CHAPTER VI

DIGESTION

=The Canal.=—The prospect presented by a widely open mouth is too

familiar to need description, but a few details may be pointed out.

The teeth are, or should be, thirty-two in number. Starting from

the middle line of either jaw, the two first are incisors, with

chisel-shaped cutting edges. If they meet, as they ought to do, their

edges are ground flat. The third tooth is the canine, with a more or

less pyramidal crown. Then two premolars, or “milk-molars,” as they

are often termed, because they are the only grinding teeth of the

first dentition. Twenty is the full complement of teeth in a child.

Lastly, three strong grinders—the molar teeth. The third molar, or

wisdom-tooth, is evidently disappearing in the human race. In civilized

people, whose brains are large and jaws small, it does not appear

until about the twentieth year. Sometimes it tries to squeeze through

the gum of a jaw not large enough to carry it, and causes trouble by

becoming “impacted” beneath the ascending ramus. Not infrequently it

fails to appear. It may be truly said that the increasing wisdom of

the human race is responsible for the postponement of its development,

although this is hardly the circumstance to which it owes its name. A

fold of mucous membrane—the frenulum linguæ—connects the under side

of the tongue with the floor of the mouth. On either side of this may

be seen the opening of a duct common to the submaxillary and sublingual

salivary glands. The opening of the duct of the parotid gland is not

so easy to find. It pierces the mucous membrane of the cheek opposite

to the base of the second molar tooth of the upper jaw. The parotid

gland lies just below the ear, behind the jaw. The saliva which it

secretes is a watery fluid containing little beside salt and a weak

ferment. It serves to moisten the food as it is being crushed by the

molar teeth. The submaxillary and sublingual secretions contain, in

addition to the ferment, ptyalin, mucus which the tongue mixes with the

masticated food as it forms it into a bolus suitable for swallowing.

The dorsal surface of the tongue is covered by papillæ, which rasp

the food against the palate. Of these the greater number are pointed,

or filiform. The remainder are flat-topped, or fungiform. The two

varieties may be distinguished with a lens, especially on the sides of

the tongue. Usually the fungiform papillæ are the redder. In fever,

when the tongue is densely furred, they stand out as bright red spots.

The back of the tongue is crossed by a =V=-shaped row of papillæ of

larger size, each surrounded by a slight fossa and a vallum, and hence

termed “circumvallate.” Very minute organs of sense—taste-bulbs—stud

the mucous membrane which lines the fosse.

The hard palate ends in a muscular curtain—the soft palate—the

central portion of which—the uvula—depends lower than the rest.

On either side the soft palate splits into two folds; the anterior,

continued to the side of the tongue; the posterior, to the pharynx.

These folds, since they bound the gateway into the pharynx, which is

known as the “fauces,” are termed the “pillars of the fauces.” The

tonsil lies between the anterior and posterior pillars of the fauces,

but does not appear as a prominence unless inflamed or enlarged.

The pharynx hangs as a bag from the base of the skull. It, like all

the rest of the alimentary tract, is lined by mucous membrane. “Mucous

membrane” is not a happy term. It does not denote that the epithelium

secretes mucus. It may or may not possess this property. Nor does it

imply that it has a different origin from the skin—that it arises

from hypoblast, the inner layer of the rudiment from which the embryo

grows. The term is applied to all internal, and therefore moist,

surfaces, whether they arise from hypoblast, as in the case of the

lining of the greater part of the alimentary tract, or whether they

are involutions of epiblast as in the case of the mouth and also of

the extreme lower end of the alimentary tract. Almost the whole of the

alimentary canal is, in the first instance, a tubular cavity in the

interior of the embryo, lined by hypoblast. This cavity communicates

with the yolk-sac, but has no openings on the exterior until it joins

up with two epiblastic pits—one the stomodæum, or mouth-cavity, at

the anterior end; and the other the proctodæum, at the posterior end

of the body. The distinction between the middle closed portion of the

alimentary canal and its two secondary openings suggests morphological

speculations, into which we have not space to enter, as to the ancestry

of the vertebrates. The majority of anatomists believe that the

primitive canal is represented in the middle portion, and that, in

prevertebrate animals, it opened to the exterior in a different way.

The pharynx is 4½ inches long. It is enclosed by three thin muscles,

which overlap from below upwards—the constrictors of the pharynx.

The anterior attachment of the superior constrictor is to the jaw; of

the middle constrictor to the hyoid bone; of the inferior constrictor

to the thyroid cartilage. Above the soft palate the nasal chambers

communicate with the pharynx by the posterior nares. Below the hyoid

bone, which is easily felt in the neck as a bony arch just above the

thyroid cartilage (Adam’s apple), the windpipe, or trachea, joins the

pharynx by a single pear-shaped orifice—the rima glottidis. When we

consider the mechanism of swallowing, we shall study the arrangements

which prevent food, passed through the fauces, from entering either the

nasal chambers above or the windpipe below and in front. At the level

of the lower border of the thyroid cartilage the pharynx becomes the

relatively narrow œsophagus. This tube, which lies behind the trachea,

and slightly to its left side, passes with a straight course to the

abdomen. It traverses the chest, lying behind the heart, pierces the

diaphragm, and just beneath it joins the stomach. Its length is about 9

inches. The stomach is a sickle-shaped bag. It has two apertures—the

cardiac orifice, or junction with the œsophagus; and the pyloric

orifice, or junction with the small intestine. It is so folded on

itself that these two apertures are not more than 4 inches apart. Its

outline may be drawn on the body-wall with a piece of charcoal from a

point an inch below and an inch to the left side of the lower end of

the breast-bone, the position of the cardiac orifice, to a point about

4 inches below the end of the breast-bone, and an inch or two to the

right side of the mid-line of the body, the position of the pyloric

orifice, with a slight curvature to represent the upper border; to

represent the lower border the same two points are joined by a bold

curve, bulging upwards to the nipple, outwards to the side of the body,

and downwards some distance on the abdomen (_cf._ Fig. 2).

[Illustration: FIG. 6.

The stomach has been cut across a short distance

from the pyloric valve, and removed, to show the

viscera which lie behind it. The descending aorta

and the vena cava rest upon the vertebral column.

They are crossed by the pancreas and the transverse

portion of the duodenum. The head of the pancreas

is enclosed by the curvatures of the duodenum. The

ducts of the liver and pancreas are seen entering

the descending duodenum side by side.]

Such an outline represents the form and position of the stomach when

distended; but it is to be understood that its dimensions depend upon

the amount of its contents. It is capable of holding about 7 pints.

The junction of œsophagus and stomach is closed by a muscular ring, or

sphincter muscle—the cardiac sphincter; the junction of stomach and

intestine is guarded by a much stronger pyloric sphincter. The average

diameter of the small intestine is about 1½ inches. It is wide enough,

therefore, to admit two fingers. The length of the tube is about 22

feet. Its first part is termed the “duodenum,” because its length

equals the breadth of twelve fingers—_i.e._, about 9 inches. The

remainder is divided arbitrarily into jejunum and ileum. The duodenum

makes three sharp curves. First it inclines upwards and to the right,

then vertically downwards, then horizontally to the left, and finally

forwards. The ducts of the liver and pancreas open by a common orifice

into the descending portion. Its horizontal portion is bound firmly to

the vertebral column. After this the whole of the small intestine is

supported by the mesentery, a double fold of peritoneum which allows it

to hang freely in the abdominal cavity. The mesentery is attached to

the back of the body-wall. Commencing on the left side of the second

lumbar vertebra, its line of attachment inclines obliquely downwards

and to the right, across the vertebral column, for about 6 inches.

