ANATOMY AND PHYSIOLOGY OF THE NORMAL PALATE.
_The hard palate—The velum and its muscles—The mucous
membrane—The blood supply—The shape and size of the hard
palate—Functions._
The palate is a more or less horizontal partition dividing the month from
the nasal cavity, and consists of a firm bony plate in front (the hard
palate) with a freely moveable membrano-muscular velum behind (the soft
palate), which under varying conditions of muscular action can either
open or close the communication between the nose and pharynx.
The bony palate forms the vaulted roof of the mouth, the central and
posterior parts of which are nearly horizontal; and on all sides, except
at the back, it is bounded by the alveolar ridge. Into its formation
several bones enter; in the adult skull one usually sees posteriorly a
cruciform suture indicating the limits of the superior maxillæ and palate
bones; but even in the adult, evidence is forthcoming in the existence
of traces of sutures to indicate that the anterior part of the palate is
formed independently of the part immediately behind it. Thus Mr. Carless
tells me that a cursory examination by him of a few dozen adult skulls
picked up at random in the Museum of the College of Surgeons revealed
the fact that in quite one half of them traces of sutures could be seen
extending outwards from the posterior part of the anterior palatine
canal; and a similar examination by him of 40 skulls from the Museum of
King’s College of many nations and various ages showed a similar result.
In almost all there was distinct evidence of the suture in the median
line; in 21, the maxillo-intermaxillary suture was indicated; whilst in
10 skulls, representing the period from infancy to young adult life,
both the above were seen in all, and 7 showed traces in addition of a
suture placed between them on either side, and which we shall describe
hereafter as the endo-mesognathic. Kölliker[34] similarly records that
out of 325 adult skulls examined, 96 of them showed definite traces
of the maxillo-intermaxillary suture. Albrecht[35] declares that nine
tenths of the skulls in the Königsberg and Kiel Museums from children
under five years of age reveal the existence of _five_ intermaxillary
sutures, proving that there are four separate portions to the so-called
intermaxilla. Each portion carries an incisor tooth, and the canine is
developed immediately at the junction between the outer portion and the
maxilla. Occasionally there are three incisors on each side, the jaw
being then called hexaprodontous; the extra tooth is developed from the
inner segment of the intermaxilla (or endognathion), the outer segment
(or mesognathion) carrying as usual only the lateral incisor. The
accompanying illustrations well indicate this arrangement of sutures and
teeth (Figs. 26 and 27); the importance of these facts will be emphasized
later. All traces of the facial aspect of these sutures disappear quite
early in life.
[Illustration: FIGS. 26 AND 27.—Diagrams to represent the normal human
upper jaw of a child, with four and six incisors respectively, and also
indicating the five intermaxillary sutures. (After _Albrecht_.)
EG, MG, XG. Endo-, meso-, and exo-gnathion. _e._ Inter-endognathic
suture. _f._ Endo-mesognathic suture. _g._ Exo-mesognathic suture. _i₁._
Central incisor. _i₂._ Lateral incisor. _iₐ._ Accessory incisor. _c._
Canine. _m₁._ First temporary molar. _m₂._ Second temporary molar.]
The bony surface of the roof of the mouth is perforated by numerous
small foramina for the transmission of the nutrient vessels to the body
of the bone, pitted for the lodgment of mucous glands, and grooved
longitudinally for the transit of vessels. At the postero-external
corners the posterior and accessory palatine canals give entrance to the
posterior palatine vessels, and nerves; and anteriorly in the median line
is the anterior palatine canal transmitting the naso-palatine vessels and
nerves.
