OF THE FUNCTION OF ABSORPTION.
Evidence of the process in the plant, in the animal—Apparatus
general and special—Experiments which prove the absorbing power of
blood-vessels and membrane—Decomposing and analysing properties
of membrane—Endosmose and exosmose—Absorbing surfaces, pulmonary,
digestive, and cutaneous—Lacteal and lymphatic vessels—Absorbent
glands—Motion of the fluid in the special absorbent vessels—Discovery
of the lacteals and lymphatics—Specific office performed by the
several parts of the apparatus of absorption—Condition of the system
on which the activity of the process depends—Uses of the function.
783. Absorption is the function by which external substances are
received into the body, and the component particles of the body are
taken up from one part of the system, and deposited in some other
part. So universal and constant is the operation, that there is not a
fluid nor a solid, not a surface nor a tissue, not an external nor an
internal organ, which is not, in its turn, the seat and the subject of
the process. By its action the component particles of the living body
are kept in a state of perpetual mutation.
784. The plant in a humid atmosphere increases in weight. The nutritive
matter of the plant diffused in the soil is taken up by its capillary
rootlets, or by the spongolæ which are attached to them, and conveyed
into the system. The fall of dew or rain upon leaves promotes the
growth of the plant. Leaves placed on water are capable of preserving
not only their own vitality, but that of the branches and twigs to
which they are attached. These phenomena show that the process of
absorption is carried on by the plant.
785. The evidence of the absorbing power possessed by the animal is
still more striking.
786. 1. If an animal be immersed in water the amount of which is
ascertained by measure, its head being kept out of the water, so that
none can enter the mouth, the body increases in weight and the water
diminishes in quantity. If certain animals, as snails, are plunged in
water impregnated with colouring matter, the fluids in the interior
of their body soon acquire the colour of the water by which they are
surrounded. Frogs, previously kept for some time in dry air, when
placed in water, absorb a quantity equal in weight to their whole body.
787. 2. In a humid atmosphere the animal increases in weight still more
than the plant.
788. 3. If a quantity of water be injected into any of the great
cavities of the body, as into that of the peritoneum, the whole of the
fluid after a certain time disappears; it is spontaneously removed.
789. 4. If in the progress of disease a fluid be poured into any cavity
of the body, as often happens in dropsy, the whole of the fluid is
removed, sometimes spontaneously and quite suddenly; but more often
slowly, under the influence of medicinal agents.
790. 5. Certain substances, whether applied to an external or an
internal surface, produce specific effects on the system, just as when
they are received into the stomach or injected into the blood-vessels.
Mercury in mere contact with the skin, but more rapidly when the
application is aided by friction, produces the same specific action
upon the salivary glands, and the same general action upon the system
as when the preparation of the metal is received into the stomach.
By the like external and local application arsenic, opium, tobacco,
and other narcotics produce their distinct and peculiar effects on
the nervous system, and their remote and general effects on the other
systems.
791. 6. If an organ or tissue be deprived of nourishment, it gradually
diminishes in bulk, and at length wholly disappears from the system.
By long-continued pressure, such as that occasioned by the pulsation
of a diseased artery, as in aneurism, or by the growth of a fleshy
tumor, portions of the firmest and strongest muscle, nay, even of the
most dense and compact bone, wholly disappear. At one time the fluids
diminish in quantity, the flesh wastes, and the weight of the body is
reduced one half or more. Under other circumstances, while the state of
the general system remains stationary, some particular part diminishes
in size, or altogether disappears.
792. 7. Healthy and strong men, engaged in hard labour and exposed to
intense heat, sometimes lose, in the space of a single hour, upwards
of five pounds of their weight. Though daily engaged for months
together in this occupation at two different periods of the day, for
the space of an hour each time, and though consequently these men lose
five pounds twice every day, yet when weighed at intervals of three,
six, or nine months, it is found that the weight of the body remains
stationary, not varying, perhaps, more than a pound or two. It follows
that the bodies of these men must absorb, twice every day, a quantity
equal in weight to that which they lose.
793. These phenomena depend on a power inherent in the body, that of
taking up and carrying into the system certain substances in contact
with its surfaces, and of transporting from one part of its system to
another its own component particles.
794. The apparatus by which these operations are carried on is general
and special.
795. The general apparatus consists of blood-vessels and membrane. The
special apparatus consists of a peculiar system of vessels, namely,
the lacteals and lymphatics, together with the system of glands termed
conglobate.
796. It is proved by direct experiment that the walls of blood-vessels
exert a power by which substances in contact with their external
surface penetrate their tissue, reach their internal surface, and mix
with the mass of the circulating fluids, and that this property is
possessed by all blood-vessels, arteries and veins, great and small,
dead and living.
797. If a portion of a vein or artery taken from the body be attached
by either extremity to two glass tubes in order to establish a current
of warm water in its interior, if the vein be then placed in a fluid
slightly acidulated, and the fluid which flows through the vessel be
collected in a flask, this latter fluid becomes, in the space of a few
minutes, sensibly acid. In this experiment there is no possibility
of communication between the current of warm water and the external
acidulated fluid, consequently the latter must penetrate the parietes
of the vessel, that is, absorption must take place through its
membranous walls.