Measured from its attached edge to the edge which bears the intestine,

it has a width of about 8 inches. Its free border has, as already

said, a length of 22 feet. Its measurements being as just stated, it

is clear that it must be folded backwards and forwards upon itself,

like a goffered frill. In the right groin the small intestine joins the

large intestine, or colon. It does not, as might have been expected,

simply dilate into the large intestine, but enters it on its mesial

side, its orifice being guarded by the ileo-colic valve. In other

words, the large intestine projects downwards beyond this orifice,

as the cæcum coli. In many animals the cæcum is of great length and

capacity. In the human embryo it begins to assume a similar form; but

a very small portion only (the so-called “cæcum” of human anatomy)

dilates to the calibre of the colon. The real cæcum retains throughout

life its embryonic calibre. It has a length of about 3½ inches, and a

diameter of not more than ¼ inch. This is the “vermiform appendix,” of

ill fame, which must be looked upon as one of Nature’s misfits. Its

great liability to become inflamed is commonly explained as due to the

tendency of such articles of food as pips, the fibre of ginger, flakes

from the inside of enamelled saucepans, etc., to become lodged in its

cavity. But whether this explanation be correct or no—and there are

reasons for thinking it somewhat fanciful—it is much to be wished

that the process of evolution would hasten the disappearance of this

functionless vestige of a cæcum. As there is no tendency towards the

inheritance of characters due to mutilation, and since the surgeon’s

knife now prevents this death-trap from claiming its toll of possible

parents, we must look upon the rudimentary cæcum, with its liability

to inflammation, as a permanent burden on the human race. In justice

to the appendix, however, it must be pointed out that it has acquired

its criminal reputation during the past twenty years. The frequency of

appendicitis has increased so enormously during this period that it

ought to be possible to correlate its prevalence with the introduction

of the cause upon which it chiefly depends.

The colon has a length of about 5 feet. Its greatest width, about

3 inches, is at its commencement, but it is everywhere much wider

than the small intestine. Whereas the wall of the small intestine is

smooth externally, the wall of the colon is sacculated. Three muscular

bands constrict it longitudinally; circular bands at intervals of

about 1 inch or 1½ inch throw it into pouches. It ascends on the

right side, lying far back against the body-wall, to which it is

bound by peritoneum, which in this part of its course covers only its

anterior surface. Having touched the under side of the liver, it loops

forwards and to the left side, crossing the middle line just above the

umbilicus. On the extreme left side it touches the spleen, getting

very near to the back of the abdominal cavity. It then descends on the

left side, again bound to the body-wall by peritoneum, although not

so closely as on the right side, until it reaches the inner lip of

the crest of the hip-bone. From here onwards the fold of peritoneum

which attaches it allows it a free movement. This portion of the large

intestine, the sigmoid flexure, may even fall over into the right

groin. Lastly it curls backwards into the pelvis, as the rectum.

Movement of the contents of the alimentary canal may be favoured by

judicious pressure, or massage. From the description of the situation

of its several parts given above, it will be understood that if the

right hand be placed on the abdomen immediately beneath the ribs, with

the fingers well round to the left side, the stomach will be covered.

Pressure from left to right will tend to drive its contents towards the

pyloric valve. The small intestine is so irregular in its course as to

preclude the possibility of following it with the hand. Pressure first

on one side and then on the other, with a general tendency to work

from above downwards, tends to press forward its contents; but, owing

to its circular form and strong muscular walls, it is not in much

need of help. Very different is the position of the large intestine

in this respect. Its calibre is much greater, its wall is sacculated,

its contents comparatively firm. If the palm of the hand be placed

above the right groin and pressure directed upwards, the cæcum coli

and ascending colon are emptied. If pressure be directed from the

extreme right side just below the ribs, across the middle line to the

left side, the transverse colon is emptied. The descending colon needs

pressure from above downwards on the left side; the sigmoid flexure,

pressure above the left groin, downwards, and towards the middle line.

The inner wall of the œsophagus is smooth, save for the wrinkles into

which it is thrown when not distended; but from the cardiac orifice

of the stomach onwards the mucous membrane of the alimentary canal

exhibits folds and other projections which serve many purposes.

They serve to delay the food, keeping it longer in contact with the

secreting surface. They increase the area pitted with tubular glands;

they increase also the area through which absorption of the products

of digestion occurs. On the inner surface of the stomach the folds

produce a reticulated pattern. In the upper portion of the small

intestine, especially the duodenum, there are prominent transverse

shelves (valvulæ conniventes). No definite folds occur below the

upper three-fourths of the small intestine, with the exception of the

constrictions of the transverse colon already referred to, which affect

the whole thickness of its wall. Throughout the whole of the small

intestine the mucous membrane projects in finger-like processes, or

villi, which give it a characteristic velvety appearance. The villi are

longest in the duodenum.

Lymph-follicles occur at intervals in the intestine. In the ileum they

are collected into patches (Peyer’s patches), on the side opposite to

the line of attachment of the mesentery. They serve both for the supply

of phagocytes, which hunt any germs that have penetrated the mucous

membrane, and also as stations to which germ-laden phagocytes retreat.

The wall of the intestine is composed of mucous membrane, submucous

tissue, and muscle. The mucous membrane is everywhere pitted with

tubular glands, termed in the stomach “gastric glands,” and in the

intestines, both small and large, “crypts of Lieberkühn.” Their

relation to the wall might be exemplified by taking a block of dough

about 6 inches thick and pushing a pencil vertically into it almost

down to the table on which it rests. The holes should be made as close

together as possible, since, especially in the stomach, extremely

little tissue intervenes between the tubes of gland-cells. If the piece

of dough were placed upon a folded cloth, the cloth would represent the

muscularis mucosæ, a layer properly regarded as a constituent of the

mucous membrane. The fibres of this coat are disposed in two or three

sheets, the fibres of one sheet crossing those of the next. By their

contractions they squeeze the ends of the crypts, and probably wobble

them about, expelling their secretion. Beneath the muscularis mucosæ is

a layer of connective tissue, the submucosa, which contains abundant

lymphatic channels, bloodvessels, and nerves. At the pyloric end of the

stomach, the tubes of gland-cells tend to pierce the muscularis mucosæ.

In the first part of the duodenum, certain tubes, having pierced this

layer, branch in the submucosa. A layer of racemose glands is thus

formed—the glands of Brunner. Outside the submucosa is the muscular

coat proper, composed of plain muscle-fibres, except in the upper part

of the œsophagus, where the fibres are striated. It consists of an

inner and an outer sheet, the fibres being disposed circularly in the

inner, longitudinally in the outer sheet, with a slight departure from

this regular arrangement in the wall of the stomach. On its outside the

canal is invested by peritoneum, a layer of flattened epithelial cells

supported by connective tissue. The abdominal wall also is lined with

peritoneum. The smooth moist surface of the peritoneum covering the

intestines glides on the peritoneum lining the abdominal wall. Between

the two is a “potential” space. In dropsy, fluid accumulates within

this space. In a healthy condition the apposed surfaces are merely

moist.

The movements of the intestines are of two kinds. At all times they

exhibit swaying movements, in the production of which the longitudinal

fibres play the chief part, although the circular fibres also contract.

The object of this undulation is to thoroughly mix the contents of

the gut with its secretions. If pills of subnitrate of bismuth are

administered, and their progress observed by the aid of Röntgen rays,

they are seen to oscillate backwards and forwards on their way down

the canal. The slower vermicular movement which squeezes the contents

forwards is called “peristalsis.” It resembles the progressive

contraction of an elastic tube which may be effected by drawing it

through a ring, but is rather more complicated. At the point at which

it is occurring the circular coat is sharply contracted. Above this

it is also somewhat contracted; below it is relaxed. The longitudinal

fibres, using the constricted portion as a _point d’appui_, pull up the

segment of the intestine which lies immediately below it, drawing it

off the contents of the tube as a glove from a finger.

When food is swallowed, it falls down the œsophagus, aided by slight

peristalsis. As soon as sufficient has accumulated on the upper surface

of the cardiac valve of the stomach, the valve relaxes; at the same

time a stronger peristalsis of the lower portion of the œsophagus

squeezes its contents into the stomach. Food remains in the stomach

until it has reached a certain stage of digestion, the chief object

of which is its subdivision into small particles. Until this stage

is reached, the pyloric valve is firmly closed. The contractions of

the wall of the stomach drive its contents round and round—down the

greater and up the lesser curvature—mixing them thoroughly with the

gastric juice (_cf._ p. 124). As the acidity of the mixture increases,

the peristaltic contractions of the stomach become more vigorous,

until, the pyloric valve relaxing, the food is little by little driven

into the duodenum.

The alimentary canal has an abundant supply of nerves from the vagus

and the sympathetic systems. It contains also within its own wall an

enormous quantity of nerve-fibres and nerve-cells. They are disposed

as two plexuses, one in the submucosa, the other between the circular

and longitudinal muscular coats. In a specimen successfully stained

with methylene-blue, they are so abundant as to give the impression

that every plain muscle-cell may have its own separate nerve-twig.

