OF THE FUNCTION OF DIGESTION.
Process of Assimilation in the plant; in the animal—Digestive
apparatus in the lower classes of animals; in the higher
classes; in man—Digestive processes—Prehension, Mastication,
Insalivation, Deglutition, Chymification, Chylification, Absorption,
Fecation—Structure and action of the organs by which these operations
are performed—Ultimate results—Powers by which those results are
accomplished—Two kinds of digestion, a lower and a higher; the former
preparatory to the latter.
548. Digestion is the function by which the aliment is converted into
nutriment. No food can nourish until it be converted into a fluid
analogous in chemical composition to that of the body by which it is
assimilated. The conversion of the crude aliment into such a fluid is
effected by a vital power peculiar to living beings, by which they
subvert the constitution of other organized bodies, and cause them to
assume their own. They accomplish this change by the agency of certain
secretions which they elaborate in their own organs, and which they
add to the substances they receive as aliment. By the action of these
secretions, the chemical composition of the aliment is brought into a
close affinity to that of the body which it nourishes.
549. This change in the chemical composition of the aliment, by means
of fluids secreted by the living bodies which receive it, is manifest
in the plant as well as in the animal. The sap, as it issues from the
root, is a colourless and limpid fluid; it has a specific gravity
a little greater than that of water; it has a sweetish taste; it
contains an acid which is sometimes free, and is either the carbonic
or the acetic; but more commonly it is combined with lime or potass.
To this crude sap, in this the first stage of its formation, vegetable
secretions, sugar and mucus, assimilative substances, are superadded,
probably by the fibres of the root.
550. As the sap ascends in the stalk, a greater quantity and a greater
number of these vegetable secretions are poured into it. In the ratio
of its elevation it acquires sugar, mucus, albumen, and an azotized
substance analogous to gluten. By the admixture of these assimilative
secretions, the crude sap is progressively assimilated nearer and
nearer to the chemical composition of the proper nutritive fluid of the
plant. Thus prepared, the sap passes to the leaf, in the upper surface
of which it undergoes a process analogous to that of digestion in the
animal (315), and is converted into proper nutrient matter.
551. The plant can only take up, by absorption, liquid food; it never
receives solid substances as aliment: it therefore needs no apparatus
for the division, solution, and fluidification of its food; its sole
work of assimilation consists in changing the innate affinities
of liquid aliment. But animals which live on vegetable and animal
substances have to modify, by their digestive juices, the affinities of
organic solids: hence assimilation in the animal must necessarily be a
more complex operation than it is in the plant.
552. Fixed immovably to the soil by its roots, the nutritive apparatus
of the plant is always in contact with its food, which is slowly but
unceasingly absorbed according to the wants of its system. But the
animal endowed with the faculty of locomotion receives its aliment into
the interior of its body, that it may transport its food along with it
in all its changes of place; and that, as in the plant, its food may be
always in contact with its nutritive apparatus. The interior nutrition
of the animal and the convergence of its nutritive apparatus to the
centre of its system, and the exterior nutrition of the plant and the
divergence of its nutritive apparatus to the peripheral extremity of
its body, are differences in their mode of nutrition, connected with
essential differences in the mode of life peculiar to the two beings.
553. Plant-like animals have a plant-like mode of nutrition. The
transition from the one class to the other is so gradual as to be
almost insensible. Fixed to the same spot in the ocean as the tree to
the land, the nutritive surface of the poriferous animal is always in
contact with the water, as the soil is with the external surface of
the plant. The cellular substance of which the bag of the poriferous
animal is composed is permeated in all directions by ramifying and
anastomosing canals, which, beginning by minute pores placed on the
external surface, terminate in larger orifices, termed vents, which
are fecal openings. These internal canals are incessantly traversed by
streams of water, which enter through the minute, and are discharged
through the larger orifices. By these currents the nutrient matter
contained in the water is conveyed to every part of the body, and
the streams that issue from the fecal orifices abound with minute
flocculent particles, the residue of the digested matter. No separate
part of the body is appropriated to the function of digestion any
more than in the plant; there is merely a general absorbent surface;
the water is to this animal what the soil is to the plant; its whole
surface is a root; every point of that surface is constantly in contact
with its food, and every point is absorbent.
554. In the class above the porifera, the margins of the superficial
pores are merely lengthened out into minute sacs, irritable and
sentient, surrounded with vibratile cilia (342). These sacs, which are
termed polypi, are so many little stomachs, which select, seize, and
digest the food brought to them in the currents of water created by the
action of the cilia (344).
[Illustration: Fig. CXLVIII.—_Hydra Viridis._
1. The Hydra with its tentacula expanded. 2. The tentacula. 3. The
body of the Hydra. 4. Disc for attachment. 5. The Hydra in the act of
creeping. 6. The Hydra with an animalcule in its digestive cavity.]
555. The fresh-water polype, the little hydra (fig. CXLVIII. 1), is one
of these minute sacs detached and endowed with the power of locomotion
(fig. CXLVIII. 5), a sentient, self-moving digestive bag. Capable of
swallowing animals many times its own size, as the red-blooded worm,
this little creature stretches its whole body like a thin elastic
membrane over its prey, so as completely to alter its own shape, and
the membranous substance of which it is composed becoming transparent
by the distention, allows the subsequent process to be distinctly seen.
The red fluid of the worm, as the process of digestion advances, is
slowly diffused over every part of the internal surface of the polype.
The whole internal surface of this minute self-moving bag is digestive;
a true and proper stomach (fig. CXLVIII. 6). By dexterous manipulation,
this internal surface may be rendered external, and the animal turned
completely inside out. Then the external begins to perform the office
of the internal surface, carrying on the function of digestion,
just as well as that which was primitively formed for it; while the
originally digestive becomes the generative surface, for the creature
buds from this surface, now the outer one; a striking and instructive
illustration of the analogy between the external covering of the animal
body or the skin, and its internal lining, or the mucous surface.
[Illustration: Fig. CXLIX.
Group of Monades; the dark spots in the interior of their bodies
representing their digestive sacs.]
556. In the monades (fig. CXLIX.), and in all the lower animalcules,
the digestive apparatus, instead of forming the entire internal
surface of the body, consists of numerous sacs, which constitute so
many separate stomachs, whence the name of the class, _polygastrica_.
When empty, or when filled with water, these digestive sacs cannot
be distinguished from the common cellular tissue of the body; but on
feeding the animals with coloured organic matter, minutely diffused in
water, the coloured particles readily enter the digestive sacs, and
render apparent their form and arrangement. In the minutest animal
hitherto appreciable, the monas termo, the 2000th part of a line
in diameter, four rounded sacs have been seen filled with coloured
particles (fig. CXLIX.). Each of these sacs, about the 6000th part of
a line in diameter, opens by a narrow neck into a funnel-shaped mouth,
surrounded with a single row of long vibratite cilia, by the action
of which the floating organic particles are brought within the reach
of the mouth. In general, even in this class, an alimentary canal
traverses the whole extent of the body, into which all the different
stomachs open. Sometimes numerous branches proceed from the main trunks
of the alimentary canal, bearing the nutritive matter to the different
parts of the body (fig. CL. 2). Often, in order to extend the digestive
surface, the alimentary canal is produced, forming rounded enlargements
called cœcal appendages, all of which act as so many additional
stomachs (fig. CLI. 3). In some individuals, observed under favourable
circumstances, nearly 200 of these cœcal stomachs, filled with coloured
matter, have been counted, and there may have been many more unseen,
because empty and collapsed. In the lowest tribes of this class
there is but one orifice to the alimentary canal, the oral; the food
entering, and the fecal matter passing out of the system by the same
aperture; but in the higher orders there is both an oral and an anal
orifice, and the mouth and the anus are placed at opposite extremities
of the body, as in the higher animals.
