DIGESTION, NUTRITION.
In plant life the permanent fabric consists of only three
elements--carbon, hydrogen, oxygen. We know that plants alone convert
inorganic or mineral substances into organic matter, and that plants
as a necessary result assimilate their inorganic food, decompose
carbonic acid, and restore its oxygen to the atmosphere.
Vegetation is constructed of cells or vesicles, and has a cellular
tissue. A cell is a living organism. It is that which makes up the
tissue of plants. For the whole life of the plant is that of the cells
which compose it; in them and by them its products are elaborated,
and all its vital processes are carried on. Cell multiplication by
division, cell growth, cell modification, exist in plants. Fluids are
transferred from cell to cell by a process called endosmose. Absorption
takes place by the roots, and the substance absorbed is carried up
into the leaves, even to the topmost bough of a tree, passing in its
course many millions of apparently water-tight partitions. Plants
exchange gases, taking in carbonic acid and giving off oxygen. They
evolve heat, have organs of reproduction, and elaborate the material
for the final evolution of the seed. This seed, whether of grain,
of vegetables, or of fruits, is composed of carbon, hydrogen, and
oxygen. And these constitute the starches and sugars which we find
have been evolved by the vegetable or plant, and which form the
food for animals. Plants, then, convert the elementary substances,
the crude material, into food. In doing so, they pass through the
processes known as the essentials of life; these are, birth, growth,
development, decline, and death.
All organic compounds are transitory. They are constantly appearing and
disappearing, composing and decomposing, organizing and disorganizing;
and they are always dependent upon a certain degree of heat and
moisture for their existence or non-existence.
The universal constituents of plant life; of organic existence,
which are indispensable to vegetation, are carbon, hydrogen, oxygen,
and nitrogen. Every vegetable substance is made up of at least
eighty-eight to ninety-nine per cent of these elements. The proper
vegetable structure, that is, the tissue itself, consists only of
three of these elements, carbon, hydrogen, oxygen; while the fourth,
nitrogen, is an essential constituent of the protoplasm, which plays
so important a part in the formation of the cell, etc.
Plants prepare or elaborate out of these chemical elements
food-substances composed of those elements--starches and sugars--upon
which animals subsist. Animals feeding upon these vegetable
substances assimilate, elaborate, them into meat substances, flesh,
or proteids. These again are composed of carbon, hydrogen, nitrogen,
and oxygen.
Nitrogen plays the important role in proteids, being the distinguishing
feature, as contrasted with substances of vegetable origin, the
carbohydrates.
Thus man is provided with two kinds of food: derived from plants,
carbohydrates; derived from animals, proteids, or albumens, besides
water and mineral salts.
These foods undergo certain preparations previous to being introduced
into the system. In the system the food undergoes farther elaboration,
to make it fit to enter into the circulation of the blood, in order
to supply suitable material for the master tissues.
We will now examine briefly the organs and their secretions that
convert food-substances into blood, and, by the blood, into tissue.
The solvents and diluents of food in the human animal economy are the
saliva of the mouth, the gastric juice of the stomach, the pancreative
juice of the pancreas, the bile of the liver, and the juices of the
intestines--the succus entericus.
The digestive apparatus consists mainly of the alimentary canal
together with various glands of which it receives the secretions.
The alimentary canal commences at the mouth and terminates at the
anus. The average length is about thirty feet, about five or six
times the length of the body.
The part situated in the head and thorax consists of the organs of
mastication, insalivation, and deglutition, and comprises the mouth
with the teeth, the salivary glands, and the æsophagus or gullet. The
parts contained in the abdomen and pelvis consist of the stomach and
the small and large intestines.
The glands which are most immediately connected with digestion are
very numerous small organs, situated in the mucous membrane of the
alimentary canal, and the larger glands, such as the salivary glands,
pancreas, and liver, whose ducts open on its inner surface.
The mouth is included between the lips and the throat, bounded by the
lips, cheeks, tongue, and hard and soft palate. It communicates behind
with the pharynx, and through the pharynx with the æsophagus. It is
lined throughout with mucous membrane.
