OF NUTRITION.
Composition of the blood—Liquor sanguinis—Recent account of the
structure of the red particles—Formation of the red particles in
the incubated egg—Primary motion of the blood—Vivifying influence
of the red particles—Influence of arterial and venous blood on
animal and organic life—Formation of human blood—Course of the new
constituents of the blood to the lungs—Space of time required for the
complete conversion of chyle into blood after its first transmission
through the lungs—Distribution of blood to the capillaries when
duly concentrated and purified—Changes wrought upon the blood while
it is traversing the capillaries—Evidence of an interchange of
particles between the blood and the tissues—Phenomena attending the
interchange—Nutrition, what, and how distinguished from digestion—How
the constituents of the blood escape from the circulation—Designation
of the general power to which vital phenomena are referrible—Conjoint
influence of the capillaries and absorbents in building up
structure—Influence of the organic nerves on the process—Physical
agent by which the organic nerves operate—Conclusion.
945. The object of the greater part of the processes hitherto described
is to form the nutritive fluid, and to bring it to the requisite state
of purity and strength. Recent researches into the composition of the
nutritive fluid confirm the general correctness of the account already
given of it, (211 _et seq._).
946. When examined as it is flowing in the finest vessels of a
transparent part of the body, or immediately after it is abstracted
from the trunk of a vein or artery, before coagulation (218) takes
place, the blood is seen to consist of a colourless fluid, through
which is diffused a countless number of minute solid particles of a red
colour. The colourless fluid is called the liquor sanguinis, and the
solid particles the blood corpuscles or the red particles.
947. By the process of coagulation, the phenomena of which have been
fully described (219 _et seq._), the blood spontaneously separates into
a clear fluid of a yellow colour called serum or blood-water, and into
a solid mass termed the clot or the crassamentum. The serum, which must
be carefully distinguished from the liquor sanguinis, is the fluid
formed from the blood by coagulation; the liquor sanguinis is the fluid
part of the blood which exists before coagulation.
948. The liquor sanguinis contains in solution a large quantity of
animal matter, fibrin (228), which separates spontaneously in a solid
form on coagulation; the serum also contains a quantity of animal
matter in solution, albumen (224), which does not separate in a
solid form spontaneously, but only on the application of heat, acids,
alcohol, &c. (224). The animal matter, the fibrin, which separates
spontaneously from the liquor sanguinis in a solid form, constitutes
one part of the clot, and the other part of it consists of the red
particles which floated in the liquor sanguinis.
949. Thus, by coagulation, the liquor sanguinis separates into a
portion which remains fluid, the serum; and into a portion which
becomes solid, the fibrin; while the fibrin, as it is passing from
the fluid to the solid state, entangles the red particles, and both
together form the clot; consequently the liquor sanguinis contains in
solution two kinds of solid matter, fibrin and albumen; while the serum
contains in solution only one kind of solid matter, albumen.
950. The solution of fibrin in the liquor sanguinis, and its
spontaneous solidification during the process of coagulation, has been
shown by Professor Müller in the following mode. Having carefully
collected blood from the femoral artery of the frog, and also from the
heart laid bare and incised, and having brought a drop of this pure
blood under the microscope, and diluted it with serum, so that the
red particles were separated from each other by distant intervals,
he observed that there formed in those intervals a coagulation of
previously dissolved matter, by which the separated red particles were
connected together. By raising, with a needle, the coagulum occupying
the intervening spaces, this solid matter was obtained free from red
particles. The blood corpuscles of the frog are rendered, by a powerful
microscope, so large, that this operation may be performed with the
greatest distinctness. In consequence of the minuteness of the red
particles of human blood they pass, with the liquor sanguinis, through
filtering-paper; but those of the frog, being four times larger, are
kept back by the filter, while the liquor sanguinis percolates through
as a clear fluid, and then coagulates. This colourless coagulum is so
transparent that it is not even detected, after its formation, until
it is raised out of the fluid with a needle. It gradually thickens and
becomes white. It is the fibrin of the blood in its purest state.
951. Professor Müller’s account of the structure of the red particles
differs in a material point from that given (231 _et seq._). He
agrees that they are rounded bodies (fig. CXII. 1), generally of
the same size, though some are seen larger than common, but never
double the mean diameter; that they are always quite flat (232); that
in a certain light they look as if they were hollowed out from the
edges to the centre (fig. CXII. 1); but, he adds, “that this spot
is a real depression, as some think, appears to me in the highest
degree improbable; for I have at last convinced myself that the blood
corpuscles of man and the mammalia contain a very small nucleus of
the diameter of the flat corpuscle. My observations prove beyond
doubt that the blood corpuscles of frogs and salamanders (fig. CXII.