[Illustration: Fig. CL.—_Fasciola Hepatica._
1. Mouth. 2. Alimentary tubes. 3. Sucker.]
557. Up to this point in the animal series the digestive sacs and the
alimentary canal are merely cavities formed in the common cellular
tissue of the body, without any lining membrane, without teeth, or
without any instruments for dividing and preparing the aliment, and
without a single gland, as far as has been ascertained, to assist the
digestive process. All the assimilative functions, the respiratory as
well as the digestive, appear to be performed by this single surface.
But in the ascending scale not only is an apparatus appropriated to
digestion, perfectly distinct from that assigned to respiration,
but even the stomach and the alimentary canal are separate organs,
distinguished from each other, both in structure and function.
Still higher in the scale new organs are successively added, as the
process becomes more complex and refined, in order to assist the main
operations carried on in particular parts of the apparatus; and as
that apparatus approaches its highest degree of perfection, not only
do the several parts of which it is composed increase in number and
complexity, but each part becomes more and more isolated from the rest,
a specific office being assigned to each in the division of labour
that is made. Viewing, however, the digestive apparatus as a whole,
whether simple or complex, whether consisting of a single uninterrupted
surface, or divided into many separate portions, its nature is
universally and invariably the same, and from the monad to man is
endowed with analogous vital energies.
[Illustration: Fig. CLI.—_Aphrodita Aculeata._
1. Proboscis in a retracted state. 2. Interior of digestive cavity.
3, 3. Cœcal appendages opening into it.]
558. Comparative anatomy, which has succeeded in tracing through the
different classes, orders, genera, and countless tribes of animals,
the modifications in form and structure of the digestive apparatus,
has shown that those modifications are invariably in strict adaptation
to the kind of food on which the apparatus is destined to act and to
the extent of the elaboration requisite to convert crude aliment into
proper animal substance. To trace this adaptation through the rising
and ever-varying series, is a most interesting and instructive study,
not only exhibiting, in the very organs that elaborate its food, the
physical and even the mental qualities assigned by the hand of nature
to each individual, but oftentimes shedding a clear and bright light on
the complex structures of the highest and most perfect organization.
Striking and beautiful illustrations are afforded by these
investigations of the principle formerly insisted on (vol. i. chap. i.
p. 28, 3), that the communication of the higher faculties exalts the
apparatus even of the very lowest processes, that the latter may work
in harmony with the former. In conformity with this principle, as the
nobler endowments exalt the animal in the scale of organization, so
even its very digestive apparatus becomes extended, isolated, complex
and refined.
559. The highest and most perfect form of the digestive apparatus is
that which is disposed in a series of chambers in free communication
with each other. In these chambers the food undergoes a succession of
changes, by which it is progressively assimilated to the nature of
animal substance. This assimilation, however, is never effected by the
sole agency of the chambers themselves; it is accomplished, to a great
extent, by the influence of special organs placed in the neighbourhood
of the digestive chambers. In the lowest animal there is but one
substance and one surface for every function; in the highest, even for
the performance of the lowest function, there is the combination of
many substances which are arranged in complex modes.
560. In man, the digestive chambers are five; the auxiliary organs are
many.
The first of these chambers is the cavity called the mouth; the
second is the bag termed the pharynx; the pharynx communicates by the
esophagus with the third chamber, the stomach; the fourth chamber
consists of the convoluted tubes named the small intestines, and the
fifth consists of the larger tubes, denominated the large intestines.
The assistant organs are, first, numerous appendages to the mouth,
namely, the tongue, the teeth, the salivary glands, and the muscles
that work the jaws; and, secondly, certain appendages to the small
intestines, namely, the pancreas, the liver, the mesenteric glands, and
the lacteal vessels.
561. By the mouth the food is softened and reduced to a pulp; by the
tongue, materially aided by the soft palate, this pulp, when duly
prepared, is transmitted to the pharynx; received by the pharynx, it
is sent on to the esophagus; by the esophagus, it is conveyed to the
stomach; in the stomach, it is converted into a peculiar substance
called chyme; the chyme, passing from the stomach into the first
portion of the small intestines, is there converted into the substance
called chyle; the chyle, carried slowly along the remaining portion of
the small intestines, is successively absorbed by the lacteals; by the
lacteals, it is conveyed through the mesenteric glands to the thoracic
duct, and by the thoracic duct it is poured into the venous blood close
to the heart. By the large intestines the refuse matter is conveyed out
of the system.
562. The function of digestion consists, then, of the following
processes:—
1. Prehension. 2. Mastication. 3. Insalivation. 4. Deglutition.
5. Chymification. 6. Chylification. 7. Absorption. 8. Fecation.
563. Prehension is the reception of the aliment; mastication is the
mechanical comminution of it; insalivation is the admixture of it with
certain juices poured into the mouth; deglutition is the transmission
of it, when duly moistened and divided, into the stomach; chymification
is the conversion of it into chyme; chylification is the conversion of
the chyme into chyle; absorption is the assumption of the chyle by the
lacteals and the transmission of it into the blood, and fecation is
the separation and discharge of the refuse matter. Each part of this
extended apparatus is modified in structure so as specially to fit it
for the performance of the office which is appropriated to it.
564. The mouth is not merely the opening between the two lips, but
consists of an oval chamber, bounded above by the upper jaw and the
palate; below by the tongue and the lower jaw; laterally by the cheeks;
behind by the soft palate; and before by the lips.
565. The upper and lower jaw, the palate bones, and the teeth,
constitute the hard or the bony parts of the mouth. The soft parts
consist of the lips, the cheeks, the soft palate, the tongue, and the
mucous membrane which lines the whole.
566. The lips and cheeks are composed principally of muscles, covered
on the outside by the skin, and lined on the inside by the mucous
membrane of the mouth. In the interspaces between the muscles is
disposed a quantity of fat, which gives form to the face, facilitates
the movements of the muscles, and protects the glands, blood-vessels,
and nerves, with which all these organs are most abundantly supplied.
567. The roof of the mouth, called the palate, consists partly of bony
and partly of membranous substance. The bony part of the palate forms
an arch in the upper jaw, the position of which in the erect posture is
horizontal: the membranous part of the palate consists of the mucous
membrane of the mouth, which affords a covering to the bony part of the
palate.
[Illustration: Fig. CLII.—_View of the Mouth, showing particularly the
Soft Palate, Tonsils, and Tongue._
1. Anterior arch of the soft palate. 2. Posterior arch. 3. Tonsils or
amygdalæ. 4. Uvula. 5. Communication between the mouth and pharynx.
6. The tongue. 7. Anterior or nervous papillæ. 8 and 9. The upper and
lower turbinated bones dividing the nostrils into (10) chambers.]
[Illustration: Fig. CLIII.—_A side view of the Mouth, Pharynx, Nose,
&c._
1. Mouth. 2. Tongue. 3. Section of the lower jaw. 4. Submaxillary
gland. 5. Sublingual gland. 6. Hyoid bone. 7. Thyroid cartilage. 8.
Thyroid gland. 9. Trachea. 10. Interior of the pharynx. 11. Section
of the soft palate. 12. The esophagus. 13. The interior of the nose.
14. The two spongy bones dividing it into three chambers. 15. The
posterior communication with the upper part of the pharynx.]
[Illustration: Fig. CLIV.—_Posterior view of the Nose, Mouth, Larynx,
and Pharynx laid open._
1. Posterior openings of the nose, communicating with the upper part
of the pharynx. 2. Posterior surface of the soft palate. 3. The uvula.
4. Back part of the mouth communicating with the pharynx. 5. The
tonsils. 6. Back part or root of the tongue. 7. Posterior surface of
the epiglottis. 8. The larynx. 9. The opening of the larynx into the
pharynx. 10. Cut edges of the pharynx. 11. Esophagus, the continuation
of the pharynx. 12. The Trachea, continuation of the larynx. 13.