The mouth contains 32 teeth, 16 in the upper jaw and 16 in the lower
jaw. The inferior maxillary bone, or lower jaw, is the only movable
bone about the head. The teeth have for their functions biting,
grinding, chewing, or triturating any hard food substance that may
be introduced into the mouth.
The tongue is a muscular organ covered with mucous membrane. By its
muscular structure it takes part in the process of mastication and
deglutition, and in the articulation of speech, while its mucous
membrane, with common and tactile sensibility, is the seat of the
sense of taste.
The tonsils are two prominent bodies which occupy the recesses formed,
one on each side of the fauces, between the anterior and posterior
palatine arches and the pillars of the fauces.
The saliva, which is poured into the mouth and there mixed with the
food during mastication, is secreted by three pairs of glands named
from their respective situation parotid, submaxillary, and sublingual.
The parotid is the largest of three salivary glands. It lies on the
side of the face, in front of the ear, and extends deeply into the
space behind the ramus of the lower jaw. Its weight varies from 5 to
8 drachms. It has a duct called the parotid or Stenson's duct. It is
about 2 1/2 inches long, and about a line and a half in thickness. Its
orifice is opposite the crown of the second molar tooth of the
upper jaw.
The submaxillary gland weighs about 2 to 2 1/2 drachms, and is
situated on the inner surface of the inferior maxillary. The duct
is named Wharton's, and is about 2 inches in length. Its orifice is
found under the tongue.
The sublingual gland weighs about a drachm. It is situated on the
floor of the mouth. The ducts are called the ducti Rivintiani. They
are from 8 to 20 in number. They may be seen when the tip of the
tongue is lifted up.
Saliva. Mixed saliva, as it appears in the mouth, is a thick, glairy,
generally frothy, turbid fluid.
The quantity of saliva secreted in 24 hours varies. The average amount
is probably from two to three pints in 24 hours.
The composition of saliva is:
Water, 994.10
Solids, 5.90
The solids are:
Pyaline, 1.41
Fat, 0.07
Epithelium and Mucus, 2.13
{ Sulphocyanide of Potassium, }
{ Phosphate of Soda, }
Salts, { ,, ,, Lime, } 2.29
{ ,, ,, Magnesia, }
{ Chloride of Sodium, }
{ ,, ,, Potassium, }
----
5.90
The specific gravity varies from 1.004 to 1.008.
The rate at which saliva is secreted is subject to considerable
variation. When the tongue and muscles concerned in mastication are at
rest, and the nerves of the mouth are subject to no unusual stimulus,
the quantity secreted is not more than sufficient, with the mucus,
to keep the mouth moist.
The purposes served by saliva are of several kinds:
1. Acting mechanically in conjunction with mucus, it keeps the mouth
in a due condition of moisture, and facilitates the movements of the
tongue in speaking, and the mastication of the food.
2. It serves also in dissolving sapid substances and rendering them
capable of exciting the nerves of taste.
3. By mixing with the food during mastication, it makes it a soft
pulpy mass, such as may easily be swallowed.
4. Saliva performs a chemical part in the digestion of food. It
transforms starchy substances into dextrine and grape sugar.
Starch is a carbohydrate--carbon 18, hydrogen 30, oxygen 15.
C18H30O15 + 3H2O = C6H12O6 + 2(C6H10O5) + 2H2O
(Grape sugar.) (Dextrine.)
Ptyaline is the salient feature of saliva. It is known as a
ferment--acting upon starch and converting it into dextrine and
grape sugar.
The action of saliva varies in intensity in different animals.
The food after having been acted upon and prepared is propelled,
by the act of deglutition, through the æsophagus into the stomach,
by way of the pharynx.
The pharynx is that part of the alimentary canal which unites the
cavities of the mouth and nose to the æsophagus. It extends from
the base of the skull to the lower border of the cricoid cartilage,
and forms a sac open at the lower end, and imperfect in front, where
it presents apertures leading into the nose, mouth, and larynx. The
pharynx is about four and a half inches in length, and is considerably
wider across than it is deep from before backwards.
