on is different according to the digestibility of the food, the bulk
of the morsels swallowed, the quantity received by the stomach, the
constitution of the individual, the state of the health, and above
all, the class of the animal, for it is widely different in different
classes. In the human stomach in about five hours after an ordinary
meal the whole of the food is probably converted into chyme.
637. The great agent in performing the process of chymification is the
gastric juice. The evidence of this is complete; for,
1. As soon as the food enters the stomach a large quantity of blood is
determined to the arteries, which secrete the gastric juice (627); and
this fluid continues to be poured into the stomach in great abundance
during the whole time the process goes on.
2. The solvent power of this fluid is demonstrated by the fact that it
sometimes dissolves the stomach itself, when death takes place suddenly
during the act of digestion in a sound and vigorous state of the
digestive organs.
3. On introducing into the stomach alimentary substances inclosed in
metallic balls perforated with holes, or in pieces of porous cloth,
it is found, on removing these bodies from the stomach, after a
certain time, that the alimentary substances contained in them are as
completely digested as if they had been in actual contact with the
surface of the stomach; the metallic ball and the cloth remaining
wholly unchanged. This experiment, which has been often performed with
the same uniform result, was the first that led to the discovery of the
true nature of the digestive process.
4. Though it be impossible to imitate out of the stomach all the
circumstances under which the food is placed within it, yet, on
procuring gastric juice from the stomachs of various animals, and
mixing it with different alimentary substances, it is found not only
to dissolve them, but to convert them into a pultaceous mass, closely
resembling chyme. Gastric juice thus procured was put into a glass
tube with boiled beef, which had been masticated; the tube was then
hermetically sealed, and exposed near the fire to a uniform heat: by
the side of this tube was placed another, containing the same quantity
of flesh immersed in water. In twelve hours, the flesh in the tube
containing the gastric juice began to lose its fibrous structure; in
thirty-five hours it had nearly lost its consistence, being reduced to
a soft homogeneous pultaceous mass. It experienced no further change
during the two following days. On the other hand, the flesh that had
been immersed in water was putrid in sixteen hours.
638. Since alimentary substances under the action of the stomach
present precisely the appearance exhibited by bodies exposed to the
influence of chemical agents, it appears that the gastric juice not
only dissolves the food, but dissolves it by a chemical agency. Its
action bears no proportion to the mechanical texture of bodies,
nor to any of their physical properties. It acts upon the densest
membrane, dissolves even bone itself; and yet produces no effect on
other substances of the most tender and delicate texture. On the skin
of fruit, on the finest fibre of flax and cotton, it is incapable of
making the slightest impression. In this selection of substances it
perfectly resembles a chemical agent acting by chemical affinity. On
certain substances its action is unquestionably of a chemical nature.
It occasions the coagulation of albuminous fluids; it prevents the
accession of putrefaction; it stops the process after it has commenced.
From the whole, it follows that the food in the stomach is converted
into chyme by the agency of a fluid secreted by the inner surface of
the stomach, and that this change is effected by a proper chemical
action.
639. It had been long ascertained that the gastric juice contains an
uncombined acid, and that if carbonate of lime be placed in a tube and
introduced into the stomach, the carbonate is dissolved just as if
it were put into weak vinegar. Several years ago, it was discovered
by Dr. Prout that this free acid is muriatic acid. Some time after
the publication of Dr. Prout’s experiments, Chevreul and Leuret and
Lassaigne in France obtained different results; but Tiedemann and
Gmelin, professors in the university of Heidelberg, in an extended
series of experiments, arrived at precisely the same conclusion as the
English physiologist, and apparently without any previous knowledge of
the researches of the latter. Tiedemann and Gmelin state, as the result
of their experiments, that the clear ropy fluid, or the gastric juice
obtained from the stomach some time after it had been without food, is
nearly or entirely destitute of acidity; that the presence of food,
or indeed of any stimulus to the mucous membrane, causes the gastric
juice to become distinctly acid; that this acidity increases according
to the indigestibility of the food; that the quantity of acid poured
out is very copious; that it consists partly of muriatic and partly
of acetic acid; and that both these acids are efficient agents in the
process of digestion. Dr. Prout, who had also recognised the presence
of acetic acid, is of opinion that its formation is an accidental
occurrence not necessary to digestion nor conducive to it; but is
either derived from the aliment, or is the result of irritation or
disease. He contends that the muriatic acid is the efficient digestive
agent.
640. The still more recent experiments of Braconnot appear to have
set this matter at rest, and to have proved, beyond all controversy,
that the stomach, when stimulated by the presence of food or other
foreign agents, possesses the power of secreting free muriatic acid
in great quantity; and that it is by this acid that the solution of
the food is effected. It is even found that muriatic acid is capable
of digesting alimentary substances out of the body. It had been long
known, that if meat and gastric juice be inclosed in a tube and kept
at the temperature of the human body, a product is obtained closely
resembling chyme (637.4). M. Blondelot, a physician at Nancy, has
recently shown that the same result may be obtained by the digestion
of the muscular fibre, in dilute muriatic acid. In both cases the
muscular fibre retains its form and its original fibrous texture; but
on the slightest motion it divides into an insoluble mass, perfectly
homogeneous and similar to the chyme of the stomach;[5] a very close
approximation to the actual digestive process, more especially when it
is considered that it is not possible to imitate out of the stomach
several circumstances materially influencing chemical action under
which the food is placed within the stomach.
[5] Dr. R. Thomson, British Annals of Medicine, No. 13.
641. Muriatic acid, the chemical agent by which the stomach dissolves
the food, is probably obtained from the muriate of soda (common salt)
contained in the blood. The soda, the basis of the salt, would appear
to be retained in the blood, to preserve the alkaline condition
essential to the maintenance of the sound constitution of the blood,
while the muriatic acid, disengaged from the soda in the process of
secretion, is poured into the stomach to act upon the food.
642. A remarkable confirmation of the correctness of the general
conclusions to which observation and experiment had thus enabled
physiologists to arrive, is afforded by the case of a young soldier
in the American army, of the name of Alexis St. Martin, who received
a wound on the left side by the accidental discharge of a musket. The
charge, which consisted of duck shot, and which was received at the
distance of one yard from the muzzle of the gun, entered the side
posteriorly in an oblique direction, forward and inward; blew off
the integument and muscles to the size of a man’s hand; fractured
and carried away the anterior half of the sixth rib; fractured the
fifth rib; lacerated the lower portion of the left lobe of the lungs;
lacerated the diaphragm, and perforated the stomach.
643. Violent fever and extensive sloughing of the parts injured ensued,
and the life of the young man was often in jeopardy, but he ultimately
recovered. At the distance of about a year from the date of the
accident, the injured parts had all become sound, with the exception
of the perforation into the stomach, which never closed, but left an
aperture permanently open, two inches and a half in circumference.
This aperture was situated about three inches to the left of the
cardia, near the left superior termination of the great curvature.
For some time the food could be retained only by constantly wearing a
compress and bandage; but at length a small fold of the mucous coat
of the stomach appeared, which increased until it completely filled
the aperture and acted as a valve, so as effectually to prevent any
efflux from within, while it admitted of being easily pushed back by
the finger from without: when the stomach was nearly empty, it was easy
to examine its cavity to the depth of five or six inches by artificial
distension; but, when entirely empty, the stomach was always contracted
on itself, and the valve generally forced through the orifice,
together with a portion of the mucous membrane equal in bulk to a hen’s
egg.
644. It chanced that the admirable opportunity thus afforded of
bringing the process of digestion, as far as it is carried on in the
stomach, under the cognizance of sense, occurred to an observant and
philosophical mind, and it was not lost.[6] The following are some of
the curious and instructive phenomena observed.
[6] Experiments and Observations on the Gastric Juice, and the
Physiology of Digestion. By W. Beaumont, M.D., Surgeon in the U. S.
Army. Boston. 1834.