Muscles acting on the pharynx.]
568. From the posterior part of the bony arch of the palate is
suspended, transversely, a moveable partition, called the soft palate
(fig. CLII. 1 and 2), which is composed of muscular fibres enclosed
in the mucous membranes of the mouth. No less than ten distinct
muscles enter into the composition of the soft palate. These muscles
are disposed in such a manner that they render the organ capable of
descending and of applying itself against the tongue (fig. CLII. 6), so
as completely to close the passage between the mouth and the pharynx
(figs. CLII. 5, and CLIV. 1), and of ascending and carrying itself
obliquely backwards towards the posterior wall of the pharynx, so as
completely to close the passage between the pharynx and the nose (fig.
CLIV. 2, 1); hence this moveable partition performs the office of a
double valve, closing the passage from the mouth to the pharynx, and
from the pharynx to the nose.
569. From the centre of the soft palate hangs pendulous the
conical-shaped body called the uvula (fig. CLII. 4), which consists of
a small muscle enveloped in the mucous membrane of the mouth. The uvula
assists in completing the valve formed by the soft palate (fig. CLIV.
2, 3); it is also an important organ in the modulation of the voice.
When destroyed by disease, both the deglutition of the food and the
sound of the voice become imperfect.
570. The lateral edges of the soft palate separate into two layers,
which enclose between them the bodies called the tonsils (fig. CLII.
3), two glands commonly about the size of an almond. These organs
co-operate with other glands in secreting the fluids of the mouth.
571. The tongue (figs. CLII. 6, and CLIII. 2) is composed of six
distinct muscles enveloped in the mucous membrane of the mouth. The
fibres of these muscles are so interwoven with each other as to form
an intricate net-work; and their number, arrangement, and exquisite
organization render the organ capable of executing a surprising variety
of movements with the most perfect precision, and with a velocity
of which the mind can scarcely form a conception: some of these
movements being requisite to bring the aliment under the operation of
mastication, and others to produce articulate speech.
572. The tongue divided into base, apex, and dorsum, is supported by a
bone called the hyoid bone (os hyoides) (figs. CXXXVI. 1, and CLIII.
6), which, unlike any other bone of the body, is placed at a distance
from the general skeleton, and completely imbedded in muscles. This
singularly posted and delicately constructed bone is not only connected
with the tongue, but with many other highly important muscles, to which
it affords a support and a lever.
573. Each jaw is provided with sixteen teeth (fig. CLV.), arranged with
perfect uniformity, eight on each side of each jaw (fig. CLV.); those
of the one side exactly corresponding with those of the other (fig.
CLV.). The teeth, from the differences they present in their size,
form, mode of connection with the jaw, and use, are divided into four
classes, namely, on each side of each jaw, two incisors (figs. CLVI.
and CLVII. 1, 2); one cuspid (figs. CLVI. and CLVII. 3); two bicuspid
(figs. CLVI. and CLVII. 4, 5); and three molars (figs. CLVI. and CLVII.
6, 7, 8).
[Illustration: Fig. CLV.
A lateral view of the whole series of the teeth, in situ, showing the
relative situation of those of the upper with those of the lower jaw.
This figure and the following figures to 159, are copied from Mr. T.
Bell’s scientific and instructive work on the Anatomy, Physiology, and
Diseases of the Teeth.]
574. The incisor, or cutting teeth, are situated in the front of the
jaw; that directly in the centre is called the central; and the next
to it the lateral incisor (fig. CLV.). Their office, as their name
imports, is to cut the food, which they do, on the principle of shears
or scissors.
575. Standing next to the lateral incisor is the cuspid, canine, or
eye-tooth (figs. CLV., CLVI., and CLVII.). It is the longest of all the
teeth. Its office is to tear such parts of the food as are too hard to
be readily divided by the incisors.
576. Next the cuspid are the bicuspid, two on each side (fig. CLV.,
CLVII.), so named from their being provided with two distinct
prominences or points. Their office is to tear tough substances
preparatory to their trituration by the next set.
[Illustration: Fig. CLVI.
Front or external view of the upper teeth. 1. The central incisor.
2. The lateral incisor. 3. The cuspid. 4. The first bicuspid. 5. The
second bicuspid. 6. The first molar. 7. The second molar. 8. The third
molar, or dens sapientiæ.]
577. The molars, or the grinders, three on each side (fig. CLVI.
and CLVII.), provided with four or five prominences on the grinding
surface, with corresponding depressions, which are so arranged that
the elevations of those of the upper are adapted to the concavities of
those of the lower jaw, and the contrary.
[Illustration: Fig. CLVII.
Front view of the lower teeth. 1. The central incisor. 2. The lateral
incisor. 3. The cuspid. 4. The first bicuspid. 5. The second bicuspid.
6. The first molar. 7. The second molar. 8. The third molar, or dens
sapientiæ.]
578. From the incisor to the molar teeth there is a regular gradation
in size, form, and use, the cuspid holding a middle place between the
incisor and the bicuspid, and the bicuspid being in every respect
intermediate between the cuspid and the molar. Thus the incisor are
adapted only for cutting, the cuspid for tearing, the bicuspid partly
for tearing and partly for grinding, and the molar solely for grinding.
The incisor has only a single root, which is nearly round, and quite
simple (fig. CLVII. 1, 2); the cuspid has only a single root, but this
is flattened and partially grooved (fig. CLVII. 3); even the bicuspid
has only a single root, but this is commonly divided at its extremity,
and is always so much grooved as to have the appearance of two fangs
partially united, the body having two points instead of one, thus
approaching it to the form of the molar (fig. CLVII. 4, 5); and these
last have always two, sometimes three, occasionally four roots, and
their body is greatly increased in size, and has a complete grinding
surface (fig. CLVII. 6, 7, 8).
579. In some animals whose food and habits require the utmost extension
of the office of a particular class of teeth, a corresponding
development of that class takes place. Thus in the carnivora, as is
strikingly seen in the tiger and the polar bear, the cuspid or canine
teeth are prodigiously elongated and strengthened, in order to enable
them to seize their food, and to tear it in pieces. On the other hand,
in the rodentia, or gnawing animals, as in the beaver, the incisors
are exceedingly elongated; while in the graminivora, and especially in
the ruminantia, the molar teeth are by far the most developed. In each
case the other kinds of teeth are of little comparative importance;
sometimes they are even altogether wanting. Thus the shark has only one
kind of tooth, the incisor; but of these there are several rows, and
all of them the creature has the power of erecting at will.
580. So intimately are these organs connected with the kind of food
by which life is sustained, and the kind of food with the general
habits of the animal, that an anatomist can tell the structure of
the digestive organs, the kind of nervous system, the physical and
even the mental endowments; that is, the exact point in the scale of
organization to which the animal belongs, merely by the inspection of
the teeth.
581. In man, the several classes of the teeth are so similarly
developed, so perfectly equalized, and so identically constructed, that
they may be considered as the true type from which all the other forms
are deviations.
582. For the accomplishment of their office the teeth must be endowed
with prodigious strength: for the fulfilment of purposes immediately
connected with the apparatus of digestion, it is necessary that they
should be placed in the neighbourhood of exceedingly soft, delicate,
irritable, and sentient organs. That they may possess the requisite
degree of strength, they are constructed chiefly of bone, the hardest
organized substance. Bone, though not as sensible as some other parts
of the body, is nevertheless sentient. The employment of a sensitive
body in the office of breaking down the hard substances used as food
would be to change the act of eating from a pleasurable into a painful
operation. It has been shown (vol. i. p. 84) that provision is made
for supplying to the animal a never-failing source of enjoyment in the
annexation of pleasurable sensations with the act of eating, and that,
taking the whole of life into account, the sum of enjoyment secured by
this provision is incalculable. But all this enjoyment might have been
lost, might even have been changed into positive pain, nay, must have
been changed into pain, but for adjustments numerous, minute, delicate,
and, at first view, incompatible.