The æsophagus or gullet, the passage leading from the pharynx into
the stomach, commences at the cricoid cartilage opposite the lower
border of the fifth cervical vertebra, descends in front of the spine,
passes through the diaphragm opposite the ninth dorsal vertebra,
and ends by an opening at the cardiac orifice of the stomach. It is
from nine to ten inches in length.
The stomach is situated in the abdominal cavity. It lies in part
against the anterior wall of the abdomen, and in part beneath the liver
and diaphragm, and above the transverse colon. It is somewhat conical
or pyriform in shape. The left part is the larger, and is named the
cardiac, or splenic, the right is named the pyloric, extremity. The
upper border is about three or four inches in length, is concave,
and is named the lesser curvature, while the lower border is much
longer, is convex, and forms the greater curvature. The dimensions
vary greatly in different subjects, and also according to the state of
distension of the organ. When moderately filled, its length is about
ten to twelve inches, and its diameter at its widest part from four
to five inches. It weighs when freed from other parts about four and
a half ounces in the male and somewhat less in the female.
The structure of the stomach consists of four coats--a serous, a
muscular, an areolar, and a mucous coat. The external or serous coat
is derived from the peritoneum. There are three kinds of muscular
fibers--longitudinal, circular, and oblique, and the internal mucous
lining is a rather thicker, soft, smooth, pulpy membrane, lying in
ridges or rugæ, and containing a large number of glands--tubular
or gastric glands, and another variety of gland called peptic,
besides others.
While the stomach contains no food, and is inactive, no gastric
fluid is secreted; and mucus, which is either neutral or slightly
alkaline, covers its surface. But immediately on the introduction of
food into the stomach, the mucous membrane, previously quite pale,
becomes slightly turgid and reddened with the influx of a large
quantity of blood; the gastric glands commence secreting actively,
and an acid fluid is poured out in minute drops, which gradually run
together and flow down the walls of the stomach, or soak into the
substance introduced. The quantity of this fluid secreted daily has
been variously estimated; but the average for a healthy adult has
been assumed to range from ten to twenty pints in twenty-four hours.
The specific gravity of gastric juice has been found to differ little
from that of water, varying from 1.001 to 1.010, and the amounts of
solid present to be very small, viz., about 56 per cent.
The chemical composition of gastric juice is:
Water, 994.40
Solids, 5.59
{ Ferment, pepsin, and a trace of ammonia, 3.19
{ Hydrochloric acid, 0.20
Solids, { Chloride of calcium, 0.06
{ ,, ,, sodium 1.46
{ ,, ,, potassium, 0.55
{ Phosphate of lime, magnesia, and iron, 0.12
On starch gastric juice per se has no effect whatever, nor has healthy
gastric juice any effect on grape sugar or cane sugar. On fats gastric
juice is powerless.
The essential property of gastric juice is the power of dissolving
proteid matters (meats, albumens, nitrogenous substances), and
converting them into a substance called peptones. Gastric juice thus
readily dissolves coagulated proteids which otherwise are insoluble,
or soluble only with difficulty in very strong acids.
Certain conditions are required for the perfection of the process,
which are all found in the stomach. The first is a temperature
of 100° F. Second, minute division and constant movement favor
digestion. Third, the greater the surface presented to the action of
the juice, the more rapid the solution.
Neutralization of the juice wholly arrests digestion.
The digestive action of gastric juice on proteids, like that of saliva
on starch, is a ferment action; in other words, the solvent action of
gastric juice is essentially due to the presence in it of a ferment
body called pepsin.
The general effect of digestion of the stomach is the conversion of
food into chyme, a substance of various compositions according to
the nature of the food, yet always presenting a characteristic thick
pultaceous grumous consistence.