Nevertheless, the contraction of the muscle-cells may take place

independently of all nerve-influence—independently, even, of the local

mechanism, the plexus referred to above. Nicotin applied to the wall

of the intestine paralyses the local nerves; yet rhythmic contractions

still occur. They are, however, no longer progressive. They do not

drive the contents of the intestine forwards. Co-ordinated contraction

is observed so long as the local mechanism is intact, even though all

external nerves have been cut. The intestines have their own nerve

cells and fibres, which, acting as a linked system of reflex centres,

provide for the harmonious contraction of their walls. External nerves,

sympathetic and splanchnic, convey impulses which either intensify the

movements or inhibit them, as need may be.

In the matter of its nerve-supply, the alimentary canal stands apart

from the other organs of the body. It may be supposed that it presents

a more primitive condition. Its muscular fibres have the power of

contracting spontaneously. The pressure of the contents of the tube

acts as a stimulus. When the fibres are stretched, they contract.

When the tube is dilated, its muscles endeavour to restore it to its

normal calibre. Such direct action would not, however, provide for the

forward passage of its contents. To bring about peristalsis, a nervous

mechanism is needed, as abundant and complicated as that which ensures

the progress of a slug or a worm. To deal satisfactorily with the

various contents of the tube—liquid, solid, gaseous—the mechanism

must be capable of complicated adjustments. The dilated portions of

the tube—stomach, cæcum coli, rectum—require special arrangements of

muscle and nerve. Nor is the canal altogether independent of the rest

of the body. To a large extent its work is carried on without regard

to the activities of other organs, yet it is not wholly free from the

control of the central nervous system. It is regulated by means of both

afferent and efferent nerves of the vagus and sympathetic. Even the

brain has something to say with regard to the way in which it shall

contract. It is a matter of common experience that emotional influences

may affect the movements of the stomach and intestines—“His bowels

yearned.”

Normally, vomiting is due to irritation of the endings of the vagus

nerve in the stomach, although the afferent impulses may have other

sources. Touching the upper surface of the epiglottis with the finger

will provoke the reflex. So also will stimulation of the olfactory

nerves by a foul smell. In this latter case the emotion of disgust to

which the odour gives rise brings about the reflex action. A flow of

saliva precedes the act of vomiting. A deep inspiration is then taken,

in order that for a time the lungs may be independent of a fresh supply

of air. The glottis is closed, the diaphragm fixed. Contraction of the

abdominal wall presses the stomach against the diaphragm; its cardiac

sphincter relaxes, and its contents are squirted into the œsophagus,

which undergoes a forcible retrogressive peristalsis.

It is interesting to note the difference between carnivora and

herbivora in regard to vomiting. Carnivora swallow fur and other

indigestible materials, as well as many unwholesome things which they

need to be able to return. A dog can, apparently, vomit at will.

Never, while in a state of nature, do herbivora need to return the

contents of the stomach. No provision is made for vomiting. A heifer

which has strayed into a dewy clover-field is not unlikely to die

from the effects of distension of its paunch, if relief be not given

by opening it with a knife. In a horse the cardiac sphincter is

strong, the pyloric weak. Pressure on the stomach tends to drive its

contents through the pyloric valve into the duodenum, not backwards

into the œsophagus. The stomach is not so placed as to allow of its

being compressed between the wall of the abdomen and the diaphragm.

Horses cannot vomit. It is a mistake to suppose that they suffer from

sea-sickness. In rough weather they sweat, their limbs tremble, they

go off their feed; but these symptoms are probably due to the fatigue

which results from excessive anxiety to maintain their balance, and

to fear. We can never know their feelings, but there is no reason for

supposing that they experience the sensation of nausea.

Vomiting is a frequent symptom of cerebral disturbance. The

fluctuations of pressure which the brain experiences as it rocks about

on its “water-bed” within the skull is the cause of sea-sickness. Yet

the motion of a ship may produce violent headache without nausea, the

brain only, not the stomach, appearing to be troubled by the motion.

Not that headache is a pain “inside the head.” Nor is it properly

described as a pain in the scalp, although the messages which are felt

in consciousness as headache originate in the endings of the nerves

of the skin which covers the skull. The excessive sensitiveness of

these nerves is due to vaso-motor conditions, usually the dilation,

occasionally the constriction, of the bloodvessels of the scalp. But

the vaso-motor condition is sympathetic with the disturbance of the

brain; and the special urgency or efficiency of the messages from the

skin results from their being delivered into excited brain-tissue.

Nausea and headache are equally symptoms of the irritability of the

brain caused by the motion of the ship. In one case messages from the

stomach, in the other case messages from the scalp, acquire undue

importance, owing to the agitated condition of the brain-tissue

through which they pass. Not uncommonly the voyager, who wakes in

the morning reconciled to the changes of pressure which he has

experienced while recumbent, finds, when he stands upright, that the

base of his brain is as sensitive as ever. Visual sensations also

contribute to the brain-disturbance. So, too, do the movements of

endolymph in the semicircular canals (_cf._ p. 335). It is, indeed,

possible that this last factor is more important than the variations

in pressure on the surface of the brain. Probably it accounts for the

after-image of rolling which almost everyone experiences for at least

a day after leaving the ship. Its cause being cerebral, the tendency

to sea-sickness can be controlled by drugs which, like the bromides,

chloral, alcohol, etc., deaden the brain.

=Salivary Glands.=—The secretion which accumulates in the mouth is

the combined product of the sublingual, submaxillary, and parotid

glands. It is a very thin, watery solution containing not more than 0·5

per cent. of solid substance. If red litmus-paper is moistened with

saliva, it becomes blue, showing that the secretion is alkaline. It

contains a ferment, ptyalin, which digests starch. The action of this

ferment can be demonstrated by holding in the mouth for half a minute

some warm starch mucilage—boiled arrowroot, for example. It quickly

loses its viscidity owing to the conversion of starch into sugar.

Chemically this change may be demonstrated by adding iodine-water to

a specimen of the starch before and after action. Before the starch

is taken into the mouth the iodine turns it blue (a characteristic

reaction for starch). After it has been exposed to the digestive action

of the saliva, iodine fails to colour the mixture, which now contains

no starch. All the starch has been converted into dextrin and sugar.

If unboiled arrowroot is placed in the mouth, some sugar is produced,

but the process of conversion is very slow. It is almost impossible to

digest raw starch in the mouth sufficiently to render it insusceptible

to the colouring action of iodine. The sugar produced by the action of

ptyalin is of the same nature as that which appears during the malting

of barley. It is therefore termed “maltose.” It closely resembles

grape-sugar, but is not identical with it.

=The Secretion of Saliva.=—The accessibility of the salivary glands,

and especially of the submaxillary, has led to their being used for a

very large number of experiments. They have been studied with the aim

of coming to an understanding of the mechanism of secretion in general.

The glands consist of tubes of gland-cells, each tube suspended in

a basket of connective tissue, in a bath of lymph (_cf._ Fig. 3).

Innumerable capillary bloodvessels traverse the lymph-bath. The

arteries which carry blood to the gland are supplied with nerves, which

regulate their calibre, and therefore determine the amount of blood

which passes through the capillaries into which they break up. The

glands also are supplied with nerves which influence their functional

activity. Nutrient substances and oxygen pass out of the blood into

the lymph. Carbonic acid passes into the blood from the lymph. Waste

products are either carried away in the lymph-stream, or make their

way through the walls of the capillaries into the blood. Many problems

present themselves for solution. How does the amount of work done by

the gland affect its supply of blood? Does the quantity of saliva

secreted vary directly with the pressure of lymph in the spaces by

which the gland is surrounded? Is this pressure wholly dependent upon

the pressure of the blood? Are the substances secreted by the gland

supplied as such by the blood, or does the gland make the ptyalin and

mucus which it secretes? If it makes its secernable products, what

materials does it abstract from the blood for the purpose of their

manufacture? Does it use the whole of these materials, whatever they

may be, or does it use part only and return the residue to the lymph?