583. Had a highly-organized and sensitive body been made the instrument
of cutting, tearing, and breaking down the food, every tooth, every
time it comes in contact with the food, would produce the exquisite
pain now occasionally experienced when a tooth is inflamed. Yet a
body wholly inorganic and therefore insensible, could not perform the
office of the instrument; first, because a dead body cannot be placed
in contact with living parts without producing irritation, disease, and
consequently pain; and, secondly, because such a body being incapable
of any process of nutrition, must speedily be worn away by friction,
and there could be no possibility of repairing or of replacing it. The
instrument in question, then, must possess hardness, durability, and,
to a certain extent, insensibility; yet it must be capable of forming
an intimate union with sentient and vital organs, must be capable of
becoming a constituent part of the living system.
584. To communicate to it the requisite degree of hardness, the hard
substance forming its basis is rendered so much harder than common bone
that some physiologists have even doubted whether it be bone, whether
it really possess a true organic structure. That there is no ground for
such doubt the evidence is complete. For,
1. The tooth, like bone in general, is composed partly of an earthy
and partly of an animal substance; the earthy part being completely
removable by maceration in an acid, and the animal portion by
incineration, the tooth under each process retaining exactly its
original form.
2. The root of the tooth is covered externally by periosteum; its
internal cavity is lined by a vascular and nervous membrane, and both
structures are intimately connected with the substance of the tooth. If
these membranes really distribute their blood-vessels and nerves to the
substance of the tooth, which there is no reason to doubt, the analogy
is identical between the structure of the teeth and that of bone.
3. Though the blood-vessels of the teeth are so minute that they do
not, under ordinary circumstances, admit the red particles of the
blood, and though no colouring matter hitherto employed in artificial
injections has been able, on account of its grossness, to penetrate
the dental vessels, yet disease sometimes accomplishes what art is
incapable of effecting. In jaundice the bony substance of the teeth is
occasionally tinged with a bright yellow colour; and in persons who
have perished by a violent death, in whom the circulation has been
suddenly arrested, it is of a deep red colour. Moreover, when the
dentist files a tooth, no pain is produced until the file reaches the
bony substance; but the instant it begins to act upon this part of the
tooth, the sensation becomes sufficiently acute.
585. These facts demonstrate that the bony matter of the tooth, though
modified to fit the instrument for its office, is still a true and
proper organized substance.
586. Each tooth is divided into body, neck, and root (fig. CLVIII. 1,
2, 3). The body is that part of the tooth which is above the gum, the
root that part which is below the gum, and the neck that part where the
body and the root unite (fig. CLVIII.). The body, the essential part,
is the tooth properly so called, the part which performs the whole work
for which the instrument is constructed, to the production and support
of which all the other parts are subservient.
[Illustration: Fig. CLVIII.
Views of different kinds of teeth, showing their anatomical division
into, 1. The body or crown. 2. The fang or root. 3. The neck.]
[Illustration: Fig. CLIX.—Sections of Teeth, exhibiting their Structure.
1. The bony substance. 2. The enamel. 3. The internal cavity. 4. The
foramen, or hole at the extremity of the root.]
587. When a vertical section is made in the tooth, it is found to
contain a cavity of considerable size (fig. CLIX, 3), termed the dental
cavity, which, large in the body of the tooth, gradually diminishes
through the whole length of the root (fig. CLIX. 3). The dental cavity
is lined throughout with a thin, delicate, and vascular membrane,
continued from that which lines the jaw. It contains a pulpy substance.
This pulp, highly vascular and exquisitely sensible, is composed almost
entirely of blood-vessels and nerves, and is the source whence the bony
part of the tooth derives its vitality, sensibility, and nutriment. The
blood-vessels and nerves that compose the pulp enter the dental cavity
through a minute hole at the extremity of the root (fig. CLIX. 4). The
membrane which lines the dental cavity is likewise continued over the
external surface of the root, so as to afford it a complete envelope.
588. Provision having been thus made for the organization of the tooth,
for the support of its vitality, and for its connexion with the living
system, over all that portion of it which is above the gum, and which
constitutes the essential part of the instrument, there is poured a
dense, hard, inorganic, insensible, all but indestructible substance,
termed enamel (fig. CLIX. 2); a substance inorganic, composed of earthy
salts, principally phosphate of lime with a slight trace of animal
matter: a substance of exceeding density, of a milky-white colour,
semi-transparent, and consisting of minute fibrous crystals. The manner
in which this inorganic matter is arranged about the body of the tooth
is worthy of notice. The crystals are disposed in radii springing from
the centre of the tooth (fig. CLX. 3); so that the extremities of the
crystals form the external surface of the tooth, while the internal
extremities are in contact with the bony substance (fig. CLX. 3). By
this arrangement a two-fold advantage is obtained; the enamel is less
apt to be worn down by friction, and is less liable to accidental
fracture.
[Illustration: Fig. CLX.
Magnified section of a tooth, to illustrate the arrangement of the
fibrous crystals composing the enamel. 1. Cavity of the tooth. 2. Bony
substance. 3. Enamel, showing the crystals disposed in radii.]
589. In this manner an instrument is constructed possessing the
requisite hardness, durability, and insensibility; yet organized,
alive, as truly an integrant portion of the living system as the eye or
the heart.
590. No less care is indicated in fixing than in constructing the
instrument. It is held in its situation not by one expedient, but by
many.
1. All along the margin of both jaws is placed a bony arch, pierced
with holes, which constitute the sockets, called alveoli, for the teeth
(fig. CLXI.). Each socket or alveolus is distinct, there being one
alveolus for each tooth (fig. CLXI.). The adaptation of the root to
the alveolus is so exact, and the adhesion so close, that each root is
fixed in its alveolus just as a nail is fixed when driven into a board.
[Illustration: Fig. CLXI.
Upper jaw, showing the alveoli.]
2. The roots of the tooth, when there are more than one, deviate
from a straight line (fig. CLVI. 6, 7, 8); and this deviation from
parallelism, on an obvious mechanical principle, adds to the firmness
of the connexion.
3. Adherent by one edge to the bony arch of the jaw, and by the other
to the neck of the tooth, is a peculiar substance, dense, firm,
membranous, called the gum, less hard than cartilage, but much harder
than skin, or common membrane; abounding with blood-vessels, yet but
little sensible; constructed for the express purpose of assisting to
fix the teeth in their situation.
4. The dense and firm membrane covering the bony arch of the jaw is
continued into each alveolus which it lines; from the bottom of the
alveolus this membrane is reflected over the root of the tooth, which
it completely invests as far as the neck, where it terminates, and
where the enamel begins: this membrane, like a tense and strong band,
powerfully assists in fixing the tooth.
5. Lastly, the vessels and nerves which enter at the extremity of the
root, like so many strings, assist in tying it down; hence, when in the
progress of age, all the other fastenings are removed, these strings
hold the teeth so firmly to the bottom of the socket, that their
removal always requires considerable force.
591. But a dense substance like enamel, acting with force against so
hard a substance as bone, would produce a jar which, propagated along
the bones of the face and skull to the brain, would severely injure
that tender organ, and effectually interfere with the comfort of eating.
592. This evil is guarded against,
1. By the structure of the alveoli (fig. CLXII.), which are composed
not of dense and compact, but of loose and spongy bone (fig. CLXII.).
This cancellated arrangement of the osseous fibres is admirably
adapted for absorbing vibrations and preventing their propagation (90).
2. By the membrane which lines the socket.
3. By the membrane which covers the root of the tooth; and,
4. By the gum.
[Illustration: Fig. CLXII.