The small intestines commence at the pylorus and after many
convolutions terminate in the large intestines. They measure on an
average about twenty feet in length in the adult. For convenience they
have been divided into three parts--the duodenum, which extends from
eight to ten inches beyond the pylorus; the jejunum, which occupies
two-fifths, and the ilium, which occupies three-fifths, of the rest
of the canal.
The mucous membrane, the interior coat, is the most important to
the function of digestion. There are permanent folds, shelf-like
processes, of the mucous membrane, called valvular conniventes. There
are also villi and glands, as the glands of Lieberkühn, of Peyer,
and of Bruner. The glands of Lieberkühn are thickly distributed over
the whole surface of the large and small intestines. The glands
of Peyer are exclusively in the small intestine. They are found
in greatest abundance in the lower part of the ileum near to the
ileo-cæcal valve. They are met with in two conditions, viz., either
scattered singly, in which case they are termed glandulæ solitairæ,
or aggregated in groups varying from one to three inches in length
and about half an inch in width, chiefly of an oval form, their long
axes parallel with that of the intestines. In this state they are
named glandulæ agminatæ. The latter are almost always placed opposite
the attachment of the mesentery. In structure they are analogous
to lymphatics or absorbent glands, and their office is to take up
certain materials from the chyle, elaborate them, and subsequently
discharge them into the lacteals, with which vessels they appear to
be closely connected. Bruner's glands are confined to the duodenum;
they are most abundant and thickly set at the commencement of this
portion of the intestines, and are provided with permanent gland ducts.
The villi are confined exclusively to the mucous membrane of the
small intestines. They are minute vascular processes, from a quarter
of a line to a line and two-thirds in length. There are about fifty
to ninety in number to a square line. Each villus consists of a
small projection of mucous membrane, and its interior is supported
throughout by fine retiform or adenoid tissue. Two or more arteries
are distributed to each villus, and from their capillaries, which
form a dense network, proceed one or two small veins, which pass out
at the base of the villus.
The lacteal vessels enter the base of each villus, and passing up
in the middle extend nearly to the top, where it ends commonly by
a closed and somewhat dilated extremity. The office of the villi
is the absorption of chyle from the completely digested food of
the intestines.
The large intestine extends from the termination of the ileum to the
anus. It is usually about five to six feet in length, being about
one-fifth of the whole length of the intestinal canal. The large
intestine is constructed of four coats like those of the stomach and
small intestines, namely, the serous, the muscular, the areolar or
submucous, and the mucous. It is divided into the ascending colon,
transverse and descending colon, and rectum and anus.
The cæcum is a short wide pouch, communicating with the lower end of
the small intestines through an opening guarded by the ileo-cæcal
valve. The appendix vermiformis is attached to the cæcum. The
colon commences at the right groin, ascends to the liver, forms the
hepatic flexure, then crosses transversely from right to left to
the spleen, forms the splenic flexure, descends to the left groin,
forms the sigmoid flexure, passes through the pelvis as the rectum,
and terminates at the anus.
The mucous membrane of the large intestines, like that of the small
intestines, is lined throughout by columnar epithelium, but unlike
it, is quite destitute of villi and is not projected in the form of
valvular conniventes.
The peritoneum, or serous membrane of the abdominal cavity, is by
far the most extensive and complicated of serous membranes. Like the
others, it may be considered to form a shut sac, on the outside of
which are placed the viscera, which it covers. The peritoneum forms
the mesenteries and omenta for the stomach, small and large intestines,
and ligaments for the liver, spleen, uterus, and bladder.
