if a quantity of blood be exposed to a red heat in a crucible, the
greatest part will be volatilised and burnt; but a quantity of brown
ashes will be left behind, which will be attracted by the magnet. If
diluted sulphuric acid be poured on these ashes, a considerable
portion of them will dissolve; if into this solution we drop tincture
of galls, a black precipitate will take place, or if we use prussiate
of potash, a precipitate of prussian blue will be formed. These facts
prove, beyond doubt, that a quantity of iron exists in the blood.
I shall not now particularly inquire how it comes there; it may
partly be taken into the blood along with the vegetable and animal
food, which is received into the stomach; for the greatest part of
the animal and vegetable substances, which we receive as food,
contain a greater or less quantity of iron. Or it may be partly
formed by the animal powers, as would appear from the following
circumstance. The analysis of an egg, before incubation, affords not
the least vestige of iron, but as soon as the chick exists, though it
has been perfectly shut up from all external communication, if the
egg be burnt, the ashes will be attracted by the magnet.
But, however we may suppose the blood to obtain its iron, it
certainly does contain it; if the coagulable lymph and serum of the
blood be carefully freed from the red particles, by repeated washing,
the strictest analysis will not discover in either of them a particle
of iron, while the red globules thus separated will be found to
contain a considerable quantity of this metal.
That the red colour of the blood depends upon iron, appears likewise
from the experiments of Menghini, which show, that the blood of
persons who have been taking chalybeate medicines for some time, is
much more florid that it is naturally; the same is agreeable to my
own observation. A late analysis, by Fourcroy, has likewise proved,
that the red colour of the blood resides in the iron; but, though the
red colour of the blood may reside in the iron which it contains, we
shall find that this colour is likewise connected with oxidation.
If the dark coloured blood, drawn from the veins, be put under a
vessel containing oxygen gas, its surface will immediately become
florid, while the bulk of the gas will be diminished. Mr. Hewson
enclosed a portion of a vein between two ligatures, and injected into
it a quantity of oxygen gas; the blood, which was before dark
coloured, instantly assumed the hue of arterial blood. Thuvenal put a
quantity of arterial blood under the receiver of an air pump; on
exhausting the air it became of the dark colour of venous blood; on
readmitting the air, it became again florid. He put it under a
receiver filled with oxygen gas, and found the florid colour much
increased.
Dr. Priestly exposed the blood of a sheep successively to oxygen gas,
atmospheric air, and carbonic acid gas; and found, that in oxygen gas
its colour became very florid, less so in atmospheric air, and in
carbonic acid gas it became quite black. He filled a bladder with
venous blood, and exposed it to oxygen gas; the surface in contact
with the bladder immediately became florid, while the interior parts
remained dark coloured.
All these facts prove, that the red colour which the blood acquires
in the lungs, is owing to the oxygen, which probably combines with
it, and the last mentioned fact shows, that oxygen will act on the
blood, even though a membrane similar to the bladder, be interposed
between them.
The same effect, probably, takes place in the lungs; the blood is
circulated through that organ by a number of fine capillary arteries;
and it is probable that the oxygen acts upon the blood through the
membranes of these arteries, in the same manner that it does through
the bladder.
In short, it seems likely, that the blood, during its circulation
through the lungs, becomes combined with oxygen; that this oxidated
blood, on its return to the heart, is circulated by the arteries to
all parts of the body; and that, during this circulation, its oxygen
combines with the hydrogen and carbon of the blood, and perhaps with
those parts of the body with which it comes into contact; it is
therefore brought back to the heart, by the veins, of a dark colour,
and deprived of the greatest part of its oxygen.
This is the most probable theory, in the present state of our
knowledge; it was proposed by Lavoisier, who imagines the focus of
heat, or fireplace to warm the body, to be in the lungs: others,
however, have thought it more consonant to facts, to suppose, that,
instead of the oxygen uniting with carbon and hydrogen in the lungs,
and there giving out its heat, the oxygen is absorbed by the blood,
and unites with these substances during the circulation, so that heat
is produced in every part of the body; and this doctrine seems
certainly supported by several facts and experiments.
The circulation of the blood, though so simple and beautiful a
function, was unknown to the ancient physicians, and was first
demonstrated by our countryman, Harvey; when he first published his
account of this discovery, he met with the treatment which is
generally experienced by those who enlighten and improve the comfort
of their fellow creatures, by valuable discoveries. The novelty and
merit of this discovery drew upon him the envy of most of his
contemporaries in Europe, who accordingly opposed him with all their
power; and some universities even went so far, as to refuse the
honours of medicine to those students, who had the audacity to defend
this doctrine; but afterwards, when they could not argue against
truth and conviction, they attempted to rob him of the discovery, and
asserted that many of the ancient physicians, and particularly
Hippocrates, were acquainted with it. Posterity, however, who can
alone review subjects of controversy without prejudice, have done
ample justice to his memory.