Does it make its products only when it is actively secreting, or is it

always making them, and storing them in its cells in order that it may

have a supply to discharge when called upon by the stimulation which

results from the presence of food in the mouth? Is their discharge

merely a washing out due to the rush of fluid which occurs when the

bloodvessels are dilated, or can the gland-cells expel their products

in response to nervous action? In what way do the nerves of the

gland influence secretion? Do they call for increased production, or

increased output, or both? These are some of the problems which the

exposed situation of the submaxillary gland allows physiologists to

tackle.

By means of a very simple operation, the ducts of one or both parotid

or submaxillary glands can be brought to the skin, and made to pour

their secretions on to the surface instead of into the mouth. The flow

under various circumstances can be watched. The saliva can be collected

and measured.

The nerves of the submaxillary gland are easily isolated. A nerve

leaves the seventh (or facial), crosses the drum of the ear, comes

out through a minute crevice in the skull, and runs for some little

distance as a separate nerve before it applies itself to the lingual

branch of the fifth, which runs along the side of the tongue. Owing to

its passage across the tympanic cavity (drum of the ear), it is termed

“chorda tympani.” As its fibres are very small, they can be recognized

wherever they form a part of the lingual nerve. They leave the lingual

to go to a ganglion, the submaxillary ganglion, from which the gland

is supplied. The gland also receives branches from the sympathetic

nerve which ascends the neck. The last-named branches accompany the

facial artery. Stimulation of either of these nerves causes the gland

to secrete. The flow of saliva which follows stimulation of the chorda

tympani is much more copious than that which follows stimulation of

the sympathetic, and as a rule it contains far less organic matter,

although about the same amount of mineral salts. Under normal

conditions the activity of the chorda tympani is brought into play in

a reflex manner by impulses which travel up the nerves of taste (the

lingual and glosso-pharyngeal) to the cerebro-spinal axis; but almost

any other nerve will serve as an afferent path. The gland may also, as

we shall presently explain, be called into activity by the cortex of

the brain.

It is certain that in the case of the submaxillary gland secretion

is not the direct result of increased blood-pressure. It is not a

case of filtration from the blood through certain membranes and

cells into the salivary duct. Atropin (belladonna) dilates the

bloodvessels, increasing blood-pressure, but it stops secretion.

After belladonna-poisoning, the mouth, like the skin, is hot and dry.

Other drugs there are which provoke a certain amount of secretion,

even after the bloodvessels going to the gland have been tied. It is

possible, by stimulating the chorda tympani, to obtain a pressure in

the fluid in the duct very much greater than that in the bloodvessels

which supply the gland. Here we have clear proof that secretion is not

filtration. Filtration is the passage of fluid through a filter-bed

from a higher to a lower pressure. In filtration, moreover, soluble

diffusible salts accompany the water. The saliva contains only half as

much of these diffusible salts as the blood. Therefore the gland tissue

stops half the salts. Secretion is an active process carried out by

the gland-cells, under the influence of nerves, in opposition to the

laws of filtration. The gland-cells determine how much water shall pass

through them and what percentage of salts shall accompany the water.

How does a gland-cell make the substance which it secretes? There is no

reason for supposing that the ptyalin or the mucus which the salivary

glands secrete is present in the blood, either ready formed, or, as it

were, half formed, in combinations which can be easily broken up. All

the evidence obtainable points to the conclusion that the gland-cells

take out of the lymph proteid materials from which they manufacture

the peculiar substances which they secrete. During rest, granules

accumulate in the cells. During activity they disappear. It has been

shown in the case of the gastric glands that these granules consist

of the special ferment which the gland secretes, in an inactive form.

It may be that it is combined with a substance which prevents it from

exerting its digestive action on the cells within which it is made;

damped, as gunpowder is damped during transit. Or it may be that it is

not a finished ferment; it may need a further addition to its molecule.

During activity, while the granules disappear, proteins accumulate at

the bases of the cells, giving to a tube of gland-cells the appearance

of a peripheral non-granular zone. This proteid substance must have

come from the lymph, and the inference seems inevitable that the cells

have taken into their protoplasm a supply of material which will

serve for the manufacture of additional granules. Each gland-cell

is therefore an independent unit. By its own activity it takes up

materials from the lymph, out of which it manufactures its own special

products. It stores its products until they are wanted. Then by its

own activity it extrudes them into the lumen of the gland-tube. It

has, indeed, been shown that, when the nerve going to a salivary

gland is stimulated, the gland shrinks, notwithstanding the great

dilation of its bloodvessels. Under the influence of the stimulation

the granules in the gland-cells imbibe water, swell up, and escape

from the cells. The cells discharge their accumulated stores, in the

first instance, more rapidly than they take up materials (even fluid)

from the blood. For its knowledge (if the term may pass) of what is

wanted the gland-cell is dependent upon messages which reach it through

the nervous system. These messages take origin in the endings of the

sensory nerves of the mouth, pass up to the brain, and are reflected

down the nerves to the gland. So accurate is the information conveyed

to the glands, that when a horse transfers the work of mastication from

one side of its mouth to the other, as it is in the habit of doing

about every quarter of an hour, the flow of saliva from the parotid

gland on the masticating side is increased; on the other side it is

diminished. Two or three times as much saliva is poured out on the one

side as on the other.

Not only is the amount of saliva poured out in response to stimulation

proportional to the needs of mastication, but the kind of saliva is

adapted to the nature of the food. In a dog—and this is an observation

which can be made only on an animal which lives on a mixed diet—it is

possible to determine the amount of the two kinds of saliva secreted

and the relation of flow to food. When meat is given to the animal, the

submaxillary gland yields its secretion; when it is fed on biscuit,

abundance of the watery parotid saliva is poured forth. A mouthful of

sand also causes the parotid saliva to flow, in order that the sand may

be washed out of the mouth.

More remarkable than the response to direct stimulation is the effect

produced by the sight and smell of food. When meat is shown to a dog,

submaxillary saliva begins to flow; when it is offered bread, parotid

saliva is secreted. And the activity of the glands is not merely a

nervous reflex independent of the animal’s mind. The moment the dog

realizes that it is being played with—that there is no intention of

giving it the coveted food—the flow of saliva ceases. An emotion

may check secretion when every physiological condition is demanding

it. This is the explanation of the Rice Ordeal. Dry rice provokes a

flow of saliva in the mouth of all save the guilty man. Response to

mental impressions is a matter of the greatest consequence in the

physiology of digestion. It holds good in the case of the secretion

of gastric juice equally with that of saliva. The sight and smell of

food sets the juice flowing into the stomach, and the more desirable

the food, the more attractive its appearance, the more stimulating

its smell, the more rapidly does the secretion flow. Here we touch

upon a theme which hardly needs exhaustive treatment. It is not

the stoutest people who eat the most, although an impartial survey

of one’s well nourished friends will show them to be persons who

“take kindly to their victuals.” A small quantity of food perfectly

digested is more nourishing than much food which the digestive organs

do not efficiently prepare for assimilation. Good digestion waits on

appetite; and appetite, in civilized man, is something more than a mere

physical need of food. The hunger which leads to the bolting of food

without pleasurable anticipation, without mastication, without any

consideration of the quality of the viands, is a harmful craving which

ends in imperfect assimilation. It is more profitable to toy with a

_hors d’œuvre_ than to engulf, unthinking, a plateful of beef. But we

have said enough to suggest reflections to those who take no thought

as to what they shall eat or what they shall drink; and few who take

thought need to be convinced.

=The Stomach.=—The sight and smell of food, its presence in the

mouth, and the performance of mastication, which induces a secretion

of saliva, gives rise at the same time to a flow of gastric juice.

It is psychic stimulation and the act of eating which cause gastric

juice to ooze from the gland-tubes of the stomach at the commencement

of digestion, not the stimulation of nerve-endings by food which has

passed down the œsophagus. As a consequence of gunshot wounds, or

as the result of operations performed for the purpose of relieving

patients whose œsophagus has become blocked, numerous cases have been

recorded in which a fistulous opening into the stomach has made it

possible to study the interior of this organ. Such cases present an

opportunity of watching the digestion of various foods introduced

through the opening, and of collecting gastric juice for purposes

of analysis. A similar condition has been established in animals by

operative means. The œsophagus having been cut, and the cut end sutured

to the margins of an aperture in the skin, food taken by the mouth

escaped by this opening instead of passing into the stomach. A similar

opening was made into the stomach for the insertion of food, and for

the purpose of studying the effects of reflex stimulation of the

gastric glands. As soon as food was introduced into the mouth, gastric

juice began to flow. The advantage of this experimental method lies in

the fact that the juice secreted was a pure juice—not mixed with food,

as in all the earlier experiments in which, the stomach being opened

without diversion of the œsophagus, the presence of food within it was

the stimulus which led to secretion. No juice flowed in the absence of

stimulation; nor was the secretion normal in composition when provoked

by a mechanical stimulus, such as the tickling of the gastric mucous

membrane by a feather.