The liver is a very important glandular organ, very constant in the
animal series, being found in all the vertebrates, and, in a more or
less developed condition, in most invertebrate tribes. It secretes
bile, and appears to act upon the blood which is transmitted through
it. The liver is the largest gland in the body, and by far the most
bulky of the abdominal viscera. It measures from ten to twelve inches
transversely from right to left, between six and seven inches from
its posterior to its anterior border, and about three and a half
inches from above downwards where thickest, which is towards the
right and posterior part. The average bulk is about eighty-eight
cubic inches. The ordinary weight in the adult is between 50 to 60
ounces, about one-thirtieth of the weight of the whole body. The
liver is solid to the feel, and of a dull reddish-brown color, with
frequently a dark-purplish tinge along the margin. It has an upper
surface smooth and convex, and an under surface which is uneven and
concave. The liver is divided into two unequal lobes, a right and a
left, and on the under surface of the right lobe are three secondary
lobes or lobules, named the lobe of Spigolius, the caudate or tailed
lobe, and the square lobe. It has five fissures or fossæ, described
as the transverse or portal; the umbilical fissure and the fissure
of the ductus venosus, together forming the longitudinal fissure;
the fossa of the vena cava, and the fossa of the gall bladder. It is
held in position by five ligaments formed by layers of peritoneum.
The liver is situated on the right side of the body under the
diaphragm. The convex surface is protected, on the right by the six
or seven lower ribs, and in front by the cartilages of the same, and
by the ensiform cartilage, the diaphragm of course being interposed.
To the left of the longitudinal fissure the liver is in contact with
the pyloric extremity and anterior surface of the stomach, on which it
moves freely. When the stomach is quite empty, the left part of this
surface of the liver may overlap the cardiac end of that organ. To
the right of the longitudinal fissure the liver rests upon the first
part of the duodenum and the hepatic flexure of the colon. Farther
back it is in contact with the upper part of the right kidney and
suprarenal capsule.
The two blood-vessels which supply the liver are the hepatic artery and
the vena porta. The hepatic vein conveys the blood away from the liver.
The lymphatics of the liver are large and numerous, forming a deep
and a superficial set.
The nerves are derived partly from the coeliac plexus and partly from
the pneumogastric nerve, especially from the left pneumogastric.
The excretory apparatus of the liver consists of the hepatic duct,
the cystic duct, gall bladder, and common bile duct.
The hepatic duct is formed by the union of a right and left branch,
which issue from the bottom of the transverse fissure and unite at a
very obtuse angle; it descends to the right, within the gastro-hepatic
omentum. Its diameter is nearly two lines, and its length nearly two
inches. At its lower end it meets the cystic descending from the gall
bladder, and the ducts uniting together at an angle form the common
bile duct.
The cystic duct is about one and a half inches in length.
The gall bladder is a pear-shaped membranous sac, three or four inches
long, about an inch and a half across its widest part, and capable of
containing from 8 to 12 fluid drachms. The gall bladder is attached
to the liver. The neck, gradually narrowing, becoming constricted,
bends downward, and terminates in the cystic duct.
The common bile duct (ductus communis choledicus), the largest of
the ducts, being from two to three lines in width, and nearly three
inches long, conveys the bile from the liver and the gall bladder
into the duodenum by a common orifice, with the pancreatic duct on
its inner surface, about three to four inches below the pylorus.
The liver is an extremely vascular organ, and receives its blood
supply from two distinct vessels, the portal vein and the hepatic
artery, while the blood is returned from it into the inferior vena
cava by the hepatic vein. Its secretion, the bile, is conveyed from
it by the hepatic duct, either directly into the intestines, or, when
digestion is not going on, into the cystic duct, and thence into the
gall bladder, where it accumulates until required. The portal vein,
hepatic artery, and hepatic duct branch together throughout the liver,
while the hepatic vein and its tributaries run by themselves. At the
transverse fissure it is merged into the areolar investment called
Glisson's capsule, which surrounds the portal vein, hepatic artery,
and hepatic duct, as they enter at this part, and accompanies them
in their branches through the substance of the liver.
The liver is made up of small roundish or oval portions called lobules,
each of which is about 1/20 of an inch in diameter, and composed of
minute branches of the portal vein, hepatic artery, hepatic duct,
and hepatic vein; while the interstices of these vessels are filled
by liver cells. These cells, which make up a great portion of the
substance of the organ, are of rounded or polygonal form; about 1/800
to 1/1000 of an inch in diameter.