4) contain a nucleus entirely different in its chemical relations
from the outer layer. With one of Frauenhofer’s microscopes I have
seen very distinctly, in the blood corpuscles of man an exceedingly
small, round, well-defined nucleus, yellower and brighter than the
transparent circumference. When the blood corpuscles are mixed,
under the microscope, with acetic acid, the shell is almost entirely
dissolved, and these small nuclei, which are seen with great difficulty
in human blood, remain, while those of the frog appear, very evidently
the nuclei observed earlier in the blood corpuscles. In man, the nuclei
within the corpuscles are so small, that the diameter does not exceed
the thickness of the flat corpuscles.”
952. The enveloping capsule is stated to be soluble in water, while the
internal nucleus is insoluble; but the capsule is not soluble in serum;
the albumen and the salts contained in the serum probably rendering it
insoluble. The colouring matter of the capsule, which gives the red
colour to the blood, is called hæmatosin. Lecanu considers the capsular
substance as a combination of a specific colouring matter, which he
calls globulin, and of albumen; but Müller regards it as fibrin,
containing a quantity of iron. The latter physiologist states that
the opinion of Brande, that the amount of iron in hæmatosin is not
greater than in serum and other animal substances, has been refuted
by Berzelius and Engelhart. The iron is not an accidental ingredient
obtained from the food; for iron has been found in the blood of a
new-born animal that has never even sucked. According to Berzelius the
colouring matter of the blood contains a quantity of iron corresponding
to somewhat more than a half per cent. its weight of metallic iron, and
he thinks it most probable that the iron exists in the blood in the
metallic state, and not as an oxide.
953. By carefully watching the development of the chick in the
incubated egg, the first formation of the red particles can be
distinctly seen. The blood in the new being, which is elaborated before
the existence of the vessels that are to contain it, is formed from
the substance of the germ or from that of the germinal membrane, and
is augmented by the blood of the egg, which is the substance of the
yolk. First, a number of granules are produced from the substance
of the yolk. These subsequently lose their granular appearance, and
become translucent. On the translucent ring is produced the nucleus
of the blood corpuscles. When completely formed, the blood corpuscles
of the bird, as of all the animals below the bird in the scale of
organization, are of an elliptical figure, and quite flat (fig. CXII.
4, 5); but when first produced they are rounded globules, not flat, and
they gradually assume their proper and permanent form; it is only on
the sixth day of incubation that they begin to be elliptical, by the
ninth day they are all elliptical (fig. CXII. 4, 5).
954. The substance of the fluid yolk is thus changed into blood without
the action of any special organ; for, as yet, no organs such as liver,
spleen, or lungs, exist. When the formation of the blood has arrived
at a certain point, it begins to be in motion. The blood is seen to
be in motion before the heart can be observed to beat. The germinal
membrane arising out of the enlarged germinal disk soon exhibits a
thin upper layer (serous membrane) and a thicker under layer (mucous
membrane). There is also formed in the middle of the germinal membrane
around the appearing trace of the embryo a translucent space, the _area
pellucida_. The exterior of the germinal membrane remains opaque, and
this opaque portion becomes divided by a definite boundary into an
external and internal annular space in from sixteen to twenty hours.
This separation encloses one part of the opaque portion of the germinal
membrane, which surrounds the interior or translucent space of the
germinal membrane, and is termed _area vasculosa_, because the blood
and vessels form the inner half of this space.
955. As far as the area vasculosa extends, a granular layer is
presented between the two layers of the germinal membrane, which soon
divides into numerous granular isolated particles with translucent
intervals, in which the blood collects, first in the form of a
yellowish, and then of a reddish fluid; first distinctly in the
periphery of the area vasculosa, from which it is seen to flow towards
the heart before the heart beats.
956. The blood exerts its vivifying influence chiefly by the red
particles. If an animal be bled to fainting, and pure serum be injected
into its vessels, re-animation does not take place; but if the blood of
another animal of the same species be injected, the animal which was
apparently dead acquires new life at every stroke.
957. The fibrin may be removed from the blood without injuring the
red particles. If the fibrin be abstracted, and a mixture of the
red particles and the serum be brought to a proper temperature, and
injected into the veins of an animal bled to fainting, re-animation is
effected.
958. If the blood of an animal of another species be injected whose red
particles are of the same form, but of a different size, re-animation
is indeed effected, but the restoration is imperfect; the organic
functions are oppressed, and languish, and death takes place generally
within the sixth day. The same effects follow, if a mixture of serum
and red particles of the blood of a different species be injected.