798. A striking experiment demonstrates the absorbing power of the
living blood-vessels. If the trunk of a vein or artery be exposed in a
living animal, and a poisonous substance in solution be dropped on the
external surface of either, the animal is killed in a few minutes,
just as when the poison is injected into the blood-vessel itself.
Analogous experiments on the minute blood-vessels not only show that
they are endowed with the like absorbing power, but that their number,
tenuity and extent, are conditions which greatly favour the activity of
the process.
799. Membrane is an organised substance abounding with blood-vessels.
Whether the absorbing power possessed by this tissue be due only to
these vessels, or whether it be assisted in the operation by other
agents not yet fully ascertained, it is certain that the absorbing
power it exerts is highly curious and wonderful.
800. An animal membrane placed in contact with water becomes saturated
with fluid: placed in contact with a compound fluid, as with water or
spirit holding colouring matter in solution, the membrane actually
decomposes the compound and resolves it into its elementary parts,
just as accurately as can be done by the chemist. If one extremity
of a piece of membrane be placed in a vessel containing the tincture
of iodine, for example, and the other extremity be kept out of the
fluid, that portion of the membrane which is in immediate contact with
the tincture acquires a perfectly dark colour, because the iodine
completely penetrates the substance of the membrane. This dark-coloured
portion is bounded by a definite line, above which the membrane
is penetrated by a different part of the solution, by a pearly,
colourless fluid, the alcohol in which the iodine was suspended. Above
this again there are traces of a still lighter coloured fluid, which
is probably water. In like manner, if strips of membrane are placed in
glasses containing port wine, the same analytical process is effected
by the membrane. The colouring matter of the wine is imbibed by the
lower portion of the membrane; above this is the alcohol, and above
this the water.
801. These and many analogous experiments demonstrate that the
process of absorption is accompanied with the further phenomena of
decomposition and analysis; and that membrane, at the very moment
it imbibes certain compound substances, resolves them into their
constituent elements.
802. It is further established by numerous experiments that different
compound substances are decomposed and absorbed by membrane with
different degrees of facility. If strips of membrane are placed in
phials containing different kinds of fluids, one fluid rises only
a line or two; others rise to the height of many inches. There is
indubitable evidence that analogous properties are possessed by living
membrane; that the mucous membrane of the stomach at the moment
it imbibes, decomposes and analyses the alimentary and medicinal
substances in contact with its surface; and consequently that in all
animals membrane becomes a most important agent in carrying on the
digestive process.
803. But perhaps the most remarkable property possessed by membrane is
that of establishing in fluids in contact with its surfaces currents
through its parietes, which proceed in opposite directions, according
to the different natures of the fluids, and more especially according
to their different densities. If small bladders composed of membrane
are filled with a fluid of greater density than water, and securely
fastened, and then thrown into water, they acquire weight and become
swollen and tense. If the experiment be reversed; if the bladders be
filled with water and immersed in a denser fluid, the denser fluid
flows inwards to the water, and the water passes from the interior
outwards. M. Dutrochet, who was led by accident to the observation
of these phenomena, and who saw at once the possible importance of
this agency in some organic processes hitherto involved in great
obscurity, commenced an extended series of experiments with a view
to ascertain the exact facts. He took the cæca of fowls, membranous
bags already made to his hand, into which he introduced a quantity
of fluid consisting of milk, thin syrup, or gum-arabic dissolved in
water. Having securely tied the membranes, he placed the bags thus
filled in water, and found that two opposite currents are established
through the walls of the cæca. The first and strongest current, that
from without inwards, is formed by the flow of the external water
towards the thicker fluid contained in the cæca; the second and weaker
current, that from within outwards, is formed by the flow of the
thicker interior fluid towards the external water. The first or the
in-going current is termed _endosmose_, from ενδον, intus, and
ωσμος, impulsus, and the second or out-going current is termed
_exosmose_, from a similar combination of Greek words signifying an
impulse outwards.
804. The velocity and strength of these currents are capable of exact
admeasurement. The amount of endosmose is measured by an apparatus
termed an endosmometer, which consists of a small bottle, the bottom
of which is taken out and the aperture closed by a piece of bladder.
Into this bottle is poured some dense fluid; the neck of the bottle is
closed with a cork, through which a glass tube, fixed upon a graduated
scale, is passed. The bottle is then placed in pure water. The water
by endosmose penetrates the bottle in various quantities according
to the density of the fluid contained in its interior through the
membrane closing its bottom. The dense fluid in the bottle, increased
in quantity by the addition of the water, rises in the tube fitted to
its neck, and the velocity of its ascent is the measure of the velocity
of the endosmose.