The function of the liver is the secretion of bile. The bile is
a somewhat viscid fluid of a yellow, or greenish-yellow, color, a
strongly bitter taste, and when fresh a scarcely perceptible odor. It
has a neutral or slightly alkaline reaction, and its specific gravity
is 1.020.
The composition of human bile is:
Water, 859.2
Solids, 140.8
-------
1,000
The solids are:
Biliary acids combined with alkalies (bilin), 91.5
Fat, 9.2
Cholestrin, 2.6
Mucus and coloring matter, 29.8
Salts, 7.7
-----
140.8
Bile is distinguished from the other alimentary secretions by the
entire absence of proteids. The chemical composition of bilin, as
compared with the organic parts of blood, is:
Carb. Hyd. Nitr. Oxy. Sul.
Bilin atoms, 76 66 2 22
Blood, 48 36 6 14
{ Biliverdin, 16 20 2 5
Coloring matter, { Glycocholic acid, 26 43 1 6
{ Taurocholic acid, 26 45 1 7 1
There seems to be some relationship between the coloring matters
of the blood and bile; and it may be added, between these and that
of the urine also; so that it is possible they may be, all of them,
varieties of the same pigment, or derived from the same source.
The quantity of bile discharged into the intestines is estimated
to be about thirty to forty ounces secreted by an adult man in
twenty-four hours.
The purposes served by the secretion of bile may be considered to be
of two principal kinds, viz., excrementitious and digestive.
As an excrementitious substance, the bile serves especially as a medium
for the separation of excess of carbon and hydrogen from the blood.
Though one of the chief purposes of the secretion of bile may appear
to be the purification of the blood by ultimate excretion, yet there
are many reasons for believing that while it is in the intestines it
performs an important part in the process of digestion. Bile has a
slight solvent action on fats, and only a slight emulsifying power.
Its functions generally may be considered thus:
1. It assists in emulsifying fatty portions of food, thus rendering
them capable of being absorbed by the lacteals.
2. Bile facilitates the absorption of fatty matter.
3. Bile, like the gastric fluid, has a strongly antiseptic power,
and may serve to prevent the decomposition of food during the time
of its sojourn in the intestines.
4. Bile has been considered to act as a natural purgative, by prompting
an increased secretion of the intestinal glands.
5. Another very important function appears to be that of so acting upon
certain constituents of the blood passing through it, as to render
some of them capable of assimilation with blood generally, and to
prepare others for being duly eliminated in the process of respiration.
6. An important influence seems also to be exerted by the liver upon
the saccharine matters derived from the alimentary canal. The chief
purpose of the saccharine and amylaceous principles of food is,
in relation to respiration and the production of animal heat.
The pancreas is a long, narrow, flattened gland of a reddish-cream
color, larger at one end than at the other, and lying behind the
stomach opposite the first lumbar vertebra. It is usually from 6 to
8 inches long, about 1 1/2 inch in average width, and 1/2 to 1 inch
in thickness. It weighs about 2 1/4 to 3 1/2 ounces. Its principal
excretory duct is called the pancreatic duct, and runs through the
entire length of the gland from left to right. The duct opens in
a common orifice with the ductus communis choledicus on the inner
surface of the duodenum about 4 inches below the pylorus.
Healthy pancreatic juice is a clear, viscid fluid, frothing when
shaken. It has a very decided alkaline reaction. The pancreas in its
minute anatomy closely resembles the salivary glands; and the fluid
elaborated by it appears almost identical with saliva.
The composition of pancreatic juice is:
Water, 980.45
Solids, 19.55
The solids are:
Pancreatic, 12.71
Inorganic bases and salts, 6.84
-----
19.55
Action of pancreatic juice. (1) It acts on starch raw and boiled with
great energy, rapidly converting it into grape sugar. (2) On proteids
(meats) it also exercises a solvent action, so far similar to that of
gastric juice that by it the proteids are converted into peptones. (3)
On fats pancreatic juice has a twofold action: it emulsifies them, and
it splits up neutral fats into their respective acids and glycerine.
Thus pancreatic juice is remarkable for the power it possesses of
acting on all food-stuffs--on starch, fats, and proteids.