645. The inner coat of the stomach, in its natural and healthy state,
is of a light or pale pink colour, varying in its hues according to its
full, or empty state. It is of a soft or velvet-like appearance (617),
and is constantly covered with a very thin transparent, viscid mucus,
lining the whole interior of the organ (619).
646. Immediately beneath the mucous coat appear small spheroidal, or
oval-shaped glandular bodies, from which the mucous fluid appears to be
secreted (619).
647. By applying aliment or other irritants to the internal coat of
the stomach, and observing the effect through a magnifying glass,
innumerable minute lucid points, and very fine nervous or vascular
papillæ are seen arising from the villous membrane, and protruding
through the mucous coat, from which distils a pure, limpid,
colourless, slightly viscid fluid (620). This fluid, thus excited, is
invariably distinctly acid (639, _et seq._). The _mucus_ of the stomach
is less fluid, more viscid or albuminous, semi-opaque, sometimes a
little saltish, and does not possess the slightest character of acidity
(619). On applying the tongue to the mucous coat of the stomach in its
empty, un-irritated state, no acid taste can be perceived. When food
or other irritants have been applied to the villous membrane and the
gastric papillæ excited, the acid taste is immediately perceptible.
The invariable effect of applying aliment to the internal, but exposed
part of the gastric membrane, is the exudation of the solvent fluid
from the papillæ. Though the aperture of these vessels cannot be seen
even with the assistance of the best microscopes, yet the points from
which the fluid issues are clearly indicated by the gradual appearance
of innumerable very fine lucid specks rising through the transparent
mucous coat, and seeming to burst and discharge themselves upon the
very points of the papillæ, diffusing a limpid thin fluid over the
whole interior gastric surface.
648. The fluid so discharged is absorbed by the aliment in contact; or
collects in small drops, and trickles down the sides of the stomach to
the more depending parts, and there mingles with the food, or whatever
else may be contained in the gastric cavity. This fluid, the efficient
cause of digestion, the true gastric juice is secreted only when it
is needed; it is not accumulated in the intervals of digestion, to
be ready for the next meal; it is seldom if ever discharged from its
proper secreting vessels, except when excited by the natural stimulus
of aliment, the mechanical irritation of tubes, or other excitants.
When aliment is received, the juice is given out in exact proportion
to its requirements for solution, except when more food has been taken
than is necessary for the wants of the system.
649. On collecting this fluid, which it was easy to obtain, it was
found to be transparent, inodorous, saltish, and acidulous to the
taste; it consisted of water, containing free muriatic and acetic
acids, phosphates and muriates, with bases of potass, soda, magnesia,
and lime, together with an animal matter soluble in cold, but insoluble
in hot water.
650. When a portion of liquid aliment, as a few spoonsful of soup,
were introduced into the stomach at the external orifice, the rugæ
(fig. CLXVII. 1) immediately closed gently upon it; gradually diffused
it through the gastric cavity, and prevented the entrance of a second
quantity till this diffusion was effected; then relaxation again took
place, and admitted of a further supply. When solid food was introduced
in the same manner, either in large pieces or finely divided, the same
gentle contraction and grasping motions were excited, and continued
from fifty to eighty seconds, so as to prevent more from being
introduced, without considerable force till the contraction was at an
end.
651. When the position of the body was such that the cardiac portion
of the stomach was brought into view, and a morsel of food was
swallowed in the natural mode, a similar contraction of the stomach,
and closing of its fibres upon the bolus was invariably observed to
take place; and till this was over, a second morsel could not be
received without a considerable effort. Hence, in addition to the other
purposes accomplished by mastication, insalivation, and deglutition,
it is probable that these operations answer the further use of duly
regulating the time for the admission of successive portions of the
food into the stomach.[7]
[7] See Dr. Andrew Combe on the Physiology of Digestion, in whose
work a full detail of this instructive case is given. See also Mayo’s
Outlines of Physiology, 4th Edit. Appendix.
652. On watching the phenomena that take place on the contact of a
portion of food with the stomach, the circumstances described (627)
are seen; the change in the mucous coat from a pale pink to a deep red
colour, in consequence of the enlargement of the blood-vessels and
their admission of a greatly increased number of red particles; the
undulating motion of the stomach, in consequence of the contraction of
its muscular fibres, excited by the stimulus of food; the distillation
of the gastric juice from the enlarged and excited papillæ; the
continuous flow of this fluid until the complete solution of the food,
when food is present; and, on the contrary, the cessation of this
discharge in a short time when it is produced by a mechanical irritant,
as the bulb of a thermometer, although at first the gastric juice
distil from the papillæ, from the contact of such an irritant, just as
when excited by the contact of food.
653. On collecting the gastric juice and placing it in contact with an
alimentary substance out of the stomach, its solution takes place more
slowly, but not less completely, than when retained in the stomach.
An ounce of this fluid was placed in a vial with a piece of boiled,
recently salted beef, weighing three drachms; the vial was then tightly
corked, and immersed in water, raised to the temperature of 100°,
previously ascertained to be the ordinary heat of the stomach. In forty
minutes the process of solution had commenced on the surface of the
beef. In fifty minutes the texture of the beef began to loosen and
separate. In sixty minutes an opaque and cloudy fluid was formed. In
one hour and a half the muscular fibres hung loose and unconnected, and
floated about in shreds in the more fluid matter. In three hours the
muscular fibres had diminished about one half. In five hours only a
few remained undissolved. In seven hours the muscular texture was no
longer apparent; and in nine hours the solution was completed.
654. At the commencement of this experiment a piece of the same beef of
equal weight and size was suspended within the stomach by means of a
string. On examining this portion of beef at the end of half an hour,
it was found to present precisely the same appearance as the piece in
the vial; but on the removal of the string at the end of an hour and
a half the beef had been completely dissolved, and had disappeared,
making a difference of result in point of time of nearly seven hours.
In both, the solution began on the surface, and agitation accelerated
its progress by removing the external coating of chyme as fast as it
was formed.
655. An ordinary dinner having been taken, consisting of boiled salted
beef, bread, potatoes, and turnips, with a gill of pure water for
drink, a portion of the contents of the stomach was drawn off into an
open mouthed vial, twenty minutes after the meal. The vial was placed
in a water-bath, maintained steadily at a temperature of 100°. It
was continued in this temperature for five hours. At the end of that
time the whole contents of the vial were dissolved. On comparing the
solution with an equal quantity of chyme taken from the stomach, little
difference could be distinguished between the two fluids, excepting
that it was manifest that the digestive process had proceeded somewhat
more rapidly in, than out of the stomach. The food, in this experiment,
after having remained in contact with the stomach for the space of
twenty minutes, had imbibed a sufficient quantity of gastric juice to
complete its solution.
656. Fifteen minutes after half a pint of milk had been introduced into
the stomach, it presented the appearance of a fine loosely-coagulated
substance mixed with a semi-transparent whey-coloured fluid. A drachm
of warm gastric juice poured into two drachms of milk at a temperature
of 100°, produced a precisely similar appearance in twenty minutes. In
another experiment, when four ounces of bread were given with a pint of
milk, the milk was coagulated and the bread reduced to a soft pulp in
thirty minutes, and the whole was completely digested in two hours.
657. When the albumen or white of two eggs was swallowed on an empty
stomach, small white flakes began to be seen in about ten or fifteen
minutes, and the mixture soon assumed an opaque whitish appearance. In
an hour and a half the whole had disappeared. Two drachms of albumen
mixed with two of gastric juice out of the stomach underwent precisely
the same changes, but in a somewhat longer time.