805. The strength of endosmose is measured by a similar apparatus,
in which a tube is twice bent upon itself, and the ascending branch
containing a column of mercury which is raised by the fluid in the
interior of the endosmometer, as the volume of this fluid is increased
by the endosmose. By means of these two instruments it is found that
the velocity and strength of endosmose follow the same law, and that
both are proportionate to the excess of the density of the fluid
contained in the endosmometer above the density of water. By numerous
experiments it is ascertained that by employing syrup of ordinary
density (I. 33) an endosmose is obtained, the strength of which is
capable of raising water more than 150 feet.
806. But though difference of density is necessary to the production
of endosmose, yet numerous and decisive experiments show that the
different natures of fluids, irrespective of their proportionate
densities, materially influence the activity and energy of the process.
Thus, if sugar-water and gum-water of the same density be placed in
the same endosmometer, the former produces endosmose with a velocity
as seventeen and the latter only as eight. The endosmose produced
by a solution of the sulphate of soda is double that produced by a
solution of the hydro-chlorate of soda of the same density. A solution
of albumen exerts an endosmose four times greater than a solution of
gelatin of the same density.
807. With organic fluids endosmose goes on without ceasing until the
chemical nature of the fluids becomes altered by putrefaction; but
with alkalies, soluble salts, acids, and chemical agents in general,
the endosmose excited is capable only of short continuance, because
such agents enter into chemical combination with the organic tissue of
the endosmometer, and thus destroy endosmose.
808. It is remarkable that the direction of the endosmotic currents
produced by vegetable membrane is the exact reverse of that produced
by animal membrane under precisely the same circumstances. Thus oxalic
acid, when separated from water by an animal membrane, invariably
exhibits endosmose from the acid towards the water; when separated
by a vegetable membrane, from the water towards the acid: and the
same is the case with the tartaric and citric acids, and with the
sulphuric, the hydro-sulphuric, and the sulphurous acids. I filled,
says Dutrochet, a pod of the _colutea arborescens_, which being opened
at one end only, and forming a little bag, was readily attached by
means of a ligature to a glass tube, with a solution of oxalic acid,
and having plunged it into rainwater, endosmose was manifested by the
ascent of the contained acid fluid in the tube, that is to say, the
current flowed from the water towards the acid. The lower part of the
leek (_allium porrum_) is enveloped or sheathed by the tubular petioles
of the leaves. By slitting these cylindrical tubes down one side,
vegetable membranous webs of sufficient breadth and strength to be
tied upon the reservoir of an endosmometer are readily obtained. An
endosmometer, fitted with one of these vegetable membranes, having been
filled with a solution of oxalic acid and then plunged into rainwater,
the included fluid rose gradually in the tube of the endosmometer, so
that the endosmose was from the water towards the acid, the reverse
of that which takes place when the endosmometer is furnished with
an animal membrane. Vegetable membrane, then, at least with fluids
containing a preponderance of acid, produces a current, the direction
of which is the exact reverse of that produced by animal membrane.
809. The bodies of organised beings are composed in great part of
various fluids of different density, separated from each other by thin
septa, precisely the conditions which are necessary to the production
of endosmose. But such conditions never concur in inorganic bodies,
whence inorganic bodies never exhibit endosmotic phenomena. Vegetable
tissue of every kind consists of vast multitudes of aggregated cells
intermingled with tubes. The parietes of these hollow organs are
exceedingly delicate and thin; the organs themselves are at all times
filled with fluids, the densities of which are infinitely various;
consequently, by endosmose and exosmose, mutual interchanges of their
contents incessantly go on; those contents brought into contact by
currents moving now in one direction and now in another, now rapidly
and now slowly intermingle, and in consequence of their admixture
changes in their chemical composition take place. It is by these
powers that water holding in solution nutrient matter diffused through
the soil penetrates the spongeolæ of the capillary rootlets, always
filled with a denser fluid than the water contained in the soil,—that
the energetic motion by which the sap ascends is generated,—that the
ascending sap is attracted into fruits, always of greater density
than the crude sap,—that buds are capable of emptying the tissue
that surrounds them when they begin to grow, and that almost all the
phenomena connected with the motions of fluids in plants, and the
chemical changes which those fluids undergo in consequence of this
admixture, is effected. And there cannot be a question that analogous
phenomena take place in the various cells, cavities, and minute
capillary vessels of the animal body.
810. It is then established on indubitable evidence that all animal
tissues, without exception, possess an inherent property by which they
are capable of transmitting through their substance certain fluids, and
even solids, convertible into fluids; and that the great agent by which
this transmission is effected is membranous tissue, whether in the form
of blood-vessels or of proper membrane. By virtue of this property
fluids and solids are absorbed, by the animal body, with whatever
surface or organ they are in contact, whether with an external or an
internal surface, or with the eye, the mouth, the tongue, the stomach,
the lungs, the liver, or the heart.
811. But membrane is so disposed and modified, in different parts of
the body, as to admit of the introduction of fluids and solids from the
exterior to the interior of the system with widely different degrees
of facility. There may be said to be in the human body three great
absorbing surfaces, the pulmonary, the digestive, and the cutaneous,
each highly important, but each endowed with exceedingly different
degrees of absorbing power.