The soft palate is a moveable curtain, consisting of a membranous
expansion or aponeurosis attached to the posterior extremity of the hard
palate by firm fibrous tissue. Incorporated with it are five pairs of
muscles, controlling its movements; it is covered by a smooth thin mucous
membrane, and terminates posteriorly in the uvula. The arrangement of
these muscles is important, not only from their normal physiological
functions, but also from their irregular action and effects in cases
of cleft palate (Fig. 28). They may be arranged in groups: two, the
levator and tensor palati, form a superior group; the azygos uvulæ is
intermediate; and the palato-glossus and palato-pharyngeus form an
inferior set. Arising from the extremity of the petrous bone, the levator
passes downwards, and spreading out below unites with its fellow in the
whole length of the median raphe. The tensor arises from the navicular
fossa of the internal pterygoid plate, and after being reflected around
the hamular process, its action there being assisted by the interposition
of a bursa, is attached to the anterior portion of the aponeurosis and
to the hinder part of the bony palate. The combined action of these
muscles raises and makes tense the velum, and in addition influences the
Eustachian tube; but the levator is by far the more important. The azygoi
uvulæ muscles arising from the median raphe and spine of the hard palate
descend to the tip of that process, and are thus able to regulate its
length.
[Illustration: FIG. 28.—Muscles of palate dissected. The cut represents
the posterior nares and upper surface of the soft palate.
_a._ The levator palati. _b._ The inner bundle of fibres of the
palato-pharyngeus, forming the posterior pillar of the fauces, _c._ The
palato-glossus. _d._ The tensor palati; the cartilaginous extremity of
the Eustachian tube is seen in front of this latter. _e._ The posterior
extremity of the inferior turbinated bone. _f._ The septum. _g g._ The
uvula on each side stretched apart. (_Fergusson._)]
The two descending muscles are placed in the pillars of the fauces,
forming the lateral prolongations of the velum, and the tonsils lie in
a recess between them. The palato-glossi arising from the tongue ascend
in the anterior pillars of the fauces, and spreading out on the anterior
surface of the velum unite in the median raphe. The palato-pharyngei
start from the median raphe in two lamellæ enclosing the termination of
the levator muscle; they descend in the posterior pillars of the fauces,
and being attached to the pharyngeal wall between the superior and middle
constrictors, by their contraction assist in raising the pharynx during
deglutition.
The nervous supply of these muscles requires little notice here; suffice
it that the superior set and the azygos are supplied by the facial nerve,
the inferior set from the pharyngeal plexus.
The _mucous membrane_ of the hard palate is of the usual oral type, and
only differs from that of the rest of the mouth in its close attachment
to the periosteum, from which in fact it is almost impossible to separate
it. It is thick, dense, rather pale and much corrugated, especially in
front and at the sides, whilst behind over the velum it is smoother and
thinner. In it are many small glands (palatine glands) which extend down
to the periosteum. In the median line is a well-marked raphe, extending
anteriorly to a prominence indicating the position of the anterior
palatine canal. The rugose condition of the membrane over the hard palate
is not seen in young children; it supervenes later in life.
The _vascular supply_ of the palate is free and abundant, a circumstance
which is of the greatest surgical importance in that it permits of the
free detachment of the soft structures from the hard by long lateral
incisions, and the necessary manipulation of these in uranoplastic
operations without any fear of loss of vitality, provided that the
patient’s health and constitution are tolerably sound, and that
sufficient pedicle is left in front and behind.
The mucous membrane of the hard palate derives its blood supply from
two of the terminal branches of the internal maxillary artery. The
naso-palatine descend through the anterior palatine canal, and entering
the palate at the incisive foramen (foramina of Stenson) assist in
supplying the anterior portion, anastomosing with the terminations of
the more important posterior or descending palatine, which find their
way to the palate from the spheno-maxillary fossæ through the posterior
palatine canals. Each of the latter arteries on reaching the palate
sends branches to the velum and tonsils, and its main twig runs onwards
in a groove of the bone to supply the mucous membrane and glands of the
hard palate and gums. Its usual position is parallel to the alveolar
border, and about three quarters of an inch from it; but this varies
considerably. The artery can often be felt pulsating as it emerges from
the bone, and is very likely to be divided in the lateral incisions made
during the operation of uranoplasty; but the knife should be carried
external to it, if possible, so that the trunk of the vessel may be
preserved in the flap. The bony palate derives its blood supply not only
from its lower surface but also from its upper, and hence detachment of
the inferior periosteal covering does not lead to death of the bone. The
soft palate derives its blood from three sources, viz. the ascending
palatine of the facial, the ascending pharyngeal, and the posterior
palatine of the internal maxillary. The two former reach it through
the sinus of Morgagni, _i. e._ over the upper border of the superior
constrictor muscle, forming loops of anastomosis on its posterior aspect
with similar branches on the opposite side; the last supplies the
anterior palatal surface.