658. Dr. Beaumont’s observations are adverse to the opinion, founded
on numerous experiments, that the food is arranged in the stomach in
a definite manner, and that a distinct line of separation exists
between old and new food (626). In the human stomach, according to
the subject of these experiments, the ordinary course and direction
of the food are first from right to left along the small arch, and
thence through the large curvature from left to right. The bolus as
it enters the cardia turns to the left, passes the aperture, descends
into the splenic extremity, and follows the great curvature towards the
pyloric end. It then returns in the course of the smaller curvature,
makes its appearance again at the aperture, in its descent into the
great curvature, to perform similar revolutions. These revolutions
are completed in from one to three minutes. They are probably induced
in a great measure by the circular or transverse muscles of the
stomach (615), as is indicated by the spiral motion of the stem of
the thermometer, both in descending to the pyloric portion, and in
ascending to the splenic. These motions are slower at first than
after chymification has considerably advanced. The whole contents
of the stomach, until chymification be nearly complete, exhibit a
heterogeneous mass of solids and fluids, hard and soft, coarse and
fine, crude and chymified; all intimately mixed, and circulating
promiscuously through the gastric cavity like the mixed contents of a
closed vessel, gently agitated or turned in the hand.
659. In attempting to pass a long glass thermometer through the
aperture into the pyloric portion of the stomach, during the latter
stages of digestion, a forcible contraction is perceived at the point
of the hour-glass contraction of the stomach, and the bulb is stopped.
In a short time there is a gentle relaxation, when the bulb passes
without difficulty, and appears to be drawn quite forcibly, for three
or four inches, towards the pyloric end. It is then released, and
forced back, or suffered to rise again, at the same time giving to the
tube a circular or rather a spiral motion, and frequently revolving it
quite over. These motions are distinctly indicated and strongly felt
in holding the end of the tube between the thumb and finger; and it
requires a pretty forcible grasp to prevent it from slipping from the
hand, and being drawn suddenly down to the pyloric extremity. When the
tube is left to its own direction at these periods of contraction, it
is drawn in, nearly its whole length, to the depth of ten inches; and
when drawn back requires considerable force, and gives to the fingers
the sensation of a strong suction power, like drawing the piston from
an exhausted tube. This ceases as soon as the relaxation occurs, and
the tube rises again, of its own accord, three or four inches, when the
bulb seems to be obstructed from rising further; but if pulled up an
inch or two through the stricture, it moves freely in all directions
in the cardiac portions, and mostly inclines to the splenic extremity,
though not disposed to make its exit at the aperture. These peculiar
motions and contractions continue until the stomach is perfectly
empty, and not a particle of food or chyme remains, when all becomes
quiescent again.
660. The chambers in which the remaining part of the digestive process
is carried on are much less accessible, and no such favourable
opportunity as that enjoyed by Dr. Beaumont has occurred of rendering
their operations manifest to the eye. Nevertheless, the researches of
physiologists have succeeded in disclosing, with almost equal exactness
and certainty, the successive changes which the food undergoes even in
these more hidden organs, that admit of no exposure during life without
extreme danger.
661. The chyme on passing through the pylorus is received into a
chamber (fig. CLXVII. 3) which forms the first portion of the small
intestines. The small intestines, taken together, constitute a tube
about four times the length of the body. This tube is conical, the
base of the cone being towards the pylorus, and its apex at the valve
of the colon, where the small intestines terminate in the large. From
the pylorus to the valve of the colon the small intestines diminish in
capacity, in thickness, in vascularity, in the size of the villi, and
in the depth and number of the valvulæ conniventes.
[Illustration: Fig. CLXXI.
1. Esophagus. 2. Stomach. 3. Liver raised, showing the under surface.
4. Duodenum. 5. Small intestines, consisting of—6. Jejunum and ilium.
7. Colon. 8. Urinary bladder. 9. Gall bladder. 10. Abdominal muscles
divided and reflected.]
662. The first portion of the small intestine is termed the duodenum
(fig. CLXVII. 3). It is about twelve inches in length, and, unlike the
stomach, which is capable of considerable motion, it is closely tied
down to the back by the peritoneum, which imperfectly covers it. The
rest of the small intestine is divided into two portions—the upper
two-fifths of which are termed jejunum, and the three lower ilium.
663. The duodenum, the chamber which receives the chyme from the
pylorus, is a second stomach, which carries on the process commenced
in the first. It is assisted in the performance of its function by two
organs of considerable magnitude, the pancreas and the liver.
664. The pancreas is a conglomerate gland (fig. CLXXII. 5), of an
elongated form, placed in the epigastric region, lying transversely
across it, immediately behind the stomach (fig. CLXXII. 1), and resting
upon the spinal column (fig. CLXXII. 5). Its right extremity is
attached to the duodenum (fig. CLXXII. 9), and its left to the spleen
(fig. CLXXII. 4). In external appearance it resembles the salivary
glands, but it is of much larger size, and its weight, from four to six
ounces, is three times greater than that of all the salivary glands
together. It secretes a peculiar fluid called the pancreatic juice,
which is carried into the duodenum by a tube named the pancreatic duct
(fig. CLXVII. 7), which opens into the duodenum about four or five
inches from its pyloric end (fig. CLXVII. 2).
[Illustration: Fig. CLXXII.
1. Stomach raised. 2. Under surface of liver. 3. Gall bladder. 4.
Spleen. 5. Pancreas. 6. Kidneys. 7. Ureters. 8. Urinary bladder. 9.
Portion of intestine called duodenum. 10. Portion of intestine called
rectum. 11. Aorta.]
665. The liver, the largest and heaviest gland in the body, weighing
about four pounds, is placed chiefly in the right hypochondriac region
(fig. CLXXI. 3); but a portion of it extends transversely across the
epigastric, into the left hypochondriac region (figs. CV. and CVII.
3). Its upper surface is in contact with the diaphragm (fig. LX. 6, b);
its under surface with the pyloric extremity of the stomach (fig. LX.
7), and its margin can be felt under the edges of the ribs of the right
side.
666. It has been stated (473, 1.) that the fluid secreted by the liver,
unlike that formed by any other organ of the body, is elaborated from
venous blood, derived from the veins of the digestive organs, and
that these veins uniting together, form a common trunk called the
vena portæ, which penetrates the liver and ramifies through it in the
manner of an artery. Galen long ago compared this venous system to a
tree whose roots are dispersed in the abdomen, and its branches spread
out through the liver. Two comparatively small arteries, called the
hepatic, nourish the liver; the ultimate divisions of these arteries
likewise terminate in the vena portæ. The ultimate branches of the
vena portæ terminate partly in a system of veins, called the hepatic,
which like ordinary veins return the blood to the right side of the
heart; and partly in a system of tubes, termed the biliary ducts,
which contain the fluid secreted by the capillary branches of the vena
portæ. This fluid is the bile. The biliary ducts uniting from all
parts of the liver by innumerable branches, at length form a single
trunk termed the hepatic duct (fig. CLXVII. 9), which carries the bile
partly to the gall bladder (fig. CLXVII. 8) by a duct called the
cystic (fig. CLXVII. 10), and partly to the duodenum (fig. CLXVII. 3)
by a duct named the choledoch (fig. CLXVII. 6), a common trunk formed
by the union of the cystic with the hepatic (fig. CLXVII. 10 and 9).
The choledoch duct opens into the duodenum at the same point as the
pancreatic (fig. CLXVII. 7), and generally by a common orifice.
667. The duodenum, on receiving the chyme from the stomach, transmits
it slowly along its surface. The kind of motion by which the chyme is
borne along the surface of the duodenum is perfectly analogous to that
by which it is transmitted from the stomach to the duodenum, irregular,
sometimes in one direction, and sometimes in another, at one time
commencing in one part of the organ, at another time in another, always
slow, but ultimately progressive.
668. As the chyme slowly advances through the upper part of the
duodenum, the biliary and the pancreatic juices slowly distil into
the lower portion of the organ. The bile is seen to exude from the
choledoch duct, not continually, but at intervals, a drop appearing at
the orifice, and diffusing itself over the neighbouring surface, about
twice in a minute, while the flow of the pancreatic juice is still
slower.