812. The pulmonary surface, for reasons which will be readily
understood from what has been already stated relative to the structure
of the air vesicles of the lungs, is by far the most active absorbing
surface of the body. The mode in which the air vesicles are formed and
disposed has been shown to be such as to give to the lungs an almost
incredible extent of membranous surface, while the membrane of which
the cells are composed is exceedingly fine and delicate. Moreover,
there is the freest possible communication between all the branches of
the pulmonary vascular system, whether arteries or veins; the distance
between the lungs and the heart is short; the course of the blood
from the pulmonary capillaries to the central engine that works the
circulation is rapid, and the lungs are at the same time close to the
central masses of the nervous system, with which indeed they are placed
in direct communication by nerves of great magnitude and of most
extensive distribution. These circumstances account for the wonderful
rapidity with which substances are absorbed, when placed in contact
with the pulmonary surface, and for the instantaneousness and intensity
of the impression produced upon the system, when the substance thus
introduced is of a deleterious nature.
813. They also afford an explanation of a phenomenon not to have been
credited without experience of the fact, that innoxious substances,
introduced into the air cells of the lungs in moderate quantities
produce no more inconvenience there than when taken into the stomach.
A single drop of pure water, when in contact near the glottis with the
same membrane that forms the air vesicles of the lungs, excites the
most violent and spasmodic cough, and the smallest particle of a solid
substance permanently remaining there occasions so much irritation
that inevitable suffocation and death result. Yet so different is the
sensibility of this membrane in different parts of its course, that
while at the upper portion of the trachea it will not bear a drop
of water without exciting violent disturbance, in the air vesicles
it tolerates with only slight inconvenience a considerable quantity
even of solid matter. An accident of a nature sufficiently alarming,
which occurred to Dessault, affords a striking illustration of this
curious fact. This celebrated surgeon had to treat a case in which the
trachea and esophagus were cut through. It was necessary to introduce
a tube through the divided esophagus into the stomach, and to sustain
the patient by food introduced in this manner. On one occasion the
tube, instead of being passed through the esophagus to the stomach,
was introduced into the trachea down to the division of the bronchi.
Several injections of soup were actually thrown into the lungs before
the mistake was discovered; yet no fatal, and even no dangerous
consequences ensued. Since that period, in various experiments on
animals, several substances of an innoxious nature have been thrown
into the lungs without producing any inconvenience beyond slight
disturbance of the respiration and cough. The reason is, that after a
short time the substances are absorbed by the membrane composing the
air vesicles, and are thus removed from the lungs and borne into the
general circulating mass. At every point of the pulmonary tissue there
is a vascular tube ready to receive any substance imbibed by it, and to
carry it at once into the general current of the circulation.
814. Hence the instantaneousness and the dreadful energy with which
poisons and other noxious substances act upon the system when brought
into contact with the pulmonary tissue. A solution of nux vomica
injected into the trachea produces death in a few seconds. A single
inspiration of the concentrated prussic acid kills with the rapidity
of a stroke of lightning. This acid in its concentrated form is so
potent a poison, that it requires the most extreme care in the use of
it, and more than one physiologist has been poisoned by it through
the want of proper precaution while employing it for the purpose of
experiment. If the nose of an animal be slowly passed over a bottle
containing this poison, and the animal happen to inspire during the
moment of the passage, it drops down dead instantaneously, just as
when the poison is applied in the form of liquid to the tongue or the
stomach. The vapour of chlorine possesses the property of arresting
the poisonous effects of prussic acid, unless the latter be introduced
into the system in a dose sufficiently strong to kill instantly; and,
hence, when an animal is all but dead from the effects of prussic acid,
it is sometimes suddenly restored to life by holding its mouth over the
vapour of chlorine.
815. Examples of the transmission of gaseous bodies through the
pulmonary membrane have been already fully described in the account of
the passage of atmospheric air to the lungs, and of carbonic acid gas
from the lungs, in natural respiration. But foreign substances may be
mixed with or suspended in the atmospheric air, which it is the proper
office of the pulmonary membrane to transmit to the lungs, and may be
immediately carried with it into the circulating mass. Thus, merely
passing through a recently-painted chamber gives to the urine the odour
of turpentine. The vapour of turpentine diffused through the chamber is
transmitted to the lungs with the inspired air, and passing into the
circulation through the pulmonary membrane, exhibits its effects in the
system more rapidly than if it had been taken into the stomach, and
thence absorbed.