The normal shape of the palate is a regular arch, bounded laterally by
the gums and alveoli into which the teeth are implanted so as to describe
a parabolic curve, being normally uninterrupted at any spot by spaces
or diastemata. The height and curvature of the palate vary considerably
in different individuals, not only from inherited peculiarities, but
also from acquired conditions dependent on the teeth. A person with a
good set of sound teeth will probably own a regular well-formed palate;
whilst if sundry of the upper permanent teeth are lost during the stage
of adolescence, the palate is likely to become high and narrow from the
falling in of the jaw. This is especially the case if the incisor teeth
are lost.
The shape of the palate in a child of two years does not differ so
markedly as one would at first expect from that of an adult except in
length, and the reason for this is plainly the existence in the latter of
three additional teeth on each side. Its increase in length is from 20
to 30 millimetres, whilst its breadth is only augmented by 10 to 15 mm.,
and this mainly posteriorly. When once the permanent incisors, canines,
and premolars are developed, the anterior portion of the palate alters
but little in shape, unless any of these teeth be lost, and the gaps not
artificially maintained.
Dr. Ehrmann[36] states that the alveolar border in front of the canine
teeth forms a nearly regular semicircle, with a posterior transverse
diameter of 22-26 mm.; thence the alveoli diverge regularly, adding
to the diameter about 2-4 mm. for each tooth. He gives the following
measurements as the mean of many observations:
|From 2-6 yrs.|From 7-10 yrs.|From 11 yrs.
------------------------------+-------------+--------------+------------
Interval between canines |22-25 mm. |23-27 mm. |25-28 mm.
------------------------------+-------------+--------------+------------
” 1st premolars|24-29 ” |25-30 ” |27-30 ”
------------------------------+-------------+--------------+------------
” 2nd ” |26-31 ” |28-32 ” |31-34 ”
------------------------------+-------------+--------------+------------
” 1st molars |— |— |32-37 ”
Oakley Coles[37] has carefully investigated the size of the palate in
several series of skulls in the Museum of the College of Surgeons, and
gives the results as follows:
Of 34 adult skulls of European origin, the average length was 49 mm., the
average width at the second bicuspid was 35 mm., and the average height
from the margins of the alveoli 9 mm.
Of 32 adult skulls of mixed races, the average length was 54 mm., the
width 35 mm., and the height 12 mm.
The frequent association of inherited mental and nervous weakness with
a high arched palate is now a well-established clinical fact. Thus
Savage states that in “Genetous Idiocy” (_i. e._ idiocy which starts in
fœtal life, and cannot be traced to any specific disease) the palate is
usually keel-shaped, the molar teeth being closely approximated; they
are also late in appearing and deficient in number. “Although this kind
of palate may be present in healthy individuals or in those suffering
from ordinary insanity, if it be associated with weak-mindedness or moral
peculiarities in youth I believe one is justified in saying that the
tendency to moral or intellectual deficiency is congenital.”[38] Only
recently Dr. F. Warner has reported[39] to the Psychological Section of
the British Medical Association the results of an investigation as to the
occurrence of deformities amongst school children, and their relationship
to defective vital and mental conditions. Out of 5344 children examined,
physical deformity was noted in 399 cases, and of these 274 were boys
and 125 girls, _i. e._ in the proportion 9·8 per cent. and 5 per cent.
respectively. It was found that of these 25 per cent. exhibited evidences
of low nutrition, 36 per cent. evidences of nervous weakness, and 31
per cent. of mental dulness. 117 cases were noted of deformity of the
palate, 77 boys and 40 girls; and of these 35 per cent. gave signs of low
nutrition, 39 per cent. of nerve weakness, and 35 per cent. of mental
dulness. These defects were more marked and more frequent in the pauper
than in the elementary public schools, in the proportion of 4·2 to 2·2.