My lord the stomach! He is not the only, nor is he the chief, agent

in digestion; but with him rests the decision as to whether the food

offered to the alimentary tract is suitable in quality and quantity. He

is offended if it be not offered with all the circumstance and ceremony

which becomes his rank. As an intimation that he is about to receive

food, he accepts the news from the mouth that its nerve-endings are

subject to mechanical stimulation. But the chewing of indiarubber would

produce a like effect. The stomach, therefore, confers with the organs

of taste and smell. If their report is favourable, he argues that the

substance which the teeth are crushing will justify an outflow of

gastric juice. He responds most generously when prolonged mastication

assures him that he may trust to receiving the food in a sufficiently

subdivided state. At our peril we neglect to propitiate my lord. Not

always debonair when treated with consideration, he is morose or

petulant when slighted. Never content with lip-service, he exacts the

labour of teeth and tongue and palate. The tribute we offer may be of

the best—savoury, wholesome, well cooked, well chewed—but if it be

not tendered with some degree of love, if thoughts are concentrated

on other things, if no attention is devoted to the meal, if no sense

of liking accompanies our offering, my lord the stomach on his part

affords the viands an indifferent reception. In consulting our own

tastes we are to a large extent consulting the needs of the stomach.

Ravenous and excessive feeding is not an exhibition of taste; it is a

return to the instinct of the savage, who was never sure that he would

get his full share, and was afraid to trust that another meal would be

obtainable when nature declared it due. Some degree of epicureanism

is favourable to digestion. The flow of gastric juice in the stomach

occurs reflexly in response to the emotion of appetite, to stimulation

of the nerves of taste and smell, to the obscure sensations which

accompany the activity of the muscles of mastication.

The gastric juice secreted in a day amounts probably to about 8 or 9

pints. To this we must add, when considering the quantity of fluid

which passes through the stomach, the saliva, which certainly reaches

as much as 2 pints, and the beverages taken with food.

Gastric juice collected in the manner described above is a clear,

colourless, inodorous fluid. It is very acid, and so powerfully peptic

as to digest its own weight of coagulated white of egg. Its solid

constituents amount to 0·5 per cent. They consist of the two ferments

pepsin and rennin, with traces of proteins and mucin, and various

inorganic salts. Its acidity is due to free hydrochloric acid to the

amount of 0·2 per cent. This acid is more or less in combination with

the pepsin. In pure gastric juice hydrochloric acid is the only acid

present; but when mixed with food the juice contains other acids also,

especially lactic.

When food first reaches the stomach, the alkaline saliva which

accompanies it neutralizes the acidity of the gastric juice. For some

time, probably about half an hour, the conversion of starch into sugar

is still carried on by the ptyalin of the saliva, owing chiefly to the

difficulty which the gastric juice encounters in permeating the masses

of masticated food. The _Bacillus acidi lactici_ is always present in

the stomach. It converts some of the sugar into lactic acid; of this a

small quantity is further changed into butyric and acetic acids, with

the formation of carbonic acid and hydrogen gas. After a while the

lactic acid is absorbed, and hydrochloric acid alone remains.

The secretion by the gastric glands of so powerful a mineral acid

as hydrochloric has always aroused interest. How is it possible for

the gland-cells to produce it without injury to them selves, or for

the stomach to contain it without self-digestion? Many chemical and

physical theories have been advanced in the belief that they rendered

the process of its production less difficult to understand. All such

theories are, however, inadequate to explain the secretion as a

discontinuous process, which occurs only as a response to demand. That

the source of the acid is the sodic chloride which the gland-cells

take from the blood does not need assertion, but we cannot picture

the process by which this exceedingly stable compound is decomposed

otherwise than on the assumption that weaker acids, or, rather, acid

salts, are also absorbed by the cells, and that, in accordance with the

laws which govern the composition of salts in solution, an exchange

of acids occurs. If sodic chloride and any acid salt—acid phosphate

of sodium, for example—are in solution in water, the salts do not

retain their form as we know them when isolated by crystallization.

The mixture contains “free” hydrochloric as well as “free” phosphoric

acid. It may be assumed that within secreting cells a similar exchange

of acids takes place. By a process which we term “vital,” the acids are

kept apart, and the hydrochloric acid is extruded by the cells. In the

present state of knowledge this vital action is mysterious; but it is

no more mysterious than the isolation of pepsin, or any other metabolic

event which occurs within a cell.

The proteolytic ferment pepsin is active only in an acid medium. Yet

apart from its digestive function as an ally of pepsin, hydrochloric

acid by itself also exerts a valuable disintegrating action on certain

constituents of the food. Possibly the most important results of the

presence of free hydrochloric acid in the great chamber into which food

is first received are due to its disinfective property. It destroys

all the putrefactive germs which accompany the food, and many germs

which, if introduced into the blood, would give rise to disease. It

also destroys the germs which multiply in the stomach towards the end

of each interval between two meals. When withdrawn from the body,

gastric juice will keep an indefinite time, if evaporation of the acid

be prevented.

=Pancreas.=—In structure the pancreas presents a marked resemblance

to the salivary glands. Probably this resemblance is merely

superficial. Minute examination reveals points, apparently of great

morphological importance, in which they differ. In the gland-tubes of

the salivary glands, and, indeed, in all glands with the exception

of the pancreas, secreting cells project into the lumen. The

secreting cells of the pancreas are invested internally by a layer of

flattened scales (intra-acinar cells). They lie, therefore, between

the basement membrane which invests them externally and this second

layer of flattened cells which separates them from the lumen of the

tube. At a very early date in embryonic life the gland-cells of the

pancreas are filled with highly refracting granules. As this occurs

long before any digestive action is called for, it may be taken as

indicating that the pancreas has functions which other glands—the

salivary, for example—do not possess. These granules do not, however,

appear in all parts of the tubes. Certain portions of the tubes

remain undeveloped—fail, that is to say, to acquire a secreting

function—even in adult life. Such patches of cells, not disposed in

gland-tubes, are known as islands of Langerhans. When the pancreas is

over-stimulated by artificial means, leading to its extreme exhaustion,

large portions of its glandular substance return to this primitive

condition. The gland-cells not only discharge their stores of granules,

but they lose the greater part of their cell-protoplasm. It would seem

that, in their effort to meet the demand for ferments, they use up

their own cell-substance in their manufacture. Having exhausted their

coal, they stoke the furnace with the looms and furniture of the mill.

It may be that other glands would do the same if it were possible to

stimulate them as strongly as the pancreas can be stimulated. The

result is probably due to the extreme susceptibility of the pancreas to

the action of secretin, a substance made in the intestine. Secretin can

be isolated and injected into the blood. We shall refer again to this

chemical stimulation of the pancreas when tracing the progress of food

through the alimentary canal.

The secretion of the pancreas is a clear, colourless, alkaline

liquid of syrupy consistence. The quantity of juice secreted is

relatively small, but the organic substances which it contains are

in a concentrated form. They constitute as much as 10 per cent. of

the pancreatic juice. Proteins are present, if the juice be fresh. If

it has stood for any length of time, they are found as peptones. The

digestive ferments of pancreatic juice are the most powerful which are

secreted into the alimentary canal.

=Bile.=—In its most important functions the liver has no relation

to digestion. It is a storehouse of absorbed food. This organ will

therefore be treated in a separate chapter. The bile which the liver

secretes into the alimentary canal has no chemical action on any of

the constituents of food, with the exception of a feeble tendency to

digest starch. Yet it is in some degree accessory to digestion. Poured

into the second portion of the duodenum through an orifice common to

the liver and the pancreas, it mingles with the semi-digested food,

or “chyme,” which, about two hours after a meal, passes through the

pyloric valve. Gastric digestion has converted the greater part of the

proteid constituents of the food into peptones or intermediate stages.