816. Vegetable and animal matter in a state of decomposition generates
a poison, which when diffused in the atmosphere, and transmitted
to the lungs in the inspired air, produces various diseases of the
most destructive kind. The exhalations arising from marshes, bogs,
and other uncultivated and undrained places, constitute a poison of
a vegetable nature, which produces principally intermittent fever
or ague. Exhalations accumulating in close, ill-ventilated, and
crowded apartments in the confined situations of densely-populated
cities, where no attention is paid to the removal of putrefying and
excrementitious matters, constitute a poison chiefly of an animal
nature, which produces continued fever of the typhoid character. It is
proved by fatal experience that there are situations in which these
putrefying matters, aided by heat and other peculiarities of climate,
generate a poison so intense and deadly that a single inspiration
of the air in which they are diffused is capable of producing
instantaneous death; and that there are other situations in which a
less highly concentrated poison accumulates, the inspiration of which
for a few minutes produces a fever capable of destroying life in from
two to twelve hours. In dirty and neglected ships, in which especially
the bilge-water is allowed to remain uncleansed; in damp, crowded, and
filthy gaols; in the crowded wards of ill-ventilated hospitals filled
with persons labouring under malignant surgical diseases, or some forms
of typhus fever, an atmosphere is generated which cannot be breathed
long, even by the most healthy and robust, without producing highly
dangerous fever.
817. The true nature of these poisonous exhalations is demonstrated by
direct experiment. If a quantity of the air in which they are diffused
be collected, the vapour may be condensed by cold and other agents, and
a residuum of vegetable or animal matter obtained, which is found to
be highly putrescent, constituting a deadly poison. A minute quantity
of this concentrated poison applied to an animal previously in sound
health, destroys life with the most intense symptoms of malignant
fever. If, for example, ten or twelve drops of a fluid containing
this highly putrid matter be injected into the jugular vein of a
dog, the animal is seized with acute fever; the action of the heart
is inordinately excited, the respiration is accelerated, the heat
increased, the prostration of strength extreme, the muscular power so
exhausted, that the animal lies on the ground wholly unable to stir or
to make the slightest effort; and, after a short time, it is actually
seized with the black vomit, identical, in the nature of the matter
evacuated with that which is thrown up by an individual labouring
under yellow fever. It is possible, by varying the intensity and the
dose of the poison thus obtained, to produce fever of almost any type,
endowed with almost any degree of mortal power. These facts, of which
practical applications of the highest utility are hereafter to be made,
may suffice to show the importance of the pulmonary membrane as an
absorbing surface. By the extent and energy of its absorbing power, it
is one of the great portals of life and health, or of disease and death.
818. The digestive surface is of much less extent than the pulmonary;
it is less vascular; it is further removed from the centre of the
circulating system, and it is covered with a thick mucus, which is
closely adherent to it; hence its absorbing power is neither so great
as that of the pulmonary membrane, nor do noxious substances in contact
with it affect the system so rapidly. An appreciable interval commonly
elapses between the introduction of a poison into the stomach and
its action upon the system. An emetic is commonly a quarter of an
hour before it begins to operate: arsenic itself is generally half an
hour, and sometimes three quarters of an hour, before it produces
any decided effect on the system: but at length a noxious substance,
applied to any part of the digestive membrane is introduced into the
circulating mass and produces its appropriate effects on the system,
just as when it is in contact with the pulmonary tissue.
819. Over the external surface of the body or the skin, there is spread
a thin layer of solid, inorganic, insensible matter, like a varnish of
Indian rubber. The obvious effect of such a barrier placed between the
external surface of the body and external objects, is to moderate the
entrance of substances from without, and the transmission of substances
from within, that is, to regulate both the absorbing and the exhaling
power of the skin. Hence the comparative slowness with which substances
enter the system by the cutaneous surface; the impunity with which the
most deadly poisons may remain for a time in contact with the skin,
with which prussic acid, arsenic, corrosive sublimate, may be touched
and even handled. The internal surface of the body is protected from
the action of acrid substances introduced into the alimentary canal by
a layer of mucus through which an irritant must penetrate before it can
pain the sentient nerve or irritate the capillary vessel; but were not
a still denser shield thrown over the external surface, pain, disease,
and death must inevitably result from the mere contact of innumerable
bodies, which now are not only perfectly innoxious, but capable of
ministering in a high degree to human comfort and improvement.
820. Immediately beneath the cuticle is a surface as vascular as it is
sensitive, from which absorption takes place with extreme rapidity.
Poison in very minute quantity introduced beneath the cuticle kills
in a few minutes. Arsenic applied to surfaces from which the cuticle
has been removed by ulceration produces its poisonous effects upon
the system just as surely as when introduced into the stomach.
The poisonous matter of small-pox and of cow-pox placed in almost
inappreciable quantity by the lancet beneath the cuticle produces in a
given time its specific action upon the system. When, in certain states
of disease, with the view of bringing the system rapidly under the
influence of a medicinal agent, the cuticle is removed by a blister,
and the exposed surface is moistened with a solution of the substance
whose action is required, the constitutional effects are developed with
such intensity, that if extreme care be not taken in the employment of
any deleterious substance in this mode the result is fatal in a few
minutes.
821. The phenomena which have been stated may suffice to illustrate the
absorbing power of the general tissues and surfaces of the body; but
superadded to this, there is carried on in particular parts of the
system a specific absorption for which a special apparatus is provided.
[Illustration: Fig. CXCII.
An enlarged view of an absorbent vessel.—1. External surface, with the
jointed appearance produced by the valves.—2. The same vessel laid
open, showing the arrangement of the valves.]