959. If blood with circular particles be injected into the vessels
of an animal whose blood corpuscles are elliptical, the most violent
effects are instantly produced; such blood acts upon the nervous system
like the strongest poisons; and death usually follows with extreme
rapidity after the injection of a very small quantity. Thus, if a few
drops of the blood of the sheep be injected into the vessels of the
bird, the bird is killed instantaneously. It is very remarkable, that
the blood of the mammalia should be thus fatal to the bird. The effect
cannot be dependent on any mechanical principle. The injection of a
fluid with particles, the diameter of which is greater than that of the
capillary blood-vessels would of course destroy life by stopping the
circulation; but the blood corpuscles of the mammalia are much smaller
than those of the bird; yet the pigeon is killed by a few drops of
mammiferous blood; and the blood of the fish is rapidly fatal to all
the mammalia as well as to birds.
960. It is manifest, both from observation and experiment, that
arterial blood is far more necessary to the support of the animal than
of the organic life. When in asphyxia the communication of atmospheric
air with the lungs is suspended, the functions of the brain are
abolished; sensibility and voluntary motion are lost the moment venous
blood circulates in the arteries of the brain. It has been shown (476),
that if this state continue, the animal life is destroyed in a minute
and a half; but that the organic life is not extinguished for many
minutes, and sometimes not even for several hours.
961. It sometimes happens that the communication between the pulmonary
artery and the aorta, and between the right and left auricle, which
naturally exist in the fœtus, is continued after birth. In persons
having this state of the circulation, called ceruleans, some portion
of venous blood is always mixed with arterial blood. In this case the
various processes of secretion and nutrition, the entire circle of
organic functions, are but little disturbed; while the animal functions
are deranged in a remarkable degree. The mind is weak and inactive,
and the muscular power is so feeble, that the least exertion produces
a sense of suffocation; and, if the muscular effort be continued,
occasions fainting, and even suspended animation.
962. But while venous blood is in no case capable of supporting
sensation and voluntary motion, there are decided cases in which
secretion is effected, at least in part, from venous blood, as the bile
from the venous blood that circulates through the liver in man and all
the mammalia, and the urine which is formed from venous blood in some
of the lower orders of animals.
963. The proper nutritive fluid of the human body is directly formed
from chyle, lymph, and venous blood; that is, partly from new matter
introduced into the system from the external world, and partly from
matter which has already formed a constituent part of the body.
The new matter, the white chyle, is prepared partly by the action
of the digestive fluids upon the food, and partly by the addition
to the digested food of highly animalized substances, endowed with
assimilative properties, by which the product is progressively
approximated to the chemical composition of the blood. The old matter
consists partly of the clear lymph, contained in the lymph vessels,
and derived from the interior of the organized parts, particles which
have already formed an integrant portion of the tissues and organs; and
partly of the dark venous blood, the residue of the proper nutritive
fluid, after the latter has yielded to the system the new matter
required by it, and has given off from the system its superfluous and
noxious particles.
964. In the duodenum and jejunum the new matter, the chyle, contains
albumen; but it is without coagulable fibrin: it acquires fibrin in the
lymph vessels on its way to the veins.
965. In the chyle globules appear; but the chyle corpuscles are white,
are without an external envelop, are comparatively few in number, are
somewhat more than half the size of the blood corpuscles, and, like the
nuclei of the latter, are insoluble in water.
966. The fatty or oleaginous matter contained in the chyle is in a free
state, not intimately combined.
967. The chyle is alkaline, but is much less alkaline than the blood;
and the iron contained in the chyle is much less intimately combined
than it is in the blood.
968. Lymph contains in solution more animal matter than chyle, and the
white globules are more abundant in lymph. But though lymph contain
in solution more albumen and fibrin than chyle, it is not so richly
loaded with these substances as blood. Still, however, the solution of
albumen and fibrin in lymph approximates lymph so closely to the blood,
that the lymph very much resembles the clear liquor sanguinis of which
the blood consists when the red particles are abstracted from it. The
colourless liquor sanguinis is the lymph of the blood. Lymph is blood
without red particles; and blood, lymph with red particles.
969. The chyle is transmitted into the lymph-vessels to mingle with the
lymph before it flows into the veins to mingle with the blood.