The proteoses or propeptones—a name is needed for the intermediate

products of proteid digestion which does not commit us to any theory

as to their chemical constitution—are quickly peptonized by the

pancreatic juice. But portions of the proteins have escaped the action

of gastric juice, or have at most been affected by its acid only;

these are precipitated by the bile-salts on the mucous membrane of

the small intestine, which is raised into projecting flanges for the

purpose of delaying the passage of the chyme, in order that it may

be thoroughly submitted to the digestive action of pancreatic juice.

Bile-salts also favour the digestion of fat, and its passage through

the intestinal wall. The action of bile-salts in spreading fats is well

known to artists. Ox-gall is smeared upon glass when it is desired

to apply oil-paints to its surface. When mixed with oil, it causes

its emulsification, or breaking up into microscopic globules. In the

absence of bile, but little fat passes into the lymph-vessels which

convey digested food from the intestine to the thoracic duct, and so to

the great veins of the neck. Its action is mechanical. It favours the

digestion of fats by rendering them easily amenable to hydrolysis by

pancreatic juice.

Bile as secreted by the liver is a clear, limpid fluid of low specific

gravity; but during its stay in the gall-bladder it is concentrated by

absorption of water, and mucin is added to it. It contains “bile-salts”

of complex constitution. These salts favour the solution of certain

by-products of cell-metabolism, cholesterin and lecithin; substances

which are formed in many cells, both in animals and plants. Cholesterin

occurs most abundantly in nerve-tissue and in blood-corpuscles.

Lecithin also is a by-product of the metabolism of nerve-tissue.

Protoplasm appears to be incapable of oxidizing these substances, as

it does other products of metabolism. Other substances of equally

complex constitution are reduced to urea if they contain nitrogen;

to water and carbonic acid if nitrogen be absent. Cholesterin and

lecithin have to be eliminated without further change. Some of the

cholesterin is excreted by the sebaceous glands of the skin. It is the

chief constituent of “lanoline” prepared from sheep’s wool; an unguent

which owes its valuable properties to the resistance which cholesterin

offers to cell action, and therefore to the action of living ferments.

Bacteria cannot turn it rancid. The sebaceous glands have the power

of directing metabolism into a channel in which cholesterin is the

chief product, but apparently all cells make it in small quantity. The

bile-salts carry cholesterin and lecithin into the alimentary canal,

from which they are not reabsorbed. Some of the bile-salts are lost to

the body, but the remainder re-enter the circulation, and recommence

their work as vehicles for these inoxidizable and insoluble substances.

In the gall-bladder cholesterin is apt to separate out from the bile

in the form of gall-stones; but whether this is due to an excess of

cholesterin in the bile, or to an abnormal, inflammatory condition of

the lining membrane of the gall-bladder, is still an open question.

Bile also contains bile-pigments. Their colour varies in different

animals, and changes according as the bile is exposed to the air, or

subject to the action of reducing agents. If oxidized, the colour

is green (biliverdin); if reduced, brownish-yellow (bilirubin).

Bile-pigment is formed from hæmoglobin, the colouring matter of the

blood, after the removal of its iron. Worn-out red blood-corpuscles

are destroyed in the spleen, in the manner already described, but it

is uncertain whether the conversion of the hæmoglobin thus set free

into bilirubin occurs in the spleen, or whether this chemical change

is reserved for the liver. Physiologists incline to the view that the

liver is the seat of the change.

=Intestinal Juice.=—The mucous membrane of the alimentary tract, as

far down as the middle of the rectum, is, as previously stated (p.

102), studded with tubular glands. They secrete a light-yellow fluid,

alkaline in reaction, and opalescent. Its most important property is

due to a ferment which converts cane-sugar into a mixture of dextrose

and levulose, and changes maltose—the sugar produced by the action on

starch of saliva and pancreatic juice—into dextrose. It is in the form

of dextrose that sugar is carried about the body and assimilated by the

tissues.

Intestinal juice also contains a ferment, erepsin, which shakes to

pieces the heavy molecules of peptones and partly formed peptones.

Under its influence they break up into comparatively simple bodies

containing the radicle of ammonia. Substances containing an NH₂

group—one H of NH₃ (ammonia) having been given up, in order that

the group may have a “free arm” with which to link on to the other

component parts of the molecule—are termed “amides.” The amides

which are most characteristic of the action of erepsin are leucin,

an amidated fatty acid; and tyrosin, an amidated aromatic acid. The

tendency of proteins to break up along these two lines—the fatty acid

line and the aromatic acid line—is of considerable interest. The one

line is represented by acetic acid, CH₃COOH; the other contains the

hexone radicle, C₆H₆. Benzoic acid, C₆H₅COOH, is representative of the

latter. It used to be thought that proteins which were shaken into

simple bodies such as amides were lost to the economy. Their downward

career was a foregone conclusion. There could be no arresting it

before they brought up at the bottom—as urea, CO(NH₂)₂—the diamide

of carbonic acid. It was even supposed that this disintegration of

proteins was a provision for getting rid of the surplus animal food

which we consume. Physiological chemists now take quite a different

view. They believe that the epithelial wall of the intestine through

which these substances are absorbed, or the liver, to which they are

carried by the portal blood-stream, has the power of recombining these

fragments into the complex protein edifice. It is even thought that

disintegration is a necessary preliminary to the rearrangement of the

sub-groups. A large variety of proteins is ingested as food. Many of

them, especially the vegetable proteins, are quite foreign to the body.

By the activity of pancreatic juice and erepsin, they are broken into

small and relatively stable groups of atoms, which are again fitted

together into the particular forms of protein which are of use to the

economy.

=The Story of a Meal.=—The chemistry of digestion will be understood

most readily if the constituents of a meal are traced from their

entrance into the mouth to their absorption through the wall of the

alimentary canal, or abandonment as indigestible.

We may describe as a typical meal one consisting of bread, vegetables,

cane-sugar, meat, milk, fat, and cheese. In the mouth the various foods

are crushed and mixed with the alkaline secretions of the salivary

glands. A certain amount of the cooked starch contained in the bread

is changed into maltose. In the stomach the digestion of starch is

continued for a time, but a large part even of the cooked starch awaits

the action of pancreatic juice. A certain amount of cane-sugar is

converted into dextrose and levulose, which are rapidly absorbed into

the blood; but this action is due to hydrochloric acid, and probably

affects a comparatively small part of the cane-sugar swallowed. Fat is

quite unaltered in the stomach. All proteins are attacked by pepsin,

but some yield to digestion more readily than others. Gluten of bread,

like all vegetable proteins, is comparatively resistant; but since

it is presented to the action of pepsin in small quantities and in a

spongy form—very suitable for digestion—it is probable that most of

it is peptonized in the stomach. Chemists experimenting with gastric

juice taken from the stomach, and reproducing the conditions as to

temperature, removal of products of action, etc., as closely as it is

possible to reproduce them in the laboratory, find that the various

foods take different times to digest. The proteins of meat are more

quickly peptonized when raw than after coagulation by heat. The same

is true of white of egg. Amongst different varieties of cooked flesh,

beef is more quickly peptonized than fish. The casein of milk is more

quickly peptonized than any other protein; and it also is no exception

to the rule that digestibility is diminished by cooking. Similar data

may be obtained for all foods. They are no doubt useful indications of

the course of action which we may expect to occur within the stomach,

but we can never be sure that my lord will obey the ruling of the

chemist. Practice with a captive golf-ball is a useful preparation

for the game; but there are conditions on the links which cannot be

reproduced on the lawn. In an artificial stomach the clean fibre of

raw fish digests more slowly than raw beef. Even when the beef is

roasted and the fish fried or boiled in the ordinary way, the beef

disappears through the dialyser (the bag of membrane suspended in a

vessel of warm water in which experimental digestion is carried out)

more quickly than the fish. Nevertheless, the living stomach is better

disposed towards a mixed meal containing a certain weight of fish than

towards a meal in which, the other constituents remaining the same,

beef takes the place of fish. Important conclusions may, no doubt,

be drawn from observations of the time occupied in the peptonization

of pure food—_i.e._, fibrin, white of egg, clean meat, etc.—under

conditions simulating those which are present in the stomach; but

they must be accepted with many reservations. In the stomach it is

not pure substances, but mixtures, that the gastric juice has to deal

with. And here a most important factor comes into play, to which

further reference will be made later on. The amount and quality of the

secretion of the gastric glands depends upon the nature of the food.