LECTURE IV.
DIGESTION, NUTRITION, &c.
The human body, by the various actions to which it is subject, and
the various functions which it performs, becomes, in a short time,
exhausted; the fluids become dissipated, the solids wasted, while
both are continually tending towards putrefaction. Notwithstanding
which, the body still continues to perform its proper functions,
often for a considerable length of time; some contrivance, therefore,
was necessary to guard against these accelerators of its destruction.
There are two ways in which the living body may be preserved; the one
by assimilating nutritious substances, to repair the loss of
different parts; the other to collect, in secretory organs, the
humours secreted from these substances.
We are admonished of the necessity of receiving substances into the
body, to repair the continual waste, by the appetites of hunger and
thirst. For the stomach being gradually emptied of its contents, and
the body, in some degree, exhausted by exercise, we experience a
disagreeable sensation in the region of the stomach, accompanied by a
desire to eat, at first slight, but gradually increasing, and at last
growing intolerable, unless it be satisfied.
When the fluid parts have been much dissipated, or when we have
taken, by the mouth, any dry food, or acrid substance, we experience
a sensation of heat in the fauces, and at the same time a great
desire of swallowing liquids. The former sensation is called hunger,
and the latter thirst.
From the back part of the mouth passes a tube, called the oesophagus
or gullet, its upper end is wide and open, spread behind the tongue
to receive the masticated aliment: the lower part of this pipe, after
it has passed through the thorax, and pierced the diaphragm, enters
the stomach, which is a membranous bag, situated under the left side
of the diaphragm: its figure nearly resembles the pouch of a bagpipe,
the left end being most capacious; the upper side is concave, and the
lower convex: it has two orifices, both on its upper part; the left,
which is a continuation of the oesophagus, and through which the food
passes into the stomach, is named cardia; and the right, through
which the food is conveyed out of the stomach, is called pylorus:
within this last orifice is a circular valve, which, in some degree,
prevents the return of the aliment into the stomach.
From the pylorus, or right orifice of the stomach, arise the
intestines, or bowels, which consist of a long and large tube, making
several circumvolutions, in the cavity of the abdomen; this tube is
about five or six times as long as the body to which it belongs.
Though it is one continued pipe, it has been divided, by anatomists,
into six parts, three small, three large. The three small intestines
are the duodenum, the jejunum, and the ileum; the duodenum commences
at the pylorus, and is continued into the jejunum, which is so called
from its being generally found empty: the ileum is only a
prolongation of the jejunum, and terminates in the first of the great
intestines, called the caecum. The other great guts are the colon and
the rectum.
The whole of what has been described is only a production of the same
tube, beginning at the oesophagus. It is called by anatomists the
intestinal canal, or prima via, because it is the first passage of
the food. It has circular muscular fibres, which give it a power of
contracting when irritated by distension; and this urges forward the
food which is contained in it. This occasions a worm like motion of
the whole intestines, which is called their peristaltic motion.
The mesentery is a membrane beginning loosely on the loins, and
thence extending to all the intestines; which it preserves from
twisting by their peristaltic motion. It serves also to sustain all
the vessels going to and from the intestines, namely the arteries,
veins, lacteals, and nerves; it also contains several glands, called,
from their situation, mesenteric glands.
The lacteal vessels consist of a vast number of fine pellucid tubes,
which arise by open mouths from the intestines, and proceeding thence
through the mesentery, they frequently unite, and form fewer and
larger vessels, which pass through the mesenteric glands, into a
common receptacle or bag, called the receptacle of the chyle. The use
of these vessels is to absorb the fluid part of the digested aliment,
called chyle, and convey it into the receptacle of the chyle, that it
may be thence carried through the thoracic duct into the blood.
The receptacle of the chyle is a membranous bag, about two thirds of
an inch long, and one third of an inch wide, at its superior part it
is contracted into a slender membranous pipe, called the thoracic
duct, because its course is principally through the thorax; it passes
between the aorta and the vena azygos, then obliquely over the
oesophagus, and great curvature of the aorta, and continuing its
course towards the internal jugular vein, it enters the left
subclavian vein on its superior part.
There are several other viscera besides those I have described, which
are subservient to digestion; among these may be mentioned the liver,
gall bladder, and pancreas. The liver is the largest gland in the
body, and is situated immediately under the diaphragm, principally on
the right side. Its blood vessels that compose it as a gland, are the
branches of the vena portarum, which, as I mentioned in the last
lecture, enters the liver and distributes its blood like an artery.
From this blood the liver secretes the bile, which is conveyed by the
hepatic duct, towards the intestines: before this duct reaches the
intestines, it is joined by another, coming from the gall bladder:
these two ducts uniting, form a common duct, which enters the
duodenum obliquely, about four inches below the pylorus of the
stomach.