822. The special apparatus of absorption, commonly termed the proper
absorbent system, consists of the lacteal and lymphatic vessels and of
the conglobate glands. The lacteals arise only from the intestines; the
lymphatics, it is presumed, from every organ, tissue, and surface of
the body. Both sets of vessels possess a structure strikingly analogous
to that of veins, the common agents of absorption. The coats of the
lacteals and lymphatics are somewhat thinner and a good deal more
transparent than those of veins; yet thin and delicate as they are,
they possess considerable strength, for they are capable of bearing,
without rupture, injections which distend them far beyond their natural
magnitude.
823. When fully distended, these vessels present a jointed appearance
somewhat resembling a string of beads (fig. CXCII. 1). Each joint
indicates the situation of a pair of valves (fig. CXCII. 2). These
valves are of a semilunar form, and are composed of a fold of the inner
coat of the vessel (fig. CXCII. 2). The convex side of the valve, in
the lacteals, is towards the intestines; in the lymphatics towards the
surfaces; in both towards the origins of the vessels. The valves allow
the contents of the vessels to pass freely towards the main trunk of
the system, but prevent any retrograde motion towards the origins of
the vessels.
824. By continued pressure the resistance of the valves may be
overcome, so that mercury may be made to pass from the trunk into the
branches. When this is done in an absorbent trunk proceeding from
certain organs, such as the liver, it is seen that the absorbents are
distributed, arborescently, in such vast numbers that the surface of
the viscus appears as if it were covered with a reticular sheet of
quicksilver.
825. The internal coat of the small intestines has been shown to
present a fleecy surface, crowded with minute elevations called villi,
which give this surface an appearance closely resembling the pile of
velvet. Each villus consists of an artery, a vein, a nerve, and a
lacteal, united and sustained by delicate cellular tissue. After a meal
the lacteals become so turgid with chyle that they completely conceal
the blood-vessels and nerves, so that the surface of the intestine
presents to the eye only a white mass, or a surface thickly crowded
with white spots (fig. CXCIII.)
[Illustration: Fig. CXCIII.
Appearance of the lacteals turgid with chyle, as seen in the jejunum
some time after a meal.]
[Illustration: Fig. CXCIV.
Magnified view of two ampullulæ turgid with chyle, terminating the
lacteal vessels.]
826. When a portion of the intestine in this condition of the lacteal
vessels is examined under the microscope, there is said to be visible
on the villus an oval vesicle, termed an ampullula (fig. CXCIV.). This
vesicle is described as having an aperture at its apex, which it is
conceived constitutes the open mouth of the lacteal, and through which
the chyle is supposed to be taken up.
[Illustration: Fig. CXCV.
View of villi, with the lacteals arising from their surface by open
mouths and forming radiated branches. The surface of one of these
villi is represented as entirely white, from the lacteals being so
turgid with chyle as completely to obscure their orifices and their
radiating branches.]
827. Mr. Cruikshank, who particularly devoted himself to the study of
this part of the absorbent system, states that he had an opportunity
of examining these vessels in a person who died suddenly some hours
after having taken a hearty meal, and who had been previously in sound
health. “In some hundred villi,” he says, “I saw the trunk of the
lacteal beginning by radiated branches (fig. CXCV.). The orifices of
these radii were very distinct on the surface of the villus as well
as the radii themselves (fig. CXCV.). There was but one trunk in each
villus. The orifices on the villi of the jejunum, as Dr. Hunter said
(when I asked him as he viewed them in the microscope how many he
thought there might be) were about fifteen or twenty in each villus,
and in some I saw them still more numerous” (fig. CXCV.).
828. The course of the lacteals, from their origin in the villi to
their termination in the thoracic duct, has been traced (687). It is
conjectured that the lymphatics take their origin from every point of
the body, but it is admitted that they have not been actually seen
even in every organ; still they have been found in so many that it is
inferred that they really exist in all, and that in those in which they
have not been hitherto detected they have eluded observation on account
of their extreme delicacy and transparency and our imperfect means of
examining them.
829. Though, like veins, lymphatics anastomose freely with each other,
yet they do not proceed from smaller to larger branches and from larger
branches to trunks, but continue of nearly the same magnitude from
their origin to their termination. They are disposed in two sets, one
of which always keeps near the external surface of the body, and the
other is deeply seated, accompanying more especially the great trunks
of the blood-vessels.
[Illustration: Fig. CXCVI.
Fig. CXCVII.
Fig. CXCVIII.
CXCVI.—1. Trunks of absorbent vessels entering a gland. 2. Gland laid
open. 3. Highly magnified views of the cells or follicles of which
the gland is supposed to consist. CXCVII.—1. Absorbent vessels called
vasa inferentia, entering (2) the gland. 3. Absorbent vessels emerging
from the gland, called vasa efferentia, and forming (4) a common
trunk.
CXCVIII.—1. Trunk of absorbent vessel entering a gland. 2. Gland
apparently composed entirely of convoluted vessels. 3. Vessels
emerging from the gland and forming (4) a common trunk.]