View of the upper and lower teeth in the alveoli; the external
alveolar plate being cut away to show the cancellated structure of the
alveoli, and the articulation of the teeth.]
These membranous substances, even more than the cancellated structure
of the alveoli, absorb vibrations and counteract the communication of
a shock to the bones of the face and head when the teeth act forcibly
on hard materials; so many and such nice adjustments go to secure
enjoyment, nay to prevent exquisite pain, in the simple operation of
bringing the teeth into contact in the act of eating.
[Illustration: Fig. CLXIII.—_View of the Muscles of Mastication, which
elevate the lower jaw._
1. The temporal muscle. 2. Its insertion passing beneath. 3. The
zygoma. 4. The masseter muscle, its anterior portion reflected to show
the insertion of the temporal. The action of these powerful muscles
is to pull the lower jaw upwards with great force against the upper
jaw, and at the same time to draw it a little forwards or backwards,
according to the direction of the fibres of the muscles.]
593. The teeth in mastication are passive instruments put in motion
by the jaws. The upper jaw is fixed, the lower only is movable. The
lower jaw is capable of four different motions; depression, elevation,
a motion forwards and backwards, and partial rotation. These simple
motions are capable, by combination, of producing various compound
motions. Numerous muscles, some of them endowed with prodigious power,
are so disposed and combined as to be able, at the command of volition,
to produce any of these motions that may be required, simple or
compound.
[Illustration: Fig. CLXIV.—_Muscles of the Jaw._
1. Portion of the zygomatic process of the temporal bone. 2. Ascending
plate of the lower jaw removed to expose, 3. External pterygoid, and,
4. Internal pterygoid muscles. The action of these muscles is to raise
the lower jaw, and to pull it obliquely towards the opposite side.
When both muscles act together, they bring the lower jaw forwards, so
as to make the fore-teeth project beyond those of the upper jaw.]
594. By the combination, succession, alternation, and repetition of
these motions, the lower is made to produce upon the upper jaw all the
variety of pressure necessary for the mastication of the food. In this
process the muscles of the tongue perform scarcely a less important
part than the muscles of the lower jaw. Some of its muscular fibres
shorten the tongue, some give it breadth, others render it concave,
and others convex: so ample is the provision for moving this organ to
different parts of the mouth and fauces, whether to bruise the softer
parts of the aliment against the palate, to mix it with the saliva, or
to place it under the pressure of the teeth.
595. By the combined action of the muscles of the lower jaw and tongue,
and that of the teeth, the food is bruised, cut, torn, and divided
into minute fragments. This operation is of so much importance that
the whole process of digestion is imperfect without it. It is proved
by direct experiment that the stomach acts upon the aliment with a
facility in some degree proportionate to the perfection with which it
is masticated. If an animal swallow morsels of food of different bulks,
and the stomach be examined after a given time, digestion is found to
be the most advanced in the smallest pieces, which are often completely
softened, while the larger are scarcely acted upon at all.
596. At the same time that, by the operation of mastication, the
aliment undergoes mechanical division, it imbibes a quantity of fluid
derived from various sources.
1. From the membrane which lines the internal surface of the mouth, and
which affords a covering to all the parts contained in it.
2. From numerous minute glands placed in clusters about the cheeks,
gums, lips, palate, and tongue. Each of these glands is furnished
with its own little duct, which, piercing the mucous membrane, opens
into the cavity of the mouth. From these glands is derived the fluid
with which the interior of the mouth is lubricated. It consists of a
glutinous, adhesive, transparent fluid, of a light grey tint, salt
taste, and slightly alkaline nature, termed mucus.
[Illustration: Fig. CLXV.—_View of the Parotid Gland with the Muscles
of the Face._
1. Parotid gland. 2. Parotid duct. 3. Masseter muscle. 4. Buccinator.
5. Triangularis, or depressor of the angle of the mouth. 6. Depressor
of the lower lip. 7. Orbicularis, or circular muscle of the mouth. 8.
Great zygomatic, or the distorter of the mouth, as in laughing. 9.
Elevator of the angle of the mouth. 10. Elevator of the upper lip,
and wing of the nose. 11. Compressor of the cartilage of the nose.
12. Orbicularis, or circular muscle of the eyelids. 13. Occipito
frontalis; elevator of the eyelids; motor of the scalp, &c., an
important muscle of expression. 14. Tendinous portion of the occipito
frontalis. 15. Elevator of the ear.]
3. From six large glands placed symmetrically, three on each side,
termed the salivary glands, namely, the parotid (fig. CLXV. 1),
situated before the ear; the submaxillary (fig. CLIII. 4), situated
beneath the lower jaw; and the sublingual (fig. CLIII. 5), situated
immediately under the tongue. Each of these glands is provided with a
duct (figs. CLXV. 2, and cliii. 4, 5), by which it pours the fluid it
elaborates, called saliva, into the mouth.
597. The other fluids of the mouth are always mixed with the saliva,
and are all commonly included under that name. The secretion of these
fluids is unceasing, and they pass into the stomach by successive acts
of deglutition at nearly regular intervals; so that the stomach, after
it has been some time without food, contains a considerable quantity
of these fluids. But they are chiefly needed during the operation of
mastication, and two provisions are made for securing their flow in the
greatest abundance at that time.
598. First, the situation of the glands is such that they are all
exposed to the action of the muscles of mastication (figs. CLXIII. and
CLXIV.), by which action the glands are excited, a large quantity of
blood is determined to them, and the quantity of fluid they secrete
is proportionate to the quantity of blood they receive. Secondly, the
glands are placed under the influence of the mind, so that the very
thought, and still more the taste, of grateful food, acting upon them
as an additional stimulus, causes an additional secretion. The quantity
of fluid formed from these different sources, and mixed with the food
during the mastication of an ordinary meal, is estimated at half a
pint. It must commonly be more than this, because, in a case described
by Dr. Gairdner, of Edinburgh, in which the esophagus had been cut
through, it was observed that from six to eight ounces of saliva were
discharged during a meal, which consisted merely of broth injected
through the divided esophagus into the stomach.
599. Saliva is a frothy, watery fluid, in its healthy state nearly
insipid, and of a slightly alkaline nature. It is composed of water,
a peculiar animal substance called salivary matter, mucus, osmazome,
a little albumen, and several salts. It produces important changes
on the food. By the water, and the salts contained in it, it softens
and dissolves the food; and thus, while it renders it easier to
be swallowed, it prepares it for the subsequent changes it is to
undergo. To this latter object, the assimilation of the food, it
seems to communicate the first tendency by the azotized substances,
the salivary, and the albuminous matter which it adds to it. From
this, the commencement of the assimilative process to its completion,
animalized substances are successively added to the food which have the
property of converting the food more and more into the nature of animal
substance.
600. Comminuted by the teeth, and softened by the saliva, the food is
reduced to a pulp. In this pulp there is a complete admixture of all
the alimentary substances with the assimilative matter secreted from
the blood, into the nature of which it is to be ultimately changed. The
mass is at the same time brought to the temperature of the blood.
601. As long as the operations of mastication and insalivation go on,
the mouth forms a closed cavity from which the food cannot escape; for
the lips enclose it before, the cheeks at the sides, the tongue below,
and the soft palate behind, the inferior edge of which being applied
in close and firm contact with the base of the tongue, prevents all
communication between the mouth and the pharynx.
602. When, by mastication, the food is sufficiently divided, and by
insalivation softened and animalized to fit it for the future changes
it is to undergo, it is collected by the tongue, and carried by that
organ to the back part of the mouth. The soft palate (fig. CLII. 1),
obedient to the stimulus of the duly prepared food, rises the instant
it is touched by it, and affords it a free passage to the pharynx
(figs. CLIII. 10, and CLIV. 10).