669. No appreciable change takes place in the chyme until it reaches
the orifice of the choledoch duct; but as soon as it comes in contact
with this portion of the duodenum, the chyme suddenly loses its own
sensible properties, and acquires those of the bile, especially its
colour and bitterness. But these properties are not long retained; a
spontaneous change soon takes place in the compound. It separates into
a white fluid and into a yellow pulp. The white fluid is the nutritive
part of the aliment; the yellow pulp is the excrementitious matter.
670. This white fluid, the proper product of the digestive process,
as far as it has yet advanced, is called chyle. If any portion of oil
or fat have been contained in the food, the chyle is of a milk-white
colour; if not, it is nearly transparent. It is of the consistence
of cream, and it bears a close resemblance to cream in its sensible
properties. It differs from chyme in being of a whiter colour, more
pellucid, and of a thicker consistence: it differs also in its chemical
nature, for, whereas chyme is acid, chyle is alkaline.
671. Three fluids are mixed with the chyme in the duodenum, each of
which contributes to the conversion of the chyme into chyle. First, the
secretion of the duodenum itself, a solvent analogous to the gastric
juice. Secondly, the secretion of the pancreas, a watery fluid holding
in solution highly important principles, namely, a large quantity of
albumen, a matter resembling casein, osmazome, and different salts.
Thirdly, the secretion of the liver, a compound fluid, consisting of
water, mucus, and several peculiar animal matters, namely, resin,
cholesterine, picromel, cholic acid, a colouring matter, probably
salivary matter, osmazome, casein, and many salts.
672. There cannot be a question that the secretion of the duodenum has
a solvent power over the chyme analogous to that of the gastric juice.
Some physiologists indeed maintain that the juice poured out from the
inner surface of the duodenum is as powerful a solvent as the gastric
juice. It is certain that substances which have escaped chymification
in the stomach undergo that process in the duodenum, and that there is
the closest analogy between the action of the duodenum on the chyme and
that of the stomach on the crude food.
613. The pancreatic secretion adds to the chyme richly azotized animal
substances, albumen, casein, osmazome (671), by which it is brought
nearer the chemical composition of the blood, and prepared for its
complete assimilation into it. The first addition of such assimilative
matter, it has been shown, is communicated by the salivary glands, but
far more important additions are now supplied from the pancreas. Hence
the larger size of the pancreas and the more copious secretion of the
pancreatic fluid, in herbivorous than in carnivorous animals; hence the
change produced in the size of the pancreas by a long continued change
in the habits of an animal; hence the smaller size of the pancreas in
the wild cat, which lives wholly on animal food, than in the domestic
cat, which lives partly on animal and partly on vegetable food.
674. The bile, the most complex secretion in the body, accomplishes
manifold purposes.
1. Like the pancreatic secretion, it communicates to the chyle richly
azotized animal substances, picromel, osmazome, and cholic acid (671);
by the combination of which with the chyme, it is brought still nearer
the chemical composition of the blood. These principles are manifestly
united with the chylous portion of the chyme, since they are not
discoverable in its excrementitious matter.
2. Bile has the property of dissolving fat; consequently, when oily
or fatty matters are contained in the food, it powerfully assists in
converting these substances into chyle.
3. The excrementitious portion of the bile is highly stimulant. The
contact of its bitter resin with the mucous membrane of the intestines
excites the secretion of that membrane; hence the extreme dryness of
the excrementitious matter when the choledoch duct of an animal has
been tied; and hence the same dryness of this matter in jaundice, when
the bile, instead of being conveyed by its appropriate duct into the
duodenum, is taken up by the absorbents, poured into the blood, and
distributed over the system.
4. The bitter resin of the bile stimulates to contraction the fibres
of the muscular tunic of the intestines: by the contraction of these
fibres the excrementitious matter is conveyed in due time out of the
body; hence the constipated state of the bowels invariably induced when
the secretion of the bile is deficient, or when its natural course into
the intestines is obstructed.
5. The excrementitious portion of the bile exerts an antiseptic
influence over the excrementitious portion of the food during its
passage through the intestines. In animals in which the choledoch duct
has been tied, the excrementitious portion of the food is invariably
found much further advanced in decay than in the natural state. This
is also uniformly the case in the human body in proportion as the
secretion of the bile is deficient, or its passage to the intestine is
obstructed.
675. Such appear to be the real purposes accomplished by the bile
in the process of digestion. Several uses have been assigned to
it, in promoting this process, which it does not serve. Seeing the
instantaneous change wrought in the chyme on its contact with the
bile, it was reasonable to suppose that the main use of the bile
was to convert chyme into chyle, a purpose apparently of sufficient
importance to account for the immense size of the gland constructed
for its elaboration. The soundness of this conclusion appeared to be
established by direct experiment. Mr. Brodie placed a ligature around
the choledoch duct of an animal: after the operation the animal ate
as usual: on killing the animal some time after it had taken a meal,
and examining the body immediately after death, it was clear that
chymification had gone on in the stomach just as when the choledoch
duct was sound, but no chyle appeared to be contained either in the
intestines or in the lacteals. In the lacteals there was found only a
transparent fluid, which was supposed to consist of lymph and of the
watery portion of the chyme. Mr. Brodie’s experiments seemed to be
confirmed by those of Mr. Mayo, who arrived at the conclusion, that
when the choledoch duct is tied, and the animal is examined at various
intervals after eating, no trace whatever of chyle is discoverable in
the lacteal vessels. But these experimentalists inferred that no chyle
existed in the intestines or lacteals, because there was present no
fluid of a milk-white colour, a colour not essential to chyle, but
dependent on the accident of oily or fatty matter having formed a
portion of the food. These experiments have been repeated in Germany
by Tiedemann and Gmelin, and in France by Leuret and Lassaigne, who
have invariably found, after tying the choledoch duct, nearly the same
chylous principles, with the exception of those derived from the bile,
as in animals perfectly sound; and the English physiologists have since
admitted that their German and French colaborateurs have arrived at
conclusions more correct than their own.
676. The bile consists then of two different portions; a highly
animalized portion, which combines with the chyme and exalts its
nature by approximating it to the condition of the blood; and an
excrementitious portion, which, after accomplishing certain specific
uses, is carried out of the system with the undigested matter of the
food. The excrementitious portion of the bile, namely, the resin, the
fat, the colouring principle, the mucus, the salts, constitute by far
the largest portion of it. These constituents of the bile for the most
part contain a very large proportion of carbon and hydrogen, and the
reasons have been already fully stated (473, _et seq._) which favour
the conclusion that the elimination of these substances under the form
of bile is one most important mode of maintaining the purity of the
blood, and that the liver is thus a proper respiratory organ, truly
auxiliary to the lungs. It is a beautiful arrangement, and like one
of the adjustments of nature, that the bile, the formation of which
abstracts from the blood so large a portion of carbon and hydrogen as
to maintain the purity of the circulating mass and to counteract its
putrescent tendency, acts on the excrementitious portion of the food,
always highly putrescent, as a direct and powerful antiseptic; that
the very matter which is eliminated on account of the putrid taint
it communicates to the blood, on its passage out of the body, stops
the putrefaction of the substances which have been ministering to the
replenishment of the blood.
677. The chyle, thick, glutinous, and adhesive, attaches itself
with some degree of tenacity to the mucous surface of the duodenum.
Nevertheless, by the successive contractions of the muscular fibres of
the duodenum the fluid is slowly but progressively propelled forwards.
The separation of the excrementitious matter becomes more complete,
and consequently the chyle more pure as it advances, until, having
traversed the course of the duodenum, it enters the second portion of
the small intestines, the jejunum.
678. The jejunum, so called because it is commonly found empty, and the
ilium, named from the number of its convolutions, on account of their
great length, are provided with a distinct membrane to support them,
and to retain them in their situation, termed the mesentery.