Hence a food, or a combination of foods, which digest readily in the

laboratory may take a long time to disappear from the stomach, and

_vice versâ_. Digestibility depends upon the nature of the food. It

depends also upon its physical state. To take simple illustrations:

Cheese contains coagulated casein, one of the most easily digestible

of proteins, but the casein is intimately mixed with fat, upon which

gastric juice can make no impression. Even when finely divided, the

particles of casein are protected from the action of the juice by

fat. In the same way the meat of pork is as digestible as mutton, but

the fat of pork is quickly melted and very liquid. In the process of

cooking the muscle-fibres become saturated with fat.

It is not the function of the stomach to complete digestion. Its

business is to initiate it. Food which reaches the stomach in fragments

is reduced to a condition in which its digestion will be readily

completed by pancreatic juice. Gastric digestion produces a much

larger proportion of intermediate products, proteoses or propeptones,

than does digestion in the duodenum. Such intermediate products are

quickly dealt with by pancreatic juice. Artificial tests of relative

digestibility do not, as a rule, take the amount of propeptones formed

in a given time into account. When considering the digestion of a

typical meal, we must bear in mind that it is not the duty of the

stomach to pass as much sugar, peptone, and fat as possible into the

blood. In fact, very few of the products of digestion are absorbed

by the bloodvessels of the stomach. The impermeability of its mucous

membrane is shown by the fact that hardly any of the water swallowed

passes through the stomach-wall. Practically all the water ingested

leaves the stomach through the pyloric valve. Various salts, some

sugar, and peptones are taken up by the vessels of the stomach; but the

bulk of all the different kinds of food passes into the duodenum in a

semi-digested state. The function of the stomach is to carry digestion

through a preliminary stage. The process will be completed in the small

intestine. It is to be noted that, although water is not absorbed by

the stomach-wall, alcohol passes through it with great rapidity. The

same is true of the various crystalline nitrogenous bodies found in

meat-extracts, and also of the essential principles of tea and coffee,

which chemically belong to the same class. All these substances are

degradation products of proteins produced by oxidation, far advanced

along the road to urea. In this selective absorption we see proof of

the activity of the cells of the mucous membrane. They take up the

substances which it is desirable to remove from the contents of the

stomach. Some may be wanted by the body for its immediate use; others

are better out of the way, because they are prejudicial to the progress

of digestion.

When contemplating the activity of the cells of the gastric mucous

membrane, we feel the need of an adjective which shall express our

recognition of the fact that they have a power which we cannot confer

upon our clumsy mechanical imitation stomach. They can discriminate.

“Vital” is the only term available, though much abused. Using it

without prejudice, as lawyers say, we speak of the “vital activity” of

the cells when we wish to imply that things happen in a living stomach

for which we cannot make provision in a model. Of the many substances

which make their appearance as digestion proceeds, some are absorbed,

others left in the mixture.

The mucous membrane shows its power of controlling digestion in yet

another way. In the neighbourhood of the pylorus its structure is

unlike that which it presents elsewhere. The gastric glands are short,

and tend to branch. Their lining cells are all of the same kind.

Over the greater part of the inner wall of the stomach the tubes

are long. They do not branch. The cells which line them are of two

kinds: small cubical cells (the term refers to their form as seen in

section), similar to those of the pyloric glands; large oval cells,

placed with their longest axes in the same direction as the axis of

the gland-tube. These oval cells do not project into the bore or

lumen of the tube, but are displaced from it by the cubical cells.

They rest on the investing, or basement, membrane. All parts of the

gastric mucous membrane secrete pepsin, although the pyloric portion

produces very little; the area which contains oval cells alone secretes

hydrochloric acid. If a short time after a meal an extract is made

from some of the mucous membrane near the pylorus, by pounding it

with salt-solution and sand to break up its cells, this extract, when

filtered and injected into the blood, stimulates the glands of the

cardiac end of the stomach. Under its influence they pour out both

pepsin and hydrochloric acid. The extract contains a substance which

acts as a chemical messenger. It is a representative of a class of

bodies which play a most important part in co-ordinating the activities

of the various organs. Hitherto physiologists have concerned themselves

with the visible or “external” secretions of glands. They have shown

how the production of these secretions is controlled by the nervous

system. Recently they have discovered that another set of influences

has to be taken into consideration. Glands, and possibly all other

tissues, take from the blood the materials out of which they make their

characteristic secretions, or, if they do not discharge secretions, the

substances which they require for the building of their own structures,

and return to the blood “internal secretions” which act as stimuli to

other tissues with which they are linked in harmonious co-operation.

The active principles of internal secretions have been termed

“hormones”—from ὁρμάω, I announce. The glands of the pyloric mucous

membrane secrete a hormone which calls upon the rest of the membrane to

pour out gastric juice (_cf._ p. 89).

What induces the cells of the pyloric mucous membrane to produce the

gastric hormone? Their activity in this respect evidently depends upon

the presence in the stomach of partially digested proteid substances.

The cells judge, as it were, when these substances come into contact

with them, that there is more work for the great bag of the stomach to

do. They call upon the part which is most active in secreting gastric

juice to pour it out quickly and get the business of digestion over.

Meat-extracts, which contain the products of protein disintegration,

have a similar influence in promoting the formation of the hormone.

Hence, no doubt, the general custom, found from experience to be

beneficial, of commencing dinner with soup; although it must be

remembered that the rapid absorption of meat-extracts makes them

peculiarly valuable as restoratives. They afford very little energy,

but what they have to give is quickly placed at the disposal of the

economy. Persons whose stomachs are unduly irritable are advised to

avoid soup. It leads to undesirable activity on the part of the gastric

glands, and especially of the acid-secreting cells. Well chewed bread

also encourages the production of the hormone.

Here it may be well to call attention to the evident division of the

stomach into two parts—the large bag, or cardiac portion, which hangs

down; and the smaller, funnel-shaped pyloric end, which is almost

vertical. The distinction between these two parts is faintly visible in

the resting stomach, but even opening the abdomen tends to obliterate

it. That it is much more evident during active digestion has been

shown by adding subnitrate of bismuth to the food, and throwing the

shadow of the stomach on a screen with Röntgen rays. When this is

done, it is seen that the two parts work in different ways. Food is

churned round and round in the cardiac portion, and pressed towards

the pylorus. Its fluid products, mixed with the abundant secretion

of the gastric mucous membrane, are wrung out of it by the pyloric

funnel. They are squeezed towards the pylorus, which opens at intervals

to let them through. If lumps of solid matter reach it, the pyloric

valve closes tightly, until the undigested food has fallen back into

the dependent bag. Dyspeptics are sometimes unpleasantly conscious of

the contractions of the pyloric funnel. In fact, putting aside pain

due to gastritis, all the discomfort of dyspepsia is felt on the right

side. Flatus accumulates beneath the pyloric valve. The valve will

not open to let it pass. The pyloric portion of the stomach contracts

strongly. Notwithstanding the general trend of movement in the opposite

direction, the gases are squeezed back into the larger bag, and escape

through the cardiac orifice.

Tables have been prepared showing the length of time which various

articles of food take to digest. They are based in part upon

observations made upon the living stomach in cases in which it has

been possible to examine its contents through a fistulous opening;

in part upon the results of artificial digestions carried out in the

laboratory. It is hardly too much to say that such observations are

absolutely without value as tests of the relative digestibility of the

several articles of diet consumed as parts of an ordinary meal. The

fact that the commencement of the flow of gastric juice depends upon

mental stimuli, and its continuance upon hormones, shows how difficult

it must be to reproduce the conditions which obtain in a healthy living

body. The most wholesome of foods taken by itself may be longer in

digesting, or may produce more irritation, than many less desirable

things taken in judicious combination. Crushed chicken, hastily

swallowed, sometimes proves more difficult of digestion than meat so

cooked and served as to stimulate appetite and to demand mastication.