As to the character of the malformations, the following are the numerical
statistics: In 105 cases, the palate was arched, narrow, high or vaulted;
in 8, it was 𝖵-shaped; in 4 it was of the flat type.
Dr. Langdon Down[40] had previously noticed and pointed out this frequent
relationship, remarking that as the result of a large number of careful
measurements of the mouths of the congenitally feeble-minded and of
intelligent persons of the same age, he found with few exceptions a
marked diminution in the transverse measurement between the posterior
bicuspids, resulting in an inordinate vaulting of the palate. There
was often noticed an actual deficiency in the bony structures of the
posterior part of the hard palate, causing the velum to hang down
abnormally, interfering with phonation.
The _function_ of the hard palate is mainly mechanical. Acting as a
partition between the nasal and buccal cavities, it prevents nasal
mucus from falling into the mouth, and, by presenting an opposing
surface to the tongue, allows of the production by the latter of the
vacuum necessary for suction, and enables the tongue to direct the food
towards the alveoli, and to disintegrate soft particles, thus assisting
mastication. It is also an accessory to the development of taste by
enabling particles to be evenly spread over the back of the tongue. For
the production of articulate speech the hard palate is an indispensable
factor, and the quality of the voice is much influenced by its contour.
The functions of the soft palate are mainly related to the acts of
respiration, deglutition, phonation, and articulation.
1. _In respiration._—If the mouth is closed, and the respiration purely
nasal, the velum hangs loosely, and allows free passage of air through
the posterior nares. If the mouth is open, the velum is raised, and air
passes freely through the fauces to or from the larynx. When air passes
simultaneously through nose and mouth, the velum hangs in a more or less
flaccid condition midway between the two extremes, and sometimes, when
absolutely relaxed, vibrates, giving rise to snoring or stertor.
2. _In deglutition._—The passage of food into the nose is prevented by
the closure of the posterior nares. This is effected by elevation and
tension of the velum, the levator and tensor muscles acting in unison,
so that its position becomes almost horizontal. The raised velum meets
the posterior wall of the pharynx, which advances as the result of the
action of the upper horizontal fibres of the superior constrictor, and
the closure is completed on either side by the approximation towards
the median line of the posterior pillars of the fauces from the action
of the palato-pharyngei muscles contained therein. These, acting from
the soft palate as a fixed point, and raising the pharynx to grasp
the bolus of food, straighten the walls of the sphincter-like isthmus
faucium, and so guide the food as down an inclined plane. The tension of
the velum also assists in this guidance. That the above is the action
of the palatal structures is proved by the results of their imperfect
development or paralysis, _e. g._ in post-diphtheritic paralysis, where
the naso-pharyngeal cavities remaining unclosed, food (especially if
fluid) regurgitates into the nose.
3. _In phonation and articulation._—The soft palate is here of
considerable importance, inasmuch as it is needed to cut off the
naso-pharynx and nasal cavities from the oral pharynx. When defective
or paralysed, a certain amount of nasal resonance is imparted to the
voice, which, however, is less noticeable during vocalisation than
in articulation. For the production of clear normal voice-sounds it
is essential that the separation between nose and mouth should be
absolute, except for the sounds _m_, _n_, and _ng_. The American twang
is probably due to a slight relaxation of the soft palate, permitting
a small percentage of voice-sounds to pass through the nose. Dr. N. W.
Kingsley[41] has recently published some excellent diagrams illustrating
the position of these parts during the production of definite sounds, and
for all, except those mentioned above, the velum is horizontal, and in
contact with the posterior pharyngeal wall.