679. The mesentery is a broad membrane composed of two layers of
peritoneum. Between these two layers, at one extremity of the
duplicature, is placed the intestines, while the other extremity is
attached to the spinal column. The mesentery being much shorter than
the intestines, the intestines are gathered or puckered upon the
membrane, by which beautiful mechanical contrivance they are held in
firm and close contact with each other, yet their convolutions cannot
be entangled, nor can they be shaken from their place by the sudden
and often violent movements of the body. It sometimes happens, in
consequence of disease, that the convolutions of the intestines are
glued together by the effusion of lymph, and then the most trifling
causes are capable of producing the severest symptoms of obstruction in
the bowels.
680. The internal surface of the small intestines is distinguished,
1. By the number of the mucous glands, which may be seen by a
magnifying glass to consist partly of a prodigious number of the
minutest follicles, not collected in groups, but equally scattered
throughout; and partly of glands of a larger dimension, disposed in
groups at particular parts of the canal.
2. By the increase in the number and size of the villi, of which there
are about four thousand to the surface of a square inch. Like those of
the stomach, the villi of the small intestine are composed of arteries,
veins, nerves, and mucous ducts; but to the villi of the small
intestine, in length about one-fourth of a line, there is added a new
vessel, the absorbent of the chyle, the lacteal (figs. 175 and 176), so
named from the milk-like chylous fluid which it contains.
3. By the great extension of the mucous coat obtained by the
disposition of the membrane into the folds called valvulæ conniventes
(fig. CLXXIII.). These folds, which rarely extend through the whole
circle of the intestine, are often joined by communicating folds (fig.
CLXXIII.). The folds are broadest in the middle, and narrowest at the
extremities (fig. CLXXIII.). In general, they are about a line and a
half broad. One edge of the fold is loose, but the other is fixed to
the intestine (fig. CLXXIII.). The office of these folds is, first,
without increasing space, to extend surface for the distribution of the
villi; and, secondly, to retard the flow of the chyle, by opposing to
its descent valves so constructed and disposed as, without arresting
its progress, to moderate and regulate its course, in order that time
may be allowed for its absorption.
[Illustration: Fig. CLXXIII.
Internal view of a portion of the jejunum, showing the arrangement of
the mucous membrane into valvulæ conniventes.]
[Illustration: Fig. CLXXIV.—_View of the Outer Coats of the Small
Intestine._
1. Peritoneal coat reflected off. 2. Muscular Coat consisting of—_a._
longitudinal fibres. _b._ Circular fibres.]
681. The onward flow of the chyle through the course of the small
intestines is effected by the action of the double layer of muscular
fibres, the circular and the longitudinal fasciculi which compose
its muscular coat (fig. CLXXIV.). The disposition of the muscular
fibres of the alimentary canal in general, and of this part of it in
particular, deserves special notice. The ordinary arrangement and
action of muscular fibres would not have produced in this case the kind
and degree of motion required. The muscular fibres that compose the
ventricles of the heart are so accumulated and disposed, that their
contraction originates, and communicates energetic impulse. The muscles
of the arm are so accumulated and disposed that their contraction
originates the like energetic impulse. Muscles so accumulated in the
alimentary canal would have produced motion, indeed, but motion not
only not accomplishing the end in view, but directly defeating it. In
order to obtain the kind and degree of motion in this case required,
the firm and thick muscle is attenuated into minute, delicate, and
thready fibres, not concentrated in a bulky mass, so as to obtain by
their accumulation a great degree of force; but spread out in such
a manner as to form a thin and almost transparent coat. The tender
fibres composing this delicate coat, by their contraction, produce two
alternate, gentle, almost constant motions, called the peristaltic,
from its resemblance to the motion of the earth-worm, and the
antiperistaltic. By the peristaltic action motion is begun at once in
several parts of the canal. Whenever the chyle is applied in a certain
quantity to any part of the intestines, that part contracts, and makes
a firm point, towards which the portions both above and below are
drawn, by means of the longitudinal fibres which shorten the canal, and
at the same time dilate the under part. By the antiperistaltic action,
which is the exact reverse of the former, the chyle is turned over and
over, and exposed to the orifices of the lacteal vessels; while, by the
motion of the chyle forwards and backwards, and backwards and forwards,
produced by these two actions constantly alternating with each other,
its slow, gentle, but ultimately progressive course is secured.
682. The chyle thus gently moved along the extended surface of the
jejunum and ilium, and still in its course acted upon in some degree
by the secretions poured out upon the mucous membrane, successively
disappears, until at the termination of the ilium (fig. CLXXI. 5) there
is scarcely any portion of it to be perceived. It is taken up by the
vessels termed lacteals.
683. The lacteal vessels (figs. 175 and 176), take their origin on
the surface of the villi, by open mouths, too minute to be visible
to the naked eye, but distinguishable under the microscope. These
minute, pellucid tubes, wholly countless in number, are composed of
membranous coats so thin and transparent that the milky colour of their
contents, from which they derive their name, is visible through them,
and yet they are firm and strong. They present a jointed appearance
(figs. CLXXVI. 4, and CLXXVII. 7). Each joint denotes the situation
of the valves with which they are provided, and which are placed at
regular distances along their entire course (fig. CXCII. 1 and 2).
These valves, which are generally placed in pairs (fig. CXCII. 2),
consist of a delicate fold of membrane of a semilunar form, one edge of
which is fixed to the side of the vessel, while the other lies loose
across its cavity (fig. CXCII. 2). So firm is this membrane, and so
accurately does it perform the office of a valve, that even after death
it is capable of supporting a column of mercury of considerable weight
without giving way, and of preventing a retrograde course of the fluid.
The lacteals are nourished by blood-vessels, and animated by nerves,
and it is conceived that they must be provided with muscular fibres, or
some analogous tissue, for they are obviously contractile, and it is
by this contractile power that their contents are moved. The delicacy
and transparency of the vessels, however, render it impossible to
distinguish the different tissues which compose their walls.
[Illustration: Fig. CLXXV.
View of the inner surface of the ilium as it appears some hours after
a meal. 1. The smaller branches of the lacteals, turgid with chyle,
covering the surface of the intestine. 2. Larger branches of the
lacteals formed by the union of the smaller branches.]
[Illustration: Fig. CLXXVI. _View of the course of the Lacteals._
1. The aorta. 2. Thoracic duct. 3. External surface of a portion of
small intestine. 4. Lacteals appearing on the external surface of
the intestine after having perforated all its coats. 5. Mesenteric
glands of the first order. 6. Mesenteric glands of the second order.
7. Receptacle for the chyle. 8. Lymphatic vessels terminating in the
receptacle of the chyle, or commencement of the thoracic duct.]
684. If the mucous coat of the small intestines be examined some hours
after a meal, the lacteals are seen turgid with chyle, covering its
entire surface (fig. CLXXV. 1). These vessels, which are sometimes
of such magnitude and in such numbers as entirely to conceal the
ramifications of the blood-vessels, unite freely with each other, and
form a net-work, from the meshes of which proceed branches which,
successively uniting, form branches of a larger size (fig. CLXXV. 2).
These larger branches perforate the mucous coat and pass for some way
between the mucous and the muscular tunics: at length they perforate
both the muscular and the peritoneal coats, when, from having been
on the inside of the intestine, they get on the outside of it (fig.
CLXXVI. 3, 4), and are included, like the intestine itself, between the
layers of the mesentery. All the different sets of lacteals converging
and uniting together, form an exceedingly complicated plexus of vessels
within the fold of the mesentery. Radiating from this plexus, the
lacteals advance forwards until they reach the glands, called, from
their being placed between the fold of the mesentery, the mesenteric
(figs. CLXXVI. 5 and 6, and clxxvii. 2 and 3); rounded, oval,
pale-coloured bodies, consisting of two sets, arranged in a double
row (figs. CLXXVI. 5 and 6, and CLXXVII. 2 and 3); the set nearest
the intestine (fig. CLXXVII. 2) being considerably smaller than the
succeeding set (fig. CLXXVII. 3).
[Illustration: Fig. CLXXVII.