830. In the human body every vessel that can be distinctly recognised
either as a lacteal or a lymphatic, passes, in some part of its course,
through a conglobate or lymphatic gland (figs. CXCVII., CXCVIII.).
These glands, small, flattened, circular or oval bodies, resembling
beans in shape, are enclosed in a distinct membranous envelope. Their
intimate structure has been already fully described (chap. xi.). They
are of various sizes, ranging from three to ten lines in diameter: they
are placed in determinate parts of the body, and are grouped together
in various ways, being sometimes single, but more often collected
in masses of considerable magnitude. Numerous absorbent vessels,
termed vasa inferentia, enter the gland on the side remote from the
heart (figs. CXCVII. 1 and CXCVIII. 1); a smaller number, called vasa
efferentia, leave it on the side proximate to the heart (fig. CXCVII.
3). If mercury be injected into the vasa inferentia (fig. CXCVI.), it
is seen to pass into a series of cells of the corresponding gland (fig.
CXCVI. 3), and then to escape by the vasa efferentia; but if the gland
be more minutely injected, as by wax, all appearance of cells vanishes;
the whole substance of the gland seems then to consist of convoluted
absorbents (fig. CXCVIII. 2), irregularly dilated, and communicating
with each other so intimately that every branch that leaves the gland
appears to have been put in communication with every branch that
entered it (fig. CXCVIII. 1, 2, 3).
831. The motion of the fluid within the absorbent vessels, though not
rapid, is energetic. If a ligature be placed around the thoracic duct
in a living animal, the tube will swell and ultimately burst, from the
rupture of its coat, in consequence of the force of the distension
that takes place below the ligature. If the thoracic duct in the neck
of a dog be opened some hours after the animal has taken a full meal,
the chyle flows from the vessel in a full stream, and in the space
of five minutes half an ounce of the fluid may be obtained. Yet this
system of vessels is beyond the influence of the circulating blood: it
has no heart to propel it; no current behind always in rapid motion
to urge it onwards; it is therefore inferred that it is moved by a
vital contractile power inherent in the vessels, analogous to, if not
identical with, muscular contractility. The flow of blood through the
arterial tubes is universally believed to be effected, in part at
least, by such a contractile power, for this, among other reasons, that
if in a living animal the trunk of an artery be laid bare, the mere
exposure of it to the atmospheric air causes it to contract to such
a degree that its size becomes obviously and strikingly diminished
(298.1). The same phenomenon has been observed in the main trunk of
the absorbent system. Tiedemann and Gmelin state that in the course of
their experiments they saw the thoracic duct contract from exposure to
the air.
832. The delicacy and transparency of the lacteals and lymphatics
long concealed them from the view of the anatomist. The lacteals had
indeed been occasionally seen in ancient times, but their office
was altogether unknown. In the year 1563 Eustachius discovered the
thoracic duct, but did not perceive its use. About half a century
afterwards, in the year 1622, the lacteals were again one day by
chance seen by Asellius, in Italy, while investigating the function of
certain nerves. Mistaking the lacteals for nerves, he at first paid no
attention to them; but soon observing that they did not pursue the same
course as the nerves, and “astonished at the novelty of the thing,” he
hesitated for some time in silence. Resolving in his mind the doubts
and controversies of anatomists, of which it chanced that he had been
reading the very day before, in order to examine the matter further,
“I took,” he says, “a sharp scalpel to cut one of these chords, but
scarcely had I struck it when I found a liquor white as milk, or rather
like cream, to leap out. At this sight I could not contain myself for
joy; but turning to the by-standers, Alexander Tadinus and the senator
Septalius, I cried out Εὕρηκα! with Archimedes; and at the
same time invited them to look at so rare and pleasing a spectacle;
with the novelty of which they were much moved. But I was not long
permitted to enjoy it, for the dog now expired, and, wonderful to
tell, at the same instant the whole of that astonishing series and
congeries of vessels, losing its brilliant whiteness, that fluid being
gone, in our very hands, and almost before our eyes, so evanished and
disappeared that hardly a vestige was left to my most diligent search.”
The next day he procured another dog, but could not discover the
smallest white vessel. “And now,” he continues, “I began to be downcast
in my mind, thinking to myself that what had been observed in the first
dog must be ranked among those rare things which, according to Galen,
are sometimes seen in anatomy.” But at length recollecting that the dog
had been opened “athirst and unfed,” he opened a third “after feeding
him to satiety; and now everything was more manifest and brilliant
than in the first case.” The zeal with which he followed out the clue
he had obtained is indicated by the number of dogs, cats, iambs, hogs,
and cows which he dissected, and by the statement that he even bought
a horse and opened it alive; but, he adds, “a living man, however,
which Erasistratus and Herophilus of old did not fear to anatomize, I
_confess_ I did not open.”
833. Nearly thirty years elapsed before the lacteals, which were long
thought to terminate in the liver, were traced to the thoracic duct;
and it was not until the year 1651, about eighty years after the
discovery of Asellius, that the lymphatics were discovered, and that
the whole of this portion of the absorbent system was brought to light.