The gall bladder, which is a receptacle of bile, is situated between
the stomach and the liver; and the bile which comes from the liver,
along the hepatic duct, partly passes into the duodenum, and partly
along the cystic duct into the gall bladder. When the stomach is
full, it presses on the gall bladder, which will squeeze out the bile
into the duodenum at the time when it is most wanted.
The bile is a thick bitter fluid, of a yellowish green colour,
composed chiefly of soda and animal oil, forming a soap; and it is
most probably in consequence of this saponaceous property that it
assists digestion, by causing the different parts of the food to
unite together by intermediate affinity. When the bile is prevented
from flowing into the intestines, by any obstruction in the ducts,
digestion is badly performed, costiveness takes place, and the
excrements are of a white colour, from being deprived of the bile.
This fluid, stagnating in the gall bladder, is absorbed by the
lymphatics, and carried into the blood, communicating to the whole
surface of the body a yellow tinge, and other symptoms of jaundice.
The jaundice therefore is occasioned by an obstruction to the passage
of the bile into the intestines, and its subsequent absorption into
the blood: this obstruction may be caused either by concretions of
the bile, called gall stones, or by a greater viscidity of the fluid,
or by a spasm, or paralysis of the biliary ducts.
The pancreas, or sweet bread, is a large gland lying across the upper
and back part of the abdomen, near the duodenum. It has a short
excretory duct, about half as wide as a crow quill, which enters the
duodenum at the same place where the bile duct enters it.
The food being received into the mouth, is there masticated or broken
down, by the teeth, and impregnated with saliva, which is pressed out
of the salivary glands, by the motions of the jaw and the muscles of
the mouth. It then descends, through the oesophagus, into the
stomach, where it becomes digested, and, in a great measure,
dissolved, by the gastric juice, which is secreted by the arteries of
the stomach. It is then pushed through the pylorus, or right orifice
of the stomach into the duodenum, where it becomes mixed with the
bile from the gall bladder and liver, and the pancreatic juice from
the pancreas. These fluids seem to complete the digestion: after
this, the food is continually moved forwards by the peristaltic
motion of the intestines.
The chyle, or thin and milky part of the aliment, being absorbed by
the lacteals, which rise, by open mouths, from the intestines, is
carried into the receptacle of the chyle, and from thence the
thoracic duct carries it to the subclavian vein, where it mixes with
the blood, and passes with it to the heart.
The food of animals is derived from the animal or vegetable kingdoms.
There are some animals which eat only vegetables, while others live
only on animal substances. The number and form of the teeth, and the
structure of the stomach, and bowels, determine whether an animal be
herbivorous, or carnivorous. The first class have a considerable
number of grinders, or dentes molares; and the intestines are much
more long and bulky; in the second class, the cutting teeth are
predominant, and the intestines are much shorter.
Man seems to form an intermediate link between these two classes: his
teeth, and the structure of the intestines, show, that he may subsist
both on vegetable and animal food; and, in fact, he is best nourished
by a proper mixture of both. This appears from those people who live
solely on vegetables, as the Gentoo tribes, and those who subsist
solely on animals, as the fish eaters of the northern latitudes,
being a feebler generation than those of this country, who exist on a
proper mixture of both. A due proportion, therefore, of the two kinds
of nourishment, seems undoubtedly the best.
Having taken a general view of the course of the aliment into the
blood, I shall now examine more particularly, how each part of the
organs concerned in digestion, or connected with that function,
contributes to that end.
The food being received into the mouth, undergoes various
preparations, which fit it for those changes it is afterwards to
undergo. By the teeth the parts of it are divided and ground,
softened and liquified by the saliva, and properly compressed by the
action of the tongue and mastication.
The mouth, in most animals, is armed with very hard substances,
which, by the motion of the lower jaw, are brought strongly into
contact. Those parts of the teeth which are above the sockets, are
not simply bony, they are much harder than the bones, and possess the
property of resisting putrefaction, as long as this hard crust
continues to cover them. The teeth are divided into three classes:
1st. The cutting teeth, which are sharp and thin, and which serve to
cut or divide the food: 2nd. The canine teeth, which serve to tear it
into pieces still smaller: 3rd. The grinders, which present large and
uneven surfaces, and actually grind the substance already broken down
by the other teeth. Birds, whom nature has deprived of teeth, have a
strong muscular stomach, called the gizzard, which serves the
purposes of teeth, and they even take into the stomach small pieces
of grit, to assist in grinding to a powder the grain that they have
swallowed.