View of the course of the Thoracic Duct from its origin to its
termination. 1. Lacteal vessels emerging from the mucous surface of
the intestines. 2. First order of mesenteric glands. 3. Second order
of mesenteric glands. 4. The great trunks of the lacteals emerging
from the mesenteric glands, and pouring their contents into—5. The
receptacle of the chyle. 6. The great trunks of the lymphatic or
general absorbent system terminating in the receptacle of the chyle.
7. The thoracic duct. 8. Termination of the thoracic duct at—9. The
angle formed by the union of the internal jugular vein with the
subclavian vein.]
685. On reaching the first series of glands (fig. clxxvii. 2), the
lacteals penetrate the substance of the gland, in the interior of
which they communicate with each other so freely, and form such
innumerable windings, that the gland seems to consist of a congeries
of convoluted lacteals. Emerging from the first series of glands, the
lacteals proceed on their course to the second series (fig., CLXXVII.
3), which they penetrate, and in the interior of which they present
the same convoluted appearance as in the first set. On passing out of
this second series of glands, the lacteals unite together, and compose
successively larger and larger branches, until at length they form two
or three trunks (fig. CLXXVII. 4), which terminate in the small oval
sac (fig. CLXXVII. 5), termed the receptacle of the chyle (receptaculum
chyli).
686. In this oval sac or receptacle of the chyle (fig. CLXXVII.
5), which rests upon the second or the first lumbar vertebra, also
terminate the trunks of the general absorbent vessels of the system
(fig. clxxvii. 6), called from the _lymph_ or the pellucid fluid which
they contain, lymphatics, as the lacteals are named from the lactitious
or milky appearance of their contents.
687. The receptacle of the chyle produced forms the thoracic duct
(fig. CLXXVII. 7), a canal about three lines in diameter. This tube
rests upon the spinal column, ascends on the right side of the aorta,
passes through the aortic opening in the diaphragm (fig. CXXXIV. 9,
10), and enters into the chest. Here it forms a transparent tube about
the size of a crow-quill; it rests upon the bodies of the dorsal
vertebræ; it continues to ascend still on the right side of the aorta,
until it reaches the sixth or fifth dorsal vertebra, when changing its
direction, it passes obliquely over to the left side (fig. CLXXVII. 7).
From this point it continues its course upwards, on the left side of
the neck, as high as the sixth cervical vertebra; when suddenly turning
forwards and a little downwards, it terminates its course in the angle
formed by the union of the internal jugular with the subclavian vein
(fig. CLXXVII. 8, 9). At its termination in these great venous trunks
are placed two valves, which prevent alike the return of the chyle, and
the entrance of the blood into the duct (fig. CLXXVIII.).
[Illustration: Fig. CLXXVIII.—_Valve at the termination of the Thoracic
Duct._
1. The Thoracic Duct. 2. Lymphatics entering the duct. 3. The vein
laid open, showing the valve at the termination of the duct. 4. The
left internal jugular vein. 5. The left subclavian vein. 6. The vein
called innominata. formed by the union of the internal jugular and
subclavian veins. 7. The right jugular vein. 8. The right subclavian
vein. 9. The superior cava formed by the union of the veins above. 10.
The inferior cava formed by the union of the veins below. 11. The two
venæ cavæ passing to the right auricle of the heart. 12. The heart.
13. The pulmonary artery dividing into right and left branches. 14.
The aorta.]
688. This account of the course of the thoracic duct is a description
of the course of the chyle. Performing a double, circuitous, and
slow circulation through the minute convoluted tubes of which the
double series of mesenteric glands are composed, the chyle, in its
receptaculum, is mixed with the contents of the lymphatic vessels,
lymph (fig. CLXXVII. 6, 5), that is, organic matter brought from every
surface and tissue of the body. Both fluids, chyle and lymph, mixed and
mingled, flow together into the thoracic duct, by which in the course
traced (687) they are poured into the blood, just as the venous torrent
is rushing to the heart (fig. CLXXVIII. 6, 9, 11).
689. Thus, the final product of digestion, the chyle; particles of
organized matter, the lymph; and venous blood, that is, blood which has
already circulated through the system commingled, flow together to the
right heart, by which it is transmitted to the lungs, where all these
different fluids are converted into one substance, arterial blood, to
be by the left heart sent out to the system for its support.
690. While these processes are going on, another and a very important
function is performed by the remaining portion of the alimentary canal.
It is the office of this part of the apparatus to carry out of the body
that portion of the aliment which is incapable of being converted into
chyle. The preparation of the excrementitious part of the aliment for
its expulsion constitutes the process of fecation. The organs in which
this process is carried on, and by which the excrementitious matter,
when duly prepared for its removal, is conveyed from the body, are the
large intestines.
691. The large intestines (fig. CLXXIX.) consist of the cæcum, the
colon and the rectum (fig. CLXXIX.). The cæcum varies in length from
two inches to six; the colon is about five feet in length, and the
rectum is about eight inches.
692. The ilium opens into the cæcum (fig. CLXXIX. 8, 10), just as the
esophagus opens into the stomach. At this point the ilium is elongated,
forming two concentric folds which join at their horns, and between
the folds are placed a number of muscular fibres. In this manner is
constructed a valve, which is termed the valve of the colon. It is
placed in a transverse direction across the intestine, and its action
as a valve is very complete. It admits of the free passage of the
contents of the small intestines into the large, but it prevents the
return of any portion of the contents of the latter into the former.
[Illustration: Fig. CLXXIX.—_View of the Abdominal Portion of the
Digestive Organs._
1. Esophagus. 2. Stomach. 3. Spleen. 4. Liver. 5. Gall-bladder
with its ducts. 6. Pancreas with its duct. 7. Duodenum. 8. Small
intestines. 9. Large intestines dividing into—10. Cæcum. 11. Ascending
colon. 12. Arch of the colon. 13. Descending colon. 14. Sigmoid
flexure here imperfectly represented. 15. Rectum.]
[Illustration: Fig. CLXXX.
Portion of the large intestine, showing the arrangement of the
muscular fibres. 1. The longitudinal fibres collected into bands, and
forming larger fasciculi. 2. The circular fibres arranged as in the
other intestines.]
693. The colon is distinguished by its capacious size, its great
length, and its longitudinal bands, which consist of strong muscular
fasciculi (fig. CLXXIX. 11). It is divided into an ascending portion
which occupies the right iliac and hypochondriac regions (fig. CLXXIX.
11); the transverse portion, called its arch, which is placed directly
across the epigastric region (fig. CLXXIX. 12), a descending portion
which occupies the left hypochondriac region (fig. CLXXIX. 13), and a
fourth portion, which being curved somewhat like the italic letter S,
is called the sigmoid flexure, which occupies the left iliac region
(fig. CLXXIX. 14). The sigmoid flexure terminates in the last portion
of the alimentary canal, called the rectum (fig. CLXXIX. 15), which is
placed in the hollow of the sacrum, and which follows the curvature
of that bone (fig. XLV. 5). The circular fibres of the rectum are
accumulated at the termination of the bowel to form the internal
sphincter of the anus. External to this is placed another set of
fibres, which constitute the external sphincter.
694. The mucous membrane of the large intestines is disposed
differently from that of the small intestines, and the mucous membrane
of the colon still differently from that of the rectum. In the colon
the mucous membrane, instead of being disposed in the form of valvulæ
conniventes, is so arranged as to divide its whole surface into
minute apartments or cells by which the descent of the fecal matter
is retarded still more than the descent of the chyle by the valvulæ
conniventes. Some particles of chyle do, however, continue to be
separated from the fecal matter, even in the large intestines; and in
order that nothing may be lost, a few valvulæ conniventes, with their
lacteals, appear here also, while the cells of the colon, by retarding
the descent of the fecal matter, allow time for the more complete
separation and absorption of the chylous particles.
695. In the rectum the mucous membrane is plaited into large transverse
folds, which disappear as the fecal matter descends into the bowel,
accumulates in it, and distends it; an arrangement which gives to this
portion of the intestine its power of distension, so closely connected
with our convenience and comfort.