970. The commingled fluids, chyle and lymph, pass into the blood very
slowly, drop by drop. The regulation of the rapidity of the admixture
seems to be the chief office of the valve placed at the termination of
the thoracic duct. When the operation is observed in a living animal,
it is seen that this valve prevents the new matter from flowing into
the blood in a full stream. If in a dog of ordinary size that has
recently eaten as much animal food as it chose, the thoracic duct be
opened in the neck, the dog being alive, there will flow from the duct
about half an ounce of fluid in five minutes (831); yet when this fluid
reaches the termination of the duct only a few inches further on, it
flows into the vein only drop by drop, at considerable intervals. One
great object of pouring the chyle and lymph into the venous system so
close to the heart (fig. CLXXVIII.), and of causing the commingled
fluid to pass under the action of that powerful engine before it is
transmitted to the lungs, seems to be, by the agitation to which it
is subjected in the right auricle and ventricle to accomplish the
most perfect admixture possible between the particles of the chyle
and lymph and the red particles of the venous blood; an object which
would be counteracted by the too rapid entrance into the current of the
circulation of the new and as yet imperfectly assimilated matter.
971. After their due admixture by the powerful action of the engine
that works the circulation, the commingled fluids are transmitted by
the right heart to the lungs. There the watery portion of the chyle
and lymph is removed; the composition of the albumen and fibrin is
completed, these substances being changed from a weak and loose into
a strong and concentrated state; the solid particles are increased in
number, augmented in size, and changed from a white into a red colour;
carbon is given off; oxygen is absorbed; azote is alternately inhaled
and exhaled; and the ultimate result is, that the three fluids—chyle,
lymph, and venous blood—are converted into one homogeneous fluid,
arterial blood, the proper nutrient fluid.
972. The particles of the chyle and lymph, on mingling with the blood,
are scattered through the mass, and become invisible, being apparently
lost among the innumerable red corpuscles; but it is not probable that
the chyle is immediately converted into blood. If the coagulation
of the blood be retarded by the addition of a small portion of the
carbonate of potass, the red particles gradually sink some lines
below the level of the fluid; and the supernatent liquid is whitish,
evidently from the chylous globules mingled with the blood. In ordinary
coagulation, the chyle globules are included among the immense number
of the red particles of the coagulum, and are thus indistinguishable;
but there is reason to believe that the chyle is not converted into
blood under at least from ten to twelve hours; it is certain, that in
that space of time after the completion of digestion, the serum of
the blood is frequently seen to be milk-white, from the quantity of
unassimilated chyle still contained in it.
973. How the red colour of the blood is obtained, and whence the
capsules of the red particles are derived, if these bodies really
possess an external envelop, is wholly unknown. But it has been shown
(953 and 955) that in incubation the blood is formed from the substance
of the fluid yolk, without the action of any special organ; that at the
period when the blood is first generated, no such organs as appear to
influence the production of the blood in the adult are in existence;
it is, therefore, reasonable to infer that the formation of blood in
the adult may not be so dependent on the action of special organs as is
commonly supposed; and that the formation of blood from chyle, of blood
corpuscles from chyle corpuscles, may take place at all periods of life
under the influence of the same general vital conditions as it does in
the incubated egg.
974. What change the matter of the blood undergoes by respiration,
whether it acquire something without which it is incapable of
maintaining life, or part with something the presence of which is
incompatible with life, is equally unknown. We only know that the
blood, during respiration, changes its colour; but of the nature of
the change produced upon its substance we are wholly ignorant. In the
present state of our knowledge, the ultimate fact is, that without the
change wrought upon the blood by respiration, the blood is incapable
of maintaining life; in fact, no proper nutrient fluid is formed.
975. Once formed, the conservation of the proper proportions of the
composition of the blood is effected by the excretory processes
already described; by the removal of its superfluous water by the
lungs, skin, and kidneys; by the removal of its superfluous carbon,
azote, and oxygen by the lungs, liver, and kidneys; by the removal of
saline and mineral matters chiefly by the kidneys; and finally by the
instantaneous removal of products of decomposition formed in the course
of the organic actions, chiefly, it would appear, by the kidneys.
976. Once formed, and duly concentrated and purified, the blood is sent
out by the left heart to the system. Driven by the heart through the
main trunks and branches of the aorta, the blood ultimately reaches the
capillary arteries, which do not divide and subdivide indefinitely,
but ultimately reach a point beyond which they no longer diminish in
size. Not all of the same magnitude, some are large enough to admit of
three or four of the red particles of the blood abreast; the diameter
of others is only sufficient to admit of two or even of one; others
are capable of transmitting only the clear and transparent liquor
sanguinis; while in many cases the membranous tunics of the capillaries
wholly disappear; the blood no longer flows in actual vessels, but is
contained in the substance of the tissues in channels which it forms
in them for itself (304).
