thicker stratum (the internal elastic lamina)lying beneath the mucous membrane and a lessprominent stratum (the external elastic lamina)lying superficial to the tracheal cartilages. Thetwo strata are connected together between thecartilaginous rings. Examination of longitudinalsections reveals a typical pattern in the fibrespassing between the cartilaginous rings and link-ing the inner and outer elastic laminae (Fig. 8).The lobules of the glands are surrounded byelastic and collagenous fibres and fibrous septawhich subdivide them into acini (Fig. 9). Wolf-Heidegger (1947) described a lattice arrangementof the fibres in the elastic laminae, the constituentfibres being oblique and crossing each other toform a meshwork. Those angles of the meshworkdirected superiorly and inferiorly vary with thelongitudinal stretching of the trachea, becoming

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FIG. S.-Longitudinal section of trachea, full-term infant,showing fibres passing between the cartilaginous rings and

connecting the inner and outer elastic laminae. Orcein,haematoxylin, and Van Gieson. 23.

more acute with tracheal extension. One mesh-work lies in a frontal plane and the other in a

sagittal plane. Longitudinal sections show thecrossing of the oblique fibres in the sagittal plane.This histological pattern may be of functionalsignificance, extension of the trachea tending tocause compression of the mucous glands lyingbetween the fasciculi of the stretched lattice.Wolf-Heidegger (1947) considered that the chieffunction of the lattice was to facilitate trachealextension, the arrangement of the connectivetissue in relation to the glandular acini being partof the general lattice pattern. However, he alsonoted that it might be concerned with themechanism of mucus secretion.To investigate this hypothesis, longitudinal

sections of the fibro-cartilaginous ventral trachealwall from newborn infants were examinedhistologically. A small longitudinal strip was

taken and divided longitudinally into two parts.

One strip was fixed (formol saline) in the extendedstate and the other in the unextended state.Thus, the form of the acini in the sub-mucosa was compared in the extended andunextended specimens. Tracheal extension

produced definite glandular compression (Figs.10-13), especially of those glands subjacent to theinner surfaces of the cartilages (Figs. 12 and 13).Tracheal extension also caused deformation of thecartilages and surrounding perichondria (Figs. 12and 13), possibly an important factor in thestimulation of sensory endings in the perichon-drium (v.s.).Assuming a functional relationship between

the subm"cosal glands and the fibrous lattice inwhich they 1i', then with deeper or more frequentrespiration (aiJd, a priori, tracheal extension) theremight be a greater secretion of mucus bymechanical expression of material from theducts and acini. This point is being investigatedfurther.

SUMMARY AND CONCLUSIONSThe importance of tracheal extension in

respiration was investigated by extending thetrachea from its upper end by head and neckextension. This reduces the extensibility avail-able for the inferior inspiratory displacement ofthe carina.

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FIG. 9.-Longitudinal section of trachea, infant aged 5 months,showing septa dividing lobules of glands. Orcein, haematoxylin,acid fuchsin. x 60.

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Fia. 10 Fia. 11

FIG. 10.-Longitudinal sec-tion of trachea, adultaged 55 years, fixed inunextended position.Note the general situa-tion of the acini.Mucicarmine. x 23.

FIG. 11.-Longitudinal sec-tion of trachea, adultaged 55 years, fixed inextended position, cf.Fig. 10. Mucicarmine.x23.

FIG. 12.-Longitudinal sec-tion of trachea, matureinfant, fixed in un-extended state. Orcein,haemalum. x 23.

FIG. 13.-Longitudinal sec-tion of trachea, matureinfant, fixed in exten-sion. cf. Fig. 12.Orcein, haemalum,Alcian blue. x 23.

FIG. 13FIG. 12

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Head extension in healthy young adults causeda diminution of the forced inspiratory volume by500 ml. over the first half second of forcedinspiration and also prolonged the inspiratoryphase in forced breathing.The flow rate in the first half second of forced

inspiratory movements was about 10% less whenthe head and neck were fully extended than whenthe head was flexed, but there was no significantdifference in the second half second. Thediminished flow rate in head and neck extensionmay be due to two factors: first, a significantnarrowing of the antero-posterior diameter of thesupraclavicular portion of the trachea, due to theanterior displacement of the membranous portionof the tracheal wall; and, secondly, stimulationof stretch receptors in the smooth muscle of thetrachea and of sensory endings in the peri-chondria. Free tracheal extension is importantin the mechanism of respiration, facilitating theinspiratory descent of the lung root.The extensibility of the human trachea was

examined over the age range of birth to 88years. There is a marked decrease in extensi-bility with age. At birth, the trachea can beextended, in vitro, by about 46% of its unextendedlength. At about 15 years of age the extensibilitybegins to diminish by about 3% every 10 yearsand declines to about 15 to 20% in old age. Thegreater extensibility in infants and children isprobably associated with a predominantlydiaphragmatic respiration.

Extension of the trachea causes compression ofthe acini of the mucous glands, especially thoselying between the cartilages and the mucousmembrane. This may be associated with themechanical secretion of mucus already present in

the ducts and acini. Tracheal extension alsocauses deformation of the cartilages and theirperichondria. The latter may give rise to nervereflexes arising from respiratory extension of thetrachea.

I would like to thank Professor J. M. Yoffey andalso Professor A. V. Neale for their interest andencouragement. I am most grateful to Dr. N. J.Brown, consultant pathologist to Southmead GeneralHospital, and to the pathologists, the Bristol RoyalInfirmary, for facilities at necropsies, also to Dr. H.de V. Heese for advice and allowing me the use ofthe spirometer.

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