603. The pharynx (fig. CLIII. 10), a muscular bag, immediately
continuous with the mouth (fig. CLIII. 1), is a vestibule into which
open several highly important organs. Before is the entrance to the
windpipe, termed the glottis (fig. CLIV. 9), leading directly to the
larynx (fig. CLIV. 8); at the sides are the mouths of two ducts, termed
the Eustachian tubes, which lead to the internal part of the organ of
hearing; above are two entrances to the nose (fig. CLIV. 1); and below
is the passage to the stomach (fig. CLIII. 12).
604. Were the food to enter the Eustachian tubes or the nose, it would
occasion great inconvenience; were it to enter the glottis, it would
cause death. It is prevented from entering the Eustachian tubes and the
nose by the soft palate (fig. CLII. 1 and 2), which by the very act of
rising to afford an opening from the mouth to the pharynx, is carried
over the other apertures so as completely to close them. By the varied
direction of the muscular fibres which enter into the composition of
this organ, it is enabled to execute the different and even opposite
motions required in the performance of its important office.
605. The food is prevented from entering the glottis partly by a
cartilaginous valve (fig. CLIV. 7), termed the epiglottis, placed
immediately above the glottis, and attached to the root of the tongue
(fig. CLIV. 6). In delivering the food to the pharynx the tongue passes
backwards (fig. CLIV. 6). In passing backwards it pushes in the same
direction the epiglottis which is attached to it, and so necessarily
carries it over the glottis, completely closing the aperture (fig.
CLIV. 9). At the same time the opening is still more securely closed
by the glottis itself, in consequence of the powerful and simultaneous
contraction of the muscles that act upon it in the production of the
voice. It is proved, by direct experiment, that the spontaneous closure
of the glottis is a more powerful agent in excluding the food from the
larynx even than the depression of the epiglottis; but both organs
concur in producing the same result; and a double security is provided
against an event which would be fatal.
606. It is deeply interesting to observe the part performed in these
operations by sensation and volition, and the boundary at which their
influence terminates and consciousness itself is lost. Mastication, a
voluntary operation, carried on by voluntary muscles, at the command
of the will, is attended with consciousness, always in the state of
health of a pleasurable nature. To communicate this consciousness, the
tongue, the palate, the lips, the cheeks, the soft palate, and even
the pharynx, are supplied with a prodigious number of sentient nerves.
The tongue especially, one of the most active agents in the operation,
is supplied with no less than six nerves derived from three different
sources. These nerves, spread out upon this organ, give to its upper
surface a complete covering, and some of them terminate in sentient
extremities visible to the naked eye. These sentient extremities,
with which every point of the upper surface, but more especially the
apex, is studded, constitute the bodies termed papillæ, the immediate
and special seat of the sense of taste. This sense is also diffused,
though in a less exquisite degree, over the whole internal surface
of the mouth. Close to the sense of taste is placed the seat of the
kindred sense of smell. The business of both these senses is with the
qualities of the food. Mastication at once brings out the qualities
of the food and puts the food in contact with the organs that are to
take cognizance of it. Mastication, a rough operation, capable of
being accomplished only by powerful instruments which act with force,
is carried on in the very same spot with sensation, an exquisitely
delicate operation, having its seat in soft and tender structures,
with which the appropriate objects are brought into contact only with
the gentlest impulse. The agents of the coarse and the delicate, the
forcible and the gentle operations are in close contact, yet they
work together not only without obstruction, but with the most perfect
subserviency and co-operation.
607. The movements of mastication are produced, and, until they have
accomplished the objects of the operation, are repeated by successive
acts of volition. To induce these acts, grateful sensations are excited
by the contact of the food with the sentient nerves so liberally
distributed over almost the whole of the apparatus. To the provision
thus made for the production of pleasurable sensation, is superadded
the necessity of direct and constant attention to the pleasure
included in the gratification of the taste. It is justly observed by
Dr. A. Combe, that without some degree of attention to the process of
eating, and some distinct perception of its gratefulness, the food
cannot be duly digested. When the mind is so absorbed as to be wholly
unconscious of it, or even indifferent to it, the food is swallowed
without mastication; then it lies in the stomach for hours together
without being acted upon by the gastric juice, and if this be done
often, the stomach becomes so much disordered as to lose its power of
digestion, and death is the inevitable result: so that not only is
pleasurable sensation annexed to the reception of food, but the direct
and continuous consciousness of that pleasurable sensation during the
act of eating is made one of the conditions of the due performance of
the digestive function.
608. With the operation of mastication and one part of the process of
deglutition, immediately to be noticed, the agency of volition and
sensation cease. Beyond this the function of digestion is wholly an
organic process. In addition to the reasons assigned (vol. i. p. 55)
why all the organic processes are placed alike beyond the cognizance of
sense and the control of the will, there is this special reason why, in
the function of digestion, they cease at the exact boundary assigned
them.
609. Every time the act of deglutition is performed the openings to the
windpipe and to the nose are closed, so that during this operation all
access of air to the lungs is stopped, consequently it is necessary
that the passage of the food through the pharynx should be rapid.
Mastication, a voluntary process, may be performed slowly or rapidly,
perfectly or imperfectly, without serious mischief; but life depends on
the passage of the food through the pharynx with extreme rapidity and
with the nicest precision. It is therefore taken out of the province
of volition and entrusted to organs which belong to the organic life,
organs which carry on their operations with the steadiness, constancy,
and exactness of bodies whose motions are determined by a physical law.
610. No sooner does the duly-prepared food touch the soft palate than
the whole apparatus of deglutition is instantly in motion. This movable
partition suddenly rises to afford to the food a free passage to the
pharynx. The pharynx itself, at the same instant, rises to receive the
morsel thrust towards it by the pressure of the tongue; and one muscle,
the stylo-pharyngeus, which concurs in producing this movement, seems
specially intended, in addition, to expand the pharynx. Three muscles
throw their fibres around the pharynx, termed its upper, middle, and
lower constrictors, which, the moment the morsel reaches the pharynx,
contract upon it, and embrace it firmly. At the same instant the
larynx, closing its aperture, springs forward towards the base of the
tongue, under which it is in a manner concealed, the additional shield
of the epiglottis being simultaneously thrown over the glottis. By this
movement of the larynx, upwards and forwards, the course of the morsel
across the dangerous passage is shortened. All these motions take
place with such rapidity that Boerhaave said the action is convulsive.
And now the food, firmly pressed by the pharynx, cannot return to the
mouth, for the root of the tongue is there stopping up the passage; it
cannot enter the Eustachian tubes or the nose, for the soft palate is
there closing the apertures; it cannot enter the larynx, for a double
guard is placed upon the glottis securing its firm closure. The food
can advance in one direction only, the direction required, that which
leads to the esophagus. Well, therefore, on the contemplation of these
complex structures and the consent and harmony with which they act,
might Paley say, “In no apparatus put together by art do I know such
multifarious uses so aptly contrived as in the natural organization of
the human mouth and its appendages. In this small cavity we have teeth
of different shape; first, for cutting; secondly, for grinding; muscles
most artificially disposed for carrying on the compound motions of the
lower jaw by which the mill is worked; fountains of saliva springing up
in different parts of the cavity for the moistening of the food while
the mastication is going on; glands to feed the fountains; a muscular
contrivance in the back part of the cavity for the guiding of the
prepared aliment into its passage towards the stomach, and for carrying
it along that passage. In the mean time, and within the same cavity,
is going on other business wholly different, that of respiration and
of speech. In addition, therefore, to all that has been mentioned,
we have a passage opened from this same cavity of the mouth into the
lungs for the admission of air, for the admission of air exclusively
of every other substance; we have muscles, some in the larynx, and,
without number, in the tongue, for the purpose of modulating that air
in its passage, with a variety, a compass, and a precision of which no
other musical instrument is capable; and, lastly, we have a specific
contrivance for dividing the pneumatic part from the mechanical, and
for preventing one set of functions from interfering with the other.