Succus entericus (intestinal juice). The precise action of this is
not known. It has been said to act upon starch, to convert proteids
into peptones, and to emulsify fats. On the other hand, each of these
actions has been denied.
The portal system of veins. The portal vein, or vena porta, collects
the blood from the stomach, intestines, pancreas, and spleen; and
carries it to the liver, from which the bile is secreted; ramifying
after the manner of an artery in the substance of the liver and
conveying to the capillaries of that organ the blood collected in
the main trunk. This blood, together with that of the hepatic artery,
after having served for the secretion of the bile and the nourishment
of the liver, is withdrawn from that organ by the hepatic veins,
and carried by them into the vena cava inferior.
Digestion begins at the mouth. Food is masticated by the movement of
the lower jaw, broken into small pieces, moistened by the saliva,
and starchy substances are converted into sugar. No change takes
place during the rapid transit through the æsophagus.
In the stomach the proteids are acted upon by the gastric juice and
converted into peptones. Fats remain unchanged, and sugars are not
acted upon. While these changes are proceeding, the thick grayish
liquid, or chyme, formed by the imperfectly dissolved food, is from
time to time ejected through the pylorus, accompanied even by large
morsels of solid less digested matter. This may occur within a few
minutes of food having been token, but the larger escape from the
stomach probably does not begin till from one to two and lasts from
four to five hours after the meal, becoming more rapid towards the
end, such pieces as most resist the gastric juice being the last to
leave the stomach. Substances can be absorbed from the cavity of the
stomach into the circulation. The presumption is, that the diffusible
sugars and peptones pass by osmosis direct into the capillaries,
and so into the gastric veins.
In the small intestines the semi-digested food, or chyme, as it passes
the biliary orifice causes a gush of bile, and at the same time the
pancreatic juice which flows freely into the intestine at the taking
of the meal, is secreted again with renewed vigor, when the gastric
digestion is completed. The conversion of starch into sugar, which
may have languished in the stomach, is resumed with great activity
by the pancreatic juice. The pancreatic juice emulsifies fats, and
also splits them into their respective fatty acids and glycerine,
and the bile is able to a certain extent to saponify the free fatty
acids. It also appears that the slight emulsifying power of the bile
is much increased by the presence of soap; and as a matter of fact,
the bile and pancreatic juice do largely emulsify the contents of the
small intestines, so that the grayish turbid chyme is changed into
a creamy-looking fluid, which has been called chyle. These products
as they are formed pass into the lacteals or the portal blood-vessels.
Through the large intestine pass off indigestible or undigested
constituents of the meal, and the gases generated.
Absorption takes place from the stomach, and occurs along the course
of the small and large intestines, especially of water. The largest
and most important part of the digested material passes away from
the canal during the transit of food along the small intestines,
partly into the lacteals, partly into the portal vein.
Digestion being, broadly speaking, the conversion of non-diffusible
proteids and starch into highly diffusible peptones and sugar, and
the emulsifying, or division into minute particles, of various fats,
it is natural to suppose that the diffusible peptones and sugars pass
by osmosis into the blood-vessels, and that the emulsified fats pass
into the lacteals. That the great mass of the fat which enters the
body from the intestines passes through the lacteals, there can be
no doubt; and there is but little doubt that a considerable quantity
of peptone and sugar does pass into the portal blood.
Chyle is a white milky-looking fluid, which after its escape
coagulates, forming a not very firm clot. The nature of the coagulation
seems to be exactly the same as that of blood.
Lymph seems to be blood minus red corpuscles, and chyle is lymph plus
a very large quantity of minutely divided fats.
It has been calculated that a quantity equal to that of the whole
blood may pass through the thoracic duct in twenty-four hours, and
of this it is supposed that about half comes from food through the
lacteals, the remainder from the body at large; but these calculations
are based on uncertain data.
Entrance of chyle into the lacteals. The lacteals begin at a
club-shaped lymphatic space lying in the center of the villus, and
connected with the smaller lymphatic spaces of the adenoid tissue
around it; it opens below into the submucous lymphatic plexus from
which the lacteals spring.