696. As soon as that portion of the alimentary matter which is
transmitted to the large intestines reaches the colon it ceases to
be alkaline, the distinctive character of the contents of the small
intestines, and becomes acid, just as the whole alimentary mass is
acid at the commencement of digestion in the stomach. It acquires
albumen; its gases are no longer the same, for whereas pure hydrogen
is contained in the small intestines, none is ever found in the large,
but in the place of it, carbureted and sulphureted hydrogen; and now
for the first time it receives its peculiar odour. As it continues
to descend, its fluid parts are progressively absorbed, so that it
becomes more and more solid, until it reaches the rectum, when it is
almost dry. Here the accumulation of it goes on to a considerable
extent, the peristaltic action at first excited by the distension of
the rectum being, it would appear, counteracted by the contraction
of the external sphincter of the anus. When, however, the distension
of the bowel reaches a certain point, it produces a sensation which
leads to the desire to expel its contents. The bowel is now thrown
into action by an effort of the will, and that action is powerfully
assisted by the descent of the diaphragm and the contraction of the
abdominal muscles, actions also induced by an effort of the will. Thus
the action of the first part of the digestive apparatus, that which is
connected with the reception and partly with the deglutition of the
food, is attended with consciousness, and is placed under the control
of the will; the main portion of the digestive apparatus, that in
which the essential part of the digestive process is carried on, is
without consciousness, and is placed beyond the influence of volition;
the last portion of the digestive apparatus, that connected with the
expulsion of the non-nutrient portion of the aliment, again acquires
sensibility and consciousness, and is placed under the control of the
will. The striking differences in the arrangement of the muscular
fibres in these different parts of the apparatus, in accordance with
the widely different function performed by them; the powerful muscles
connected with the prehension, mastication and deglutition of the food;
the delicate and transparent tissue of fibres forming the muscular
coat of the stomach and small intestines; the increase in the number
and strength of the fibres of the large intestines, and the prodigious
accession to them in the rectum, are adjustments not only exquisite and
admirable in their own nature, but so indispensable to our well-being
and comfort, that were the appropriate action of either to be suspended
but for a short period, life would be extinguished, or if it could be
protracted, it would be changed into a state of unbearable torment.
697. From the preceding account of the structure and action of the
apparatus of digestion, on a comparison of all the phenomena, it
appears that the successive stages of the process are marked by the
progressive approximation of the food to the nature of the blood. The
main constituents, of the blood are albumen, fibrin, an oily principle,
and red particles. Even in the chyme there are traces of albumen, with
globules, not indeed to be compared in number with the red particles
of the blood, smaller in size, and without colour, but still of an
analogous nature. In the chyle of the duodenum the quantity of albumen
is larger, there are traces of fibrin, and of an oily matter, and the
number of the globules is increased. In the chyle, after its exit from
the mesenteric glands, the albumen, the fibrin, the oil, the globules,
and more especially the two first and the last, are greatly increased.
But in the chyle when it reaches the thoracic duct, these principles
are so augmented, concentrated, and approximated to the state in which
they exist in the blood, that the chyle is now capable of undergoing
the characteristic process of the blood; for as the blood, when drawn
from a vein, undergoes spontaneous coagulation, so the chyle, when
drawn from the thoracic duct, separates into three parts; a solid
substance or clot, which remains at the bottom of the vessel; a fluid
which surrounds the clot; and a thin layer of matter, which is spread
over the surface of the fluid. The solid substance is analogous to the
fibrin, and the fluid to the serum of the blood; while the layer of
matter which is spread over the fluid is of an oily nature: moreover,
the chyle, when in contact with the air, quickly changes to a red
colour, and abounds with minute particles of various sizes, but the
largest of which is not yet equal to the diameter of the red particles
of the blood.
698. The changes wrought upon the food, by which it is thus
approximated to the chemical composition of the blood, are effected,
as has been shown, partly by the gastric and intestinal juices, and
partly by matters combined with the food highly animalized in their
own nature, and endowed with assimilative properties, as the salivary
secretion mixed with the food during mastication; the pancreatic
and biliary secretions mixed with the food during the conversion of
the chyme into chyle; and the mesenteric secretions mixed with the
elaborated chyle of the mesenteric glands, and lastly, organized
particles which have already formed a part of the living structures of
the body mixed with the chyle under the form of lymph in the thoracic
duct.
699. The lymph, until lately regarded as excrementitious, is really
highly animalized, partly combined with the chyle as its last and
highest assimilative matter; whence the compound formed by the
admixture of chyle and lymph is far more proximate to the blood than
the purest and most concentrated chyle; and partly returning with the
chyle to the lungs, to receive there a second depuration, and thereby a
higher elaboration.
700. There is evidence that there is a series of organs specially
provided for the elaboration of the lymph no less than of the chyle.
There are organs manifestly connected with the digestive apparatus,
to which physiologists have found it extremely difficult to assign a
specific office. These organs have a structure in some essential points
alike; that structure is strikingly analogous to the organization of
glands: like glands, they receive a prodigious quantity of arterial
blood, and are supplied with a proportionate number of organic nerves;
yet they are without an excretory duct. The organs in question are
the bodies called the renal capsules, placed above the kidneys; the
thyroid and thymus glands situated in the neck, and the spleen in close
connexion with the stomach.
701. These organs, however analogous in structure to glands, cannot,
it has been argued, be secreting organs, because they are destitute of
an excretory duct, do not manifestly form from the blood any peculiar
secretion, or, if they do, since there are no means of detecting
where it is conveyed, it is impossible to understand how it is
appropriated. But if these organs collect, concentrate, and elaborate
lymph, preparatory to its admixture with the chyle and to its being
sent a second time into the blood to undergo a second process of
depuration, they perform the function of glands; and their want of an
excretory duct, which has hitherto rendered their office so obscure, is
accounted for; they do not need distinct tubes for the transmission of
any product of secretion; the lymphatic vessels which proceed from them
and which convey the fluid they elaborate into the receptacle of the
chyle, are their excretory ducts. That one of these organs, the spleen,
is specially connected with the elaboration of the lymph, is manifest,
both from its chemical nature and from the remarkable change which
takes place in the chyle the moment the lymph from the spleen is mixed
with it. Tiedemann and Gmelin state, as the uniform result of their
observations and experiments, that the quantity of fibrin contained
in the chyle is greatly increased, and that it actually acquires red
particles as soon as the lymph from the spleen is mixed with it, and
that the lymph from the spleen superabounds both with fibrin and with
red particles. That the organs just enumerated, with the spleen,
perform a similar function, is inferred from their being, like it, of a
glandular structure, and without any excretory duct. If the spleen be
really one of a circle of organs appropriated to a function such as is
here supposed, a purpose is assigned to it adequate to its rank in the
scale of organization; inferior to few, if its importance be estimated
by the quantity of arterial blood with which it is supplied; yet this
is the organ for which Paley could find no better use than that of
serving for package.
702. But in whatever mode the lymph be elaborated, it is certain that
it consists of matter highly animalized, and that its most important
principles, its albumen, its fibrin, its globules, and even its salts,
are in a chemical condition closely resembling that in which they exist
in the blood.
703. It will appear hereafter that all the proximate principles of
which the body is composed are reducible by analysis to three, namely,
sugar, oil, and albumen: of these, sugar and oil are the least, and
albumen the most highly organized. Every alimentary substance must
contain at least one of these proximate principles, and in the various
articles which compose an ordinary meal always two, and often all
three, are afforded in abundance. From the phenomena which have been
stated, it is clear that the digestive organs, in acting on these
principles, exert the following powers.