834. Taking together the whole of the apparatus of absorption, the
specific office performed by its several parts seems to be as follows:—
835. 1. It is established that the lacteals absorb chyle, and that they
refuse to take up almost every other substance which can be presented
to them. Experimentalists are uniform in stating that however various
the substances introduced into the stomach, it is exceedingly rare to
find in the lacteals anything but chyle. These vessels appear to be
endowed with a peculiar sensibility, derived from the nervous system,
by which they are rendered capable of exerting an elective power,
readily absorbing some substances and absolutely rejecting others.
836. 2. The lymphatics absorb a far greater variety of substances
than the lacteals, but not all substances indiscriminately; chiefly
organized matter in a certain stage of purification; particles passing
through successive processes of refinement (707).
837. 3. The blood-vessels, and more especially the capillary veins,
appear to absorb indiscriminately all substances, however heterogeneous
their nature, which are dissolved or dissolvable in the fluids
presented to them.
838. 4. The absorbent glands appear by various modes, either by
removing superfluous and noxious matters, or by the addition of
secreted substances possessing assimilative properties, to approximate
the fluid which flows through them more and more closely to the nature
of the blood. Fatal effects result from the artificial infusion of
minute portions even of mild substances into the blood. Hence the
extended and winding course which Nature causes the new matter formed
from the food to undergo, even after its elaboration in the digestive
apparatus, in order that, before it is allowed to mingle with the
blood, its perfect purification and assimilation may be secured.
839. The activity or inactivity of the process of absorption is mainly
dependant on the emptiness or the plethora of the system. There is
a point of saturation beyond which the absorbent vessels, though in
immediate and continued contact with absorbable matters, will take
up no more. The nearer the system to this point the less active the
process; the further the system from this point the more active the
process. Thus, when an animal whose vessels are full to saturation is
immersed in water, or exposed to humid air, its body does not increase
in weight, and there is no sensible diminution of the water; but the
longer an animal is kept without fluid, and the more it is exposed
to the action of a dry air, the further its system is removed from
the point of saturation, and exactly in that proportion, when it is
brought in contact with water, is the diminution of the quantity of the
fluid and the increase in the weight of the body. This law explains
many circumstances of the animal economy,—why it is impossible to
dilute the blood or any other animal fluid beyond a certain point,
by any quantity of liquid which may be in contact with the external
surface, or which may be taken into the stomach; why it is impossible
to introduce nutrient matter into the system, beyond a certain point,
by any quantity of food, which the digestive organs may convert into
chyle; why, consequently, the bulk and weight of the body are incapable
of indefinite increase; why that bulk and weight are so rapidly
regained after long abstinence; and why the appetite is so keen, and
the ordinary fulness and plumpness of the body are so soon restored,
after recovery from fever and other acute diseases, when the digestive
organs have been uninjured.
840. Different portions of the absorbent apparatus accomplish specific
uses. With the absorbent action of the capillary blood-vessels and of
membranous surfaces every organic function, but more especially the
processes of digestion and respiration, are intimately connected.
841. The specific absorption carried on by the lacteals has for its
object the introduction of new materials into the system, for the
reparation of the losses which it is constantly sustaining by the
unceasing actions of life.
842. The specific absorption carried on by the lymphatics has a
two-fold object. First, the introduction of particles, which have
already formed an integrant part of the system, a second time into the
blood, in order to subject them anew to the process of respiration,
thereby affording them a second purification, and giving them new and
higher properties; and, secondly, the regulation of the growth of the
body, and the communication and preservation of its proper form.
843. It is the office of the lacteals to replenish the blood by
constantly pouring into it new matter, duly prepared for its conversion
into the nutritive fluid. It is the office of the lymphatics to preside
over the distribution of the blood as it is deposited in the system in
the act of nutrition. The lymphatics are the architects which mould and
fashion the body. They not only regulate the extension of the frame,
but they retain each individual part in its exact position, and give to
it its exact size and shape. Growth is not mere accretion, not simple
distension; it consists of a specific addition to every individual
part, while all the parts retain the same exact relation to each other
and to the whole. When a bone grows it does not increase in bulk by
the mere accumulation of bony matter; but every osseous particle is so
increased in length and breadth that the relative size of every part,
and the general configuration of the whole organ, remain precisely
the same. When a muscle grows, while the entire organ enlarges in
bulk by the augmentation of every individual part, each part retains
exactly its former proportions and its relative connexions. When the
brain grows a certain quantity of cerebral matter is added to every
individual part, but at the same time the proportionate size and
original form of each part, and the primitive configuration of the
entire organ, are retained exactly the same. How is this effected? By
a totally new disposition of every integrant particle of every part of
every organ. New matter is not deposited before the removal of the old:
the lymphatic, in the very act of removing the old, fashions a mould
for the reception of the new, and then the capillary artery brings
the new particle and deposits it with unerring exactness in the bed
prepared for it. Thus, by removing the old materials of the body in a
determinate manner, and thereby fashioning a mould for the reception of
the new, the lymphatics may be said, in the strictest sense, to be the
architects of the frame.