The thoracic duct is the common trunk which receives the absorbents
from both the lower limbs, from the abdominal viscera, from the
walls of the abdomen, from the left side of the thorax, left lung,
left side of the heart, and left upper limbs, and from the left side
of the head and neck. It is from fifteen to eighteen inches long in
the adult, and extends from the second lumbar vertebra to the root of
the neck. At the last dorsal vertebra there is usually a dilation of
the duct, of variable size, which is called the receptaculum chyli,
and is the common place of junction of the lymphatics of the lower
limbs and the trunks of the lacteal vessels. There are two sets of
absorbent vessels--the lacteals, which convey the chyle from the
alimentary canal to the thoracic duct; and the lymphatics, which
take up the lymph from all the other parts of the body and return
it into the venous system. There is a right lymphatic duct, about
a quarter to a half inch in length, which receives the lymph from
the absorbents of the right upper limb, the right side of the head
and neck, the right side of the chest, the right lung and the right
half of the heart, and the upper surface of the liver. The thoracic
duct terminates on the outer side of the internal jugular vein,
in the angle formed by the union of that vein with the subclavian,
and the subclavian empties itself in the superior vena cava.
Lymphatics and lacteals are furnished with valves serving the same
office as those of the veins, and for the most part constructed after
the same fashion.
Lymph and chyle, unlike the blood, pass only in one direction, namely,
from the fine branches to the trunk and so to the large veins, on
entering which they are mingled with the stream of blood and form
part of its constituents.
In some part of their course all lymphatic vessels pass through
certain bodies called lymphatic glands.
Analysis of lymph and chyle:
Lymph. Lymph from Chyle from
Thoracic Duct. the Lacteals.
Water, 937.32 939.70 902.37
Fibrine, 0.595 10.60 3.70
Albumen, 42.775 38.83 35.16
Fat, 6.51 a little 36.01
Extractive Matter, 5.05
Salts, 7.75 10.87 22.76
------ ------- -------
1000. 1000. 1000.
Chyle having reached the lymphatic channels, its onward progress is
determined by a variety of circumstances. Putting aside the pumping
action of the villi, the same events which cause the movement of the
lymph generally, also further the flow of the chyle, and these are
briefly as follows:
1. The wide-spread presence of valves in the lymphatic vessels causes
every pressure exercised on the tissues in which they lie, to assist
in the propulsion forward of the lymph.
2. Considering the whole lymphatic system as a set of branching tubes
passing from the extravascular regions just outside the small arteries
and veins and capillaries, to the large venous trunks, it is obvious
that the mean pressure of the blood in the subclavian at the junction
with the jugular is the cause of the movement, etc., assisted perhaps
by the respiratory movements, and other causes, as osmosis, etc.
The average quantity of solid fecal matter evacuated by the human adult
in twenty-four hours is about five ounces; an uncertain proportion
of which consists simply of the undigested or chemically modified
residue of the food, and the remainder of certain matters which are
excreted in the intestinal canal.
Gases contained in the stomach and intestines. The sources of the
gases contained in the stomach and bowels may be enumerated:
1. Air introduced in the act of swallowing either food or saliva.
2. Gas developed by the decomposition of alimentary matter, or of
the secretions and excretions mingled with it in the stomach and
intestines.
3. It is probable that a certain mutual interchange occurs between
the gases contained in the alimentary canal, and those present in
the blood of the gastric and intestinal blood-vessels.
The movement of the intestines is peristaltic or vermicular, and is
effected by the alternate contractions and dilatations of successive
portions of the intestinal coats. The contractions, which may commence
at any point of the intestine, extend in a wavelike manner along
the tube. This is due to the involuntary longitudinal and circular
muscular fibers contracting successively from above downwards and from
behind forwards, etc. The movements take place slowly, and in health
are commonly unperceived by the mind, but they are perceptible when
they are accelerated under the influence of any irritation.