The mouth, with all these intentions to serve, is a single cavity; is
one machine, with its parts neither crowded nor confined, and each
unembarrassed by the rest.” It should be added, the mouth is also the
immediate seat of one of the senses, and is in intimate communication
with a second sense; both these senses are always excited while the
principal business performed by the machine is carried on, and are
necessarily excited by the very working of the machine, and the
sensations induced in the natural and sound state of the apparatus are
invariably pleasurable.
611. The food is delivered by the pharynx to the esophagus (fig.
CLIII. 12), a tube composed partly of membrane and partly of muscle.
Its muscular fibres consist of a double layer, an external and an
internal layer; the external has a longitudinal direction; the internal
describes portions of a circle around the tube. By the contraction
of the longitudinal fibres the length, and by the contraction of the
circular fibres, the diameter of the tube is diminished. Cellular
membrane envelops these layers of fibres externally, and mucous
membrane covers them internally. When the tube is contracted, the
mucous membrane is disposed in folds, which disappear when it is
dilated, and these folds allow of the expansion of the tube without
injury to the delicate tissue that lines it. The food passes slowly
along the esophagus urged towards the stomach, not by its own gravity,
but by a force exerted upon it by the tube itself, chiefly by the
contraction of its circular fibres. Delivered at length to the
stomach, the food is incapable of returning into the esophagus in
consequence of the oblique direction in which the esophagus enters the
stomach, the obliquity of its entrance serving the office of a valve.
[Illustration: Fig. CLXVI.—_View of the Stomach with its Muscular Coats
displayed._
1. The esophagus terminating in the stomach. 2. The cardiac orifice.
3. The pylorus. 4. The commencement of the duodenum. 5. The large
curvature of the stomach. 6. The small curvature. 7. The large
extremity. 8. The small extremity. 9. The longitudinal muscular
fibres. 10. The circular muscular fibres.]
612. The stomach is a bag of an irregular oval shape (fig CLXVI.),
capable, in the adult, of containing about three pints. It is placed
transversely across the upper part of the abdomen (fig. LX. 7). It
occupies the whole epigastric (fig. CV. 3), and the greater part of
the left hypochondriac regions (fig. CVII. 3). Above, it is in contact
with the diaphragm, the arch of which extends over it (fig. LX. 7, b);
below with the intestines (fig. LX. 8, 9), on the right side with the
liver (fig. LX. 6), and on the left side with the spleen (fig. CLXVIII.
5).
[Illustration: Fig. CLXVII. _Internal View of the Stomach and Duodenum._
1. Mucous membrane, forming the rugæ. 2. Pyloric orifice opening into
the duodenum. 3. Duodenum. 4. Interior of the duodenum, showing the
valvulæ conniventes. 5. Termination of, 6. The biliary or choledoch
duct. 7. Pancreatic duct, terminating at the same point as the
choledoch duct. 8. Gall-bladder removed from the liver. 9. Hepatic
duct proceeding from the liver. 10. Cystic duct proceeding from the
gall-bladder, forming by its union with the hepatic, a common trunk,
the choledoch.]
613. Into the left extremity, which is much larger and considerably
higher than the right (fig. CLXVI. 7), the esophagus opens by an
aperture called the cardiac orifice (fig. CLXVI. 2). At the right
extremity, a second aperture called the pyloric orifice (fig. CLXVII.
2), leads into the first intestine.
614. Between the cardiac and the pyloric orifices are two curvatures,
one above, called the smaller (fig. CLXVI. 6), the other below, termed
the larger curvature (fig. CLXVI. 5).
615. Like the esophagus, the stomach is composed of two layers of
muscular fibres, the external longitudinal (fig. CLXVI. 9), the
internal circular (fig. CLXVI. 10). By the contraction of the first the
extent of the stomach, from extremity to extremity, is diminished, or
the organ is shortened; by the contraction of the second the extent of
the stomach, from curvature to curvature, is diminished, or the organ
is narrowed. During digestion, by the contraction of these muscular
fibres, the capacity of the stomach is changed alternately in both
directions, whence a gentle motion is communicated to the aliment,
which is thus brought in succession under the influence of the agent
that acts upon it.
616. A thin but strong membrane, derived from the peritoneum, the
membrane that lines the general cavity of the abdomen, forms the
external tunic of the stomach; hence its outer covering is called the
peritoneal coat.
617. The inner or mucous coat (fig. CLXVII. 1), a direct continuation
of the lining membrane of the esophagus, is sometimes called also
villous, on account of the minute bodies termed villi, with which every
point of its internal surface is studded. It is these villi which give
to this surface its pilous or velvety appearance,
[Illustration: Fig. CLXVIII.—_View of the Vascular connexion between
the Stomach, Liver, Spleen, and Pancreas._
1. Stomach raised to exhibit its posterior surface. 2. Pylorus. 3.
Duodenum. 4. Pancreas. 5. Spleen. 6. Undersurface of the liver. 7.
Gall-bladder, in connexion with the liver. 8. Large vessels proceeding
from. 9. A common trunk to supply the liver, gall-bladder, stomach,
duodenum, pancreas, and spleen.]
618. The mucous coat is far more extensive than the other two, in
consequence of its being plaited into a number of folds (fig. CLXVII.
1), termed rugæ, which are so disposed as to present the appearance
of a net-work. The object of the rugæ is to enlarge the space for the
expansion of blood-vessels and nerves, and to admit of the occasional
distension of the organ without injury to the delicate tissues of which
it is composed.
619. Immediately beneath the mucous coat are the mucous follicles
which secrete the mucous fluid by which the inner surface of the organ
is defended. These glandular bodies are extremely numerous, and vary
considerably in diameter. The largest are towards the great extremity,
the smaller towards the pylorus.
620. Altogether different from the mucous secretion is another fluid,
which also flows from the mucous surface, termed the gastric or the
digestive juice, from its being the principal agent in the digestive
process. By some anatomists the gastric juice is supposed to be
secreted by minute glands placed between the mucous and the muscular
coats, provided with ducts which pierce the mucous coat, and which bear
their fluid into the stomach precisely as the salivary glands carry the
saliva into the mouth. It is certain that this is the case with some
animals, as in certain birds, the ostrich for example, in which glands
of considerable magnitude, with ducts large enough to be visible, are
seen to pour the digestive fluid into the stomach. But as no such
glands have been discovered in the human stomach, it is generally
conceived that in man the gastric juice is secreted by minute arteries
expanded upon the villi.
621. All around the pyloric orifice (fig. CLXVII. 2) is placed a
thick, strong, and circular muscle (fig. CLXVII. 2), termed, from its
office, pylorus. It is about four times the thickness of the muscular
coat of the stomach, and presents the appearance of a prominent and
even projecting band (fig. CLXVII. 2). From the frequent action of its
fibres, the pylorus often looks as if a piece of packthread had been
tied around it (fig. CLXVI. 3). Its office is, by the contraction of
its fibres, to guard and close the opening from the stomach until the
aliment has been duly acted upon by the digestive fluid.
[Illustration: Fig. CLXIX.
View of the stomach, showing the number and magnitude of its
blood-vessels, and the mode of their distribution.]
622. The quantity of blood sent to the stomach is greater than is spent
upon any other organ except the brain. The vessels of the stomach
(fig. CLXIX.) form two distinct layers, of which the external is
distributed to the peritoneal and muscular coats, while the internal,
after ramifying on the fine cellular tissue which unites the muscular
and mucous tunics, penetrates the mucous coat, and is spent upon the
villi, where it forms an exquisitely-delicate net-work. There is,
moreover, an intimate vascular connexion between the spleen, pancreas
and liver, and the stomach (fig. CLXVIII. 8, 9). The arteries which
supply all these organs spring from a common trunk, and there is the
freest communication between them by anastomosing branches.