Returning to the story of a meal, vegetables pass almost unaltered

through the stomach. Some of the scanty proteins which they contain

are peptonized, but unless they are very well masticated or cooked

until they are soft, and therefore easily pulped by the churning action

of the stomach, the gastric juice has to reach the proteins through

cell-walls. None of the digestive juices are able to dissolve the

cellulose of vegetable cell-walls. Blocks of vegetable tissue pass

down the whole length of the alimentary canal in the form in which

they were left by the teeth. Hence the extreme indigestibility of

ill-chewed cucumber or apple. The pyloric valve of the stomach is

forbidden to allow any lumps of food to pass until the very last stage

of gastric digestion. Pieces of ill-masticated vegetable tissue lie for

a long time in the stomach, irritating the ends of the gastric nerves,

until at last the time comes for them to be shot through the pylorus

into the duodenum. Many salts which vegetables contain, especially the

earthy carbonates and phosphates, are dissolved by the acid of the

gastric juice.

Meat consists of muscle-fibres supported by connective tissue. In the

stomach the gelatiniferous connective tissue is dissolved, setting

the fibres free. Further, the fibres being surrounded by a membrane

of the same nature—sarcolemma—this is removed; and although it may

be hardly justifiable to speak of “Krause’s membranes” (_cf._ Fig.

10) as gelatiniferous septa, the fibres are certainly composed of

segments—Bowman’s discs, sarcous elements—into which they break up

under the action of gastric juice. As a result, meat-fibre is reduced

to a finely divided granular condition. The capacity of gastric juice

for dissolving collagen (the substance of which connective tissue

is composed) may be regarded as its most characteristic, as it is

one of its most valuable, properties. Collagen, when boiled or acted

on by acids, takes water into its molecule, becoming gelatin. Under

the influence of gastric juice gelatin is rapidly hydrolysed into

diffusible gelatin-peptone. Pancreatic juice is unable to act upon

collagen, unless it has been previously boiled, or swollen by the

action of dilute acids.

Fat is composed of vesicles of oil supported by connective tissue.

Gastric juice, by dissolving the connective tissue and the collagenous

walls of the vesicles, sets the oil free. The oil, even though it be as

firm as suet when cold, is liquid, or almost liquid, at the temperature

of the body.

Thus, with the exception of raw vegetables, the hard fibre of cooked

vegetables, elastic tissue of meat, and a few other indigestible

substances, the meal is reduced in the stomach to a cream-coloured,

fatty, strongly acid “chyme.” In this condition it enters the duodenum,

where it at once comes into contact with an alkaline secretion. The

passage of acid chyme down this portion of the canal provokes the

discharge of gushes of bile and pancreatic juice. By precipitating

partially digested proteins and “acid-albumin” bile renders the mixture

thicker and sticky. It colours it yellowish-brown. Under the influence

of pancreatic juice the remaining proteins and proteoses are rapidly

converted into peptones, some of which are shaken down by the violent

action of erepsin into simpler bodies, such as leucin and tyrosin, etc.

The chyme becomes alkaline, grey, and thin. All undigested starch is

changed into maltose, and this into dextrose. Cane-sugar is converted

into dextrose and levulose. These sugars are absorbed into the blood.

Milk-sugar, if not converted into lactic acid, remains as lactose

(C₁₂H₂₂O₁₁), in which condition it is absorbed without “inversion.”

Fats are split by a ferment of the pancreatic juice into fatty acid and

glycerin; some of the fatty acid combines with alkali to form soap, but

of this we shall have more to say later on.

The duct common to the liver and the pancreas opens into the second

part of the duodenum. The organs which produce bile and pancreatic

juice are comparatively remote from the place where their secretions

come into contact with the food. By what mechanism are they thrown

into activity when the assistance of their secretions is required?

As in the case of the stomach, the agent is a hormone, a chemical

messenger. The hormone, termed “secretin,” is formed by the cells of

the mucous membrane of the duodenum when acid comes in contact with

them. It is absorbed by the blood, which carries it to the pancreas and

the liver. When it reaches the pancreas, it acts as a most powerful

stimulant to the discharge of accumulated ferments, and to the

production of an additional supply. It stimulates the liver to pour

forth bile. At present we are in ignorance as to the chemical nature

of this hormone. It is not a proteid substance, nor is it a ferment.

If scrapings from the mucous membrane of the duodenum be crushed with

sand and hydrochloric acid, the mixture boiled, neutralized with

carbonate of soda, and filtered, the clear, colourless liquid which

results has a powerful effect upon the pancreas, when injected, in

even small quantities, into the blood. Apparently, the cells of the

duodenal mucous membrane are constantly producing and accumulating

a substance which is converted into secretin when acted on by acid.

It is not necessary for the acid to stimulate the living cells. If

the mucous membrane is ground up with sand and salt-solution, the

filtrate is inactive but an active extract is obtained by treating the

crushed cells with HCl. It changes some substance which they contain

(provisionally termed “prosecretin”) into the efficient hormone.

In the lower portion of the small intestine any maltose that remains

is converted into diffusible dextrose. A very large amount of water

has been poured into the canal in the various digestive juices. This,

together with water drunk, is absorbed in the large intestine. At the

lower end of the alimentary canal nothing remains but indigestible

substances taken with food, chiefly cellulose, and the pigments and

other bodies which, as already said, are eliminated in bile.

The absorption of water is checked by the ingestion of extremely

soluble salts, such as sulphate of magnesia, the heavy molecule of

which diffuses with difficulty. We attribute the fact that sulphate of

magnesia remains in the intestine, and prevents water from diffusing

out of it, to its slowness in passing through a membrane, because

this is what would happen in dialysis;[2] but we must remember that

the living wall of the intestine is not a membrane. The cells which

line the intestine take up substances far less easily diffusible than

the sulphate of magnesia which they refuse. Nevertheless, speaking

generally, it is the less diffusible salts which act as aperients, the

more diffusible which are absorbed. The forward passage of the contents

of the alimentary canal is hastened by castor-oil. The peristalsis of

the intestines is stimulated by certain drugs, such as jalap or the

burnt products of tobacco. Another class of drugs, of which aloes is

an example, increases the secretion of the intestines, small or large.

Certain purgatives, such as calomel, podophyllin, etc., used to be

regarded as cholagogues. It was supposed that they increased the flow

of bile. This is an error. Their action is complicated, but it affects

chiefly the peristalsis of the intestine. The poor misunderstood liver

still suffers from the libels of primitive medical science. It is

the most innocent of organs, in no way responsible for derangements

of digestion. It carries out its functions without haste and without

delay. With the possible exception of salicylate of soda, no drug is

known which can stimulate it to a more rapid output of bile.

=Absorption.=—All the cells which line the alimentary canal are

capable of absorbing food, if it is presented to them in a suitable

form. In a suitable form means, speaking generally, in a diffusible

condition, although it must not be supposed that the epithelial cells

are incapable, under certain circumstances, of taking up non-diffusible

substances, just as a unicellular organism—an amœba—can take in

food. If soluble proteins, such as white of egg or acid-albumin, are

injected into the large intestine, a very considerable proportion

of the substance so injected is absorbed. It is possible, indeed,

to supply in this way the whole of the nitrogenous food needed by

the system, none entering by the mouth. If milk is injected, a

certain amount of the fat also is retained. It can be shown that such

absorption takes place when no digestion of the food occurs in the

colon. The food is taken up by the epithelial cells in the form in

which it is injected.

The organs specially devoted to absorption are the villi, which project

into the contents of the small intestine. Each is a conical process

about 0·5 millimetre long. The villi are longest in the upper half of

the small intestine. Below this level they decrease in number and size.

A villus is completely covered with epithelial cells of short, columnar

form. The free border of each cell is slightly hardened, forming a

disc or cap which appears striated in optical section—an indication,

as some think, that it is traversed by pores. Others hold that the

appearance of striation is due to minute cilia-like projections which

beset the free border of each cell. In worms and other invertebrates

the cells carry motile projections of not inconsiderable size, which

no doubt free their surfaces from the unassimilable matter which tends

to accumulate upon them. Possibly they help to fix particles which are

suitable for absorption. In mammals the presence of cilia has not been

demonstrated. The extreme minuteness of the striæ seems to point to

their being merely indications that the border is permeable to fluids,

including droplets of fat.