1. A solvent power. The first action of the stomach on the alimentary
substances presented to it is to reduce them to a fluid state. No
substance is nutritious which is not a fluid, or capable of being
reduced to a fluid. The stomach reduces alimentary substances to
a fluid state by combining them with water. Water enters into the
composition of organized bodies in two states, as an essential and
as an accidental element. A quantity of water is contained in sugar
when reduced to its dryest state; this water cannot be dissipated
without the decomposition of the sugar; it is therefore an essential
constituent of the compound. Water is combined with sugar in its
moist state: of this water much may be removed without destroying the
essential properties of the sugar: this part of the water is therefore
said to be an accidental constituent of the sugar. In most cases
organized bodies contain water in both these forms; and though it is
commonly impossible to discriminate between the water that is essential
and that which is accidental, yet the mode of union among the elements
of bodies in these two states of their combination with water are
essentially different. The stomach has the power of combining water
with alimentary substances in both these forms. Thus fluid albumen,
or white of egg, presented to the stomach is immediately coagulated
or converted into a solid. Soon this solid begins to be softened, and
the softening goes on until it is again reduced to a fluid. What was
fluid albumen in the white of egg is now fluid albumen in chyme; but
the albumen has undergone a remarkable change. Out of the stomach the
albumen of the egg may be converted by heat into a firm solid; but the
albumen of the chyme is capable of being converted only into a loose
and tender solid. In passing from its state in the egg to its state
in the chyme, the albumen has combined with a portion of water which
has entered as an essential ingredient into its composition. By this
combination the compound is reduced from what may be called a strong
to a weak state. This is the first action exerted by the stomach on
most alimentary substances. They are changed from a concentrated to a
diluted, from a strong to a weak state: the power by which the stomach
effects this change is called its reducing power, and the agent by
which it accomplishes it is the gastric juice; the essential ingredient
of which has been shown to be muriatic acid, or chlorine (639, _et
seq._). The muriatic acid obtained from the common salt of the blood
is poured in the form of gastric juice into the stomach, dissolves the
food, combines it with water, reduces it from a concentrated solid to
a dilute fluid; and thus brings it into the condition proper for the
subsequent part of the process.
2. A converting power. Since whatever be the varieties of food, the
chyme invariably forms a homogeneous fluid, the stomach must be endowed
with the power of transforming the simple alimentary principles into
one another; the saccharine into the oily, and the oily into the
albuminous. The transformation of the saccharine into the oleaginous
principle is traceable out of the body in the conversion of sugar into
alcohol, which is essentially an oil. That the same transformation
takes place within the body is indubitable. The oleagenous and the
albuminous principles are already so nearly allied in nature to animal
substance that they do not need to undergo any essential change in
their composition.
3. A completing power. When the alimentary substances have been reduced
and formed into chyme, when the chyme has been converted into chyle,
and when the chyle absorbed by the lacteals is transmitted to the
mesenteric glands, it undergoes during its passage through these organs
a process the direct reverse of that to which it is subjected in the
stomach; for whereas it is the office of the stomach to combine the
alimentary substances with water, it is one office of the mesenteric
glands to remove the superfluous water of the chyle; to abstract
whatever particles of matter may be contained in the compound which are
not indispensable to it, and to concentrate its essential constituents;
and consequently these organs exert on the digested aliment a
completing, in contradistinction to a reducing power.
4. A vitalizing power. When sugar is converted into oil, when oil is
converted into albumen, when albumen, by the successive processes
to which it is subjected is completed, that is, when the alimentary
substances are made to approximate in the closest possible degree to
the nature of animal substance, they must undergo a still further
change, more wonderful than any of the preceding, and far more
inscrutible; they must be endowed with vitality; must be changed from
dead into living matter. Living substance only is capable of forming
a constituent part of living substance. The ultimate action of the
digestive organs is the communication of life to the food, to which
last and crowning process the reducing, converting, and completing
processes are merely subordinate and preparatory. Of the agency by
which this process is effected we are wholly ignorant; we know that
it goes on; but the mode in which it is accomplished is veiled in
inscrutable darkness.
704. Blood is alive; blood is formed from the food; life is
communicated to the food before it is mixed with the blood. The blood
is essentially albumen, which it contains in the form of albumen
properly so called, in that of fibrin, and in that of red particles.
In the thoracic duct the strong albumen of the lymph is mixed with
the weaker albumen of the chyle. At the point where the thoracic duct
terminates in the venous system, lymph and chyle are mixed with venous
blood, and all commingled are borne directly to the lungs. There the
carbon with which the venous blood is loaded is expelled in the form
of carbonic acid gas; the particles of the lymph undergo some, as yet,
unknown change, exalting their organization; and the water hitherto
held in chemical union with the weak albumen of the chyle, is separated
and carried out of the system together with the carbonic acid gas in
the form of aqueous vapour. By this removal of its aqueous particles
the ultimate completion is given to the digested aliment; and the weak
and delicate albumen of the chyle is converted into the strong and firm
albumen of the blood.
705. It has been stated (539), that though gelatin enters abundantly
into the composition of many tissues of the body, and performs most
important uses in the economy, it is never found in the blood; that
it is formed from the albumen of the blood by a reducing process, in
consequence of which carbon is evolved, which unites with the free
oxygen of the blood, forming carbonic acid, thus conducing, among other
purposes, to the production of animal heat. It is equally remarkable,
that though the lymphatics or absorbents arise in countless numbers
from every tissue of the body, and are endowed with the power of taking
up every constituent particle of every organ, solid as well as fluid,
yet gelatin is never found in the lymphatic vessels. The lymphatics
contain only albumen in a form far more proximate to the blood than
that of the chyle; consequently, before the gelatin of the body is
taken up by the lymphatics, it must be reconverted into albumen; that
is, the absorbed gelatin must undergo a process analogous to that which
gelatin and other matters undergo in the stomach and duodenum; it
follows that the digestive process is not confined to the stomach and
duodenum, but is carried on at every point of the body. Hence there are
two processes of digestion, a crude and a refined process. The crude
process is carried on in the stomach and duodenum, in which dead animal
matter is converted into living substance, as yet, however, possessing
only the lowest kind of vitality. The capillary arteries receiving the
substance thus prepared for them, build it up into structure perhaps
the lowest and coarsest, the least organized, and capable of performing
only the inferior functions.
706. Capillary arteries in countless numbers terminate in the tissues
in membraneless canals (304 and 310). Particles of the blood are seen
to quit the arterial stream and to enter into the tissues, becoming a
component part of them: other particles are seen to quit the tissues
and to enter the current of the blood. The latter are probably organic
particles, to which a certain degree of elaboration has been already
given, now transmitted to the capillary veins, to be carried back to
the lungs to undergo there a further depuration, fitting them on their
return to the system for a higher organization.
707. Thus the lymphatic vessels, analogous in so many other respects
to the veins, are probably similar to them in this also—that they take
up from the tissues particles already organized, in order to submit
them to processes which communicate to them a progressively higher
organization. The notion that the contents of the lymphatics consist of
worn-out particles, capable of accomplishing no further purpose in the
economy, is not tenable:—
1. Because it is not analogous to the ordinary operations of nature to
mix wholly excrementitious matter with a substance for the production,
elaboration, and perfection of which, she has constructed such an
expensive apparatus.
2. Because, on the other hand, the admixture of matter already highly
animalized with matter, as yet but imperfectly animalized, exalts the
nature of the latter, and is conducive to its complete animalization.
3. Because the lymph, almost wholly albuminous, is already closely
allied in nature to the blood; it is, therefore, reasonable to infer,
that it is matter passing through an advancing stage of purification
and exaltation.
4. Because this plan of progressive organization is in harmony with the
ordinary operations of nature, in which there is traceable a successive
ascent from the low to the high, the former being preparatory and
necessary to the latter. The tender and delicate organs of animal life,
the brain, the nerves, the apparatus of sense, the muscles, inasmuch as
they perform the highest functions, probably require to be constructed
of a more highly organized material, for the production of which the
matter primarily derived from crude aliment is subjected to different
processes, rising one above the other in delicacy and refinement; by
each of which it is made successively more and more perfect, until it
acquires the highest qualities of living substance, and is capable of
becoming the instrument of performing its most exalted functions.