[Illustration: Fig. CLXX.—_View of the Organic Nerves of the Stomach._
1. Under surface of the liver turned up, to bring into view the
anterior surface of the stomach. 2. Gall bladder. 3. Organic nerves
enveloping the trunks of the blood-vessels. 4. Pyloric extremity of
the stomach and commencement of the duodenum. 5. Contracted portion
of the pylorus. 6. Situation of the hour-glass contraction of the
stomach, here imperfectly represented. 7. Omentum.]
623. Equally abundant is its supply of nerves, some of which are
derived from the organic or non-sentient system, and others from
the animal or sentient system. The organic nerves are spread out in
countless numbers upon the great trunks of the arteries, so as to give
them a complete envelope (fig. CLXX. 3); these nerves, never quitting
the arteries, accompany them in all their ramifications, and the fibril
of the nerve is ultimately lost upon the capillary termination of the
artery. It is by these organic nerves that the stomach is enabled to
perform its organic functions, which, for the reason assigned (vol. i.
p. 82), is placed beyond volition, and is without consciousness. By the
nerves derived from the sentient system which mingle with the organic
(fig. XVI.), the function of nutrition is brought into relation with
the percipient mind, and is made part of our sentient nature. By the
commixture of these two sets of nerves, derived from these two portions
of the nervous system, though we have no _direct_ consciousness of the
digestive process—consciousness ceasing precisely at the point where
the agency of volition stops (vol. i. p. 82, et seq.), yet pleasurable
sensation results from the due performance of the function. Hence
the feeling of buoyancy, exhilaration, and vigour, the pleasurable
consciousness to which we give the name of health, when the action of
the stomach is sound: hence the depression, listlessness, and debility,
the painful consciousness which we call disease, when the action of the
stomach is unsound: hence, too, the influence of the mental state over
the organic process; the rapidity and perfection with which the stomach
works when the mind is happy—when the repast is but the occasion and
accompaniment of the feast of reason and the flow of soul; the slowness
and imperfection with which the stomach works when the mind is harassed
with care struggling against adverse events; or is in sorrow and
without hope; when the friend that sat by our side, and with whom we
were wont to take sweet counsel, is gone, and therefore gone that which
made it life to live.
624. Renovation is the primary and essential office of the stomach,
and its organic nerves enable it to supply the ever-recurring wants of
the system. Gratification of appetite is a secondary and subordinate
office of the stomach, and its sentient nerves enable it to produce
the state of pleasurable consciousness when its organic function is
duly performed. By the double office thus assigned it, the stomach is
rendered what Mr. Hunter named it, the centre of sympathies.
625. From the whole length of the great arch of the stomach, and
partly also from the commencement of the duodenum (fig. CLXX.), the
peritoneal coat of the stomach is produced, forming a thin, delicate
membranous bag, called the omentum, or cawl (fig. CLXX. 7). The omentum
extends from the great arch of the stomach to below the umbilicus,
and completely covers a large portion of the anterior surface of the
abdominal viscera (fig. CLXX. 7). Between the two fine membranous
layers of which it is composed is contained a quantity of fat, of which
substance it serves as a reservoir, and by the transudation of which it
appears to lubricate the intestines, and to assist in preventing their
accretion.
626. The food, on reaching the stomach, does not occupy indifferently
any portion of it, but is arranged in a peculiar manner always in one
and the same part. If the stomach be observed in a living animal, or
be inspected soon after death, it is seen that about a third of its
length towards the pylorus is divided from the rest by the contraction
of the circular fibre called the hour-glass contraction (fig. CLXX.
6). The stomach is thus divided into a cardiac and a pyloric portion
(fig. CLXX. 6). The food, when first received by the stomach, is always
deposited in the cardiac portion, and is there arranged in a definite
manner. The food first taken is placed outermost, that is, nearest the
surface of the stomach; the portion next taken is placed interior to
the first, and so on in succession, until the food last taken occupies
the centre of the mass. When new food is received before the old is
completely digested, the two kinds are kept distinct, the new being
always found in the centre of the old.
627. Soon after the food has been thus arranged, a remarkable change
takes place in the mucous membrane of the stomach. The blood-vessels
become loaded with blood; its villi enlarge, and its cryptæ, the minute
cells between the rugæ, overflow with fluid. This fluid is the gastric
juice, which is secreted by the arterial capillaries now turgid with
blood. The abundance of the secretion, which progressively increases as
the digestion advances, is in proportion to the indigestibility of the
food, and the quietude of the body after the repast.
628. In the food itself no change is manifest for some time; but at
length that portion of it which is in immediate contact with the
surface of the stomach begins to be slightly softened. This softening
slowly but progressively increases until the texture of the food,
whatever it may have been, is gradually lost; and ultimately the most
solid portions of it are completely dissolved.
629. When a portion of food thus acted on is examined, it presents the
appearance of having been corroded by a chemical agent. The white of a
hard-boiled egg looks exactly as if it had been plunged in vinegar or
in a solution of potass. The softened layer, as soon as the softening
is sufficiently advanced, is, by the action of the muscular coat of
the stomach, detached, carried towards the pylorus, and ultimately
transmitted to the duodenum; then another portion of the harder and
undigested food is brought into immediate contact with the stomach,
becomes softened in its turn, and is in like manner detached; and this
process goes on until the whole is dissolved.
630. The solvent power exerted by the gastric juice is most apparent
when the stomach of an animal is examined three or four hours after
food has been freely taken. At this period the portion of the food
first in contact with the stomach is wholly dissolved and detached; the
portion subsequently brought into contact with the stomach is in the
process of solution, while the central part remains very little changed.
631. The dissolved and detached portion of the food, from every
part of the stomach flows slowly but steadily beyond the hour-glass
contraction, or towards the pyloric extremity (626), in which not a
particle of recent or undissolved food is ever allowed to remain. The
fluid, which thus accumulates in this portion of the stomach, is a
new product, in which the sensible properties of the food, whatever
may have been the variety of substances taken at the meal, are lost.
This new product, which is termed chyme, is an homogeneous fluid,
pultaceous, greyish, insipid, of a faint sweetish taste, and slightly
acid.
632. As soon as the chyme, by its gradual accumulation in the pyloric
extremity amounts to about two or three ounces, the following phenomena
take place.
633. First, the intestine called duodenum, the organ immediately
continuous with the stomach, contracts. The contraction of the duodenum
is propagated to the pyloric end of the stomach. By the contraction
of this portion of the stomach, the chyme is carried backwards from
the pyloric into the cardiac extremity, where it does not remain,
but quickly flows back again into the pyloric extremity, which is
now expanded to receive it. Soon the pyloric extremity begins again
to contract; but now the contraction, the reverse of the former,
is in the direction of the duodenum; in consequence of which, the
chyme is propelled towards the pylorus. The pylorus, obedient to the
demand of the chyme, relaxes, opens, and affords to the fluid a free
passage into the duodenum. As soon as the whole of the duly prepared
chyme has passed out of the stomach, the pylorus closes, and remains
closed, until two or three ounces more are accumulated, when the same
succession of motions are renewed with the same result; and again cease
to be again renewed, as long as the process of chymification goes on.
634. When the stomach contains a large quantity of food, these
motions are limited to the parts of the organ nearest the pylorus; as
it becomes empty, they extend further along the stomach, until the
great extremity itself is involved in them. These motions are always
strongest towards the end of chymification.
635. The stomach during chymification is a closed chamber; its cardiac
orifice is shut by the valved entrance of the esophagus, and its
pyloric orifice by the contraction of the pylorus.