Among those parts of the mouth which contribute to the preparation of
the food, we must reckon the numerous glands which secrete saliva,
and which have therefore been called salivary glands. The saliva is a
saponaceous liquor, destitute of taste or smell, which is squeezed
out from these glands, and mixed with the food during mastication. In
the mouth, therefore, the food becomes first broken down by the
teeth, impregnated with saliva, and reduced to a soft pasty
substance, capable of passing with these, through the oesophagus,
into the stomach. It is here that it undergoes the change, which is
particularly termed digestion.
Digestion comprehends two classes of phenomena, distinct from each
other: 1st. Physical and chemical: 2nd. Organic and vital. The object
of the first, is to bring the alimentary substances to such a state
as is necessary, that they may be capable of the new combinations
into which they are to enter, to obtain the animal character. The
object of the second is, to produce those combinations which some
have thought to be very different from those produced by simple
chemical attractions.
The physical and chemical phenomena of digestion, relate chiefly,
1st. To the action of heat; 2ndly. To the dissolution of the
alimentary substances. The heat of the animal is such, as is well
fitted to promote solution.
That digestion is performed by solution, is evident, from several
experiments, particularly those made by Dr. Stevens, who enclosed
different alimentary substances in hollow spheres of silver, pierced
with small holes. These were swallowed, and after remaining some time
in the stomach, the contents were found dissolved. The great agent of
solution is the gastric juice, which possesses a very strong solvent
power. This juice is secreted by the arteries of the stomach; it may
be collected in considerable quantity, by causing an animal that has
been fasting for some time, to swallow small hollow spheres, or tubes
of metal filled with sponge.
This liquid does not act indiscriminately upon all substances; for if
grains of corn be put into a perforated tube, and a granivorous bird
be made to swallow it, the corn will remain the usual time in the
stomach without alteration; whereas if the husk of the grain be
previously taken off, the whole of it will be dissolved. There are
many substances likewise which pass unaltered through the intestines
of animals, and consequently are not acted upon by the gastric juice.
This is the case frequently with grains of oats, when they have been
swallowed by horses entire, with their husks on. This is the case
likewise with the seeds of apples and other fruits, when swallowed
entire by man; yet if these substances have been previously ground by
the teeth, they will be digested. It would appear therefore, that it
is chiefly the husk or outside of these substances which resists the
action of the gastric juice.
This juice is not the same in all animals; for many animals cannot
digest the food on which others live. Thus sheep live wholly on
vegetables, and if they are made to feed on animals, their stomachs
will not digest them: others again, as the eagle, feed wholly on
animal substances, and cannot digest vegetables.
The accounts of the experiments made on gastric juice are very
various: sometimes it has been found of an acid nature, at other
times not. The experiments of Spallanzani show, however, that this
acidity is not owing to the gastric juice, but to the food. The
result of his experiments, which have been very numerous, prove, that
the gastric juice is naturally neither acid nor alkaline. No
conclusion, however, can be drawn from these experiments made out of
the stomach, with respect to the nature of the gastric juice; nor do
the analyses which have been made of it throw any light on its mode
of action. But, from the experiments which have been made on
digestion, in the stomach, particularly by Spallanzani, the following
facts have been established.
The gastric juice attacks the surfaces of bodies, and combines
chemically with their particles. It operates with more energy and
rapidity, the more the food is divided, and its action increased by a
warm temperature. By the action of digestion, the food is not merely
reduced to very minute parts, but its chemical properties become
changed; its sensible properties are destroyed, and it acquires new
and very different ones. This juice does not act as a ferment; so far
from it, it is a powerful antiseptic, and even restores flesh which
is already putrid.
When the alimentary substances have continued a sufficient time in
the stomach, they are pushed into the intestines, where they become
mixed with the bile and pancreatic juice, as was before observed.
What changes are caused by these substances, we have yet to learn;
but there is no doubt, that they serve some important purposes. By
the peristaltic motion of the bowels, the alimentary matters thus
changed are carried along, and applied to the mouths of the lacteal
vessels, which open into the intestines, like a sponge, and by some
power, not well understood, absorb that part which is fitted for
assimilation, while the remainder is rejected as an excrement.
The lacteal vessels are furnished with valves, which allow a free
passage to the chyle from the intestines, but prevent its return. The
most inexplicable thing in this operation, is the power which these
vessels possess of selecting from the intestinal mass, those
substances which are proper for nutrition, and rejecting those which
are not.
These lacteal vessels, as was before observed, pass through the
mesentery, and their contents seem to undergo some important change
in the mesenteric glands. The chyle which passes through vessels,
appears to be an oily liquor, less animalised than milk, and its
particles seem to be held in solution by the intermedium of a
mucilaginous principle. It is conveyed along the thoracic duct in the
manner already described, and enters the blood slowly, and, as it
were, drop by drop, by the subclavian vein; in this way it becomes
intimately mixed with the blood, and combining with oxygen in the
lungs, it acquires a fibrous character, and becomes fit to nourish
the body.