977. Under the microscope, says Müller, the blood corpuscles are
seen distinctly pouring from the smallest ramifying arteries into
vessels which grow no smaller. After leaving these, they again
assemble in the origins of veins formed in collected branches. The
blood corpuscles flow in the finest capillaries, one after another,
and often interruptedly. They are colourless when they flow singly;
accumulated more thickly, they appear yellow, and in still greater
quantity, yellowish red or red. In animals that have lost their
strength, the globules flow without stoppage: when the animal is weak
and the motion is retarded, the globules move by starts; they move
on, but go more rapidly by fits. In a still weaker animal they only
advance during the impulse of the heart, and then fall back a little.
When several arterial currents unite in an anastomosis, one current
always predominates and traverses the anastomosis alone, to mingle its
blood in the other currents. Thus the currents meet and divide in the
reticulate capillaries till all are collected again in veins. Sometimes
the direction of the current changes, when another current becomes
stronger, and the previous leader weaker, according to the pressure
exerted on the part.
978. While the blood is thus traversing the capillaries, its colour
changes from a bright scarlet to a dark red. This change in the colour
of the blood is the certain sign that particles have been abstracted
from the circulating mass, and have been applied to the formation
and support of the fluid and solid parts through which the stream is
flowing. Some physiologists have satisfied themselves that they have
seen the actual escape of particles from the circulating current; that
they have witnessed the immediate combination of those particles with
the substance of the tissues, and even that they have beheld other
particles quitting the tissues and mingling with the flowing blood.
Other physiologists doubt whether the most patient observation, aided
by the most skilful management of the best glasses, can ever have
rendered such phenomena matters of sense. “I imagined,” says Müller,
“at an early period, that I had seen something like this in the setting
circulation; but by prolonging the observation I saw the globules move
on if the current continued.”
979. But whether the human eye have ever actually seen or not an
interchange of particles between the blood and the tissues, it is
absolutely certain that such an interchange does take place. For,—
1. Indubitable evidence has been stated (786, _et seq._) of continual
absorption from all parts of the body, yet there is no loss of
substance; there must therefore of necessity be a proportionate
deposition.
2. Equal evidence has been adduced (688) that constant additions are
made to the blood through the organs of digestion, yet the quantity
of the blood in the body does not progressively and permanently
increase; it follows that a quantity must be abstracted from the blood
proportionate to the quantity added to it.
3. The human germ, from a scarcely visible point, by the successive
additions of new matter progressively acquires the bulk of the adult
man.
4. Organs whose special office it is to abstract particles from the
blood for the elaboration of specific secretions consist almost
entirely of congeries of blood-vessels. The agents are multiplied in
proportion to the extent of the labour assigned them.
5. Growth, which is merely excess of deposition above absorption, is
active in proportion to the quantity of blood which circulates through
the growing part in a given time. The blood-vessels of a growing part
increase in number and augment in size is proportion to the rapidity of
the growth. In morbid growth, it is sometimes sufficient to stop the
process merely to tie the main trunks of the arteries distributed to
the part.
980. By every organ and every tissue; by the membrane, the muscle, the
bone; by the brain, the heart, the liver, the lungs, particles are
abstracted from the countless streams that bathe them, or that flow
through them. In every case in which particles are thus abstracted by a
tissue the following phenomena take place:—
1. Only those constituents of the blood are abstracted by the tissue
which are of the same chemical nature as its own.
2. The constituents of the blood abstracted by a tissue, identical in
chemical composition with its own, are immediately incorporated into
its substance.
3. The constituents of the blood abstracted by a tissue, as they are
incorporated into its substance, are not disposed fortuitously, but are
arranged according to the specific organization of the tissue, and thus
receive its own peculiar structure.
4. The constituents of the blood which thus receive the peculiar
organization and structure of the tissue by which they are
appropriated, acquire all its peculiar vital endowments.
981. It is manifest, then, that the tissues assimilate the blood just
as the digestive fluids assimilate the aliment. And this is nutrition,
the assimilation of the blood by the tissues and organs. Digestion is
the conversion of the food into blood; nutrition is the conversion of
blood into living fluids and solids.
982. For the reasons assigned (757 and 758), it is probable that
the living fluids and solids, formed from the blood by the act of
nutrition, are not generated at the parts of the body where they
appear, but that, pre-existing in the blood, they are merely evolved
at those parts. Hence the variety and complexity of the processes for
the elaboration of the blood which have been described, and all of
which appear to be indispensable to bring the blood to a proper state
of purity and strength. The great effort of the system is put forth
in effecting the constitution of the blood. When the blood is once
formed, all the rest of the work appears to be easy; because, before it
reaches any part of the organization which it is destined to support,
the blood is already adapted, mechanically, chemically, and vitally, to
afford that support. Still since there are cases, as in the production
of gelatin, in which the substance does not appear to be pre-existent
in the blood, we are under the necessity of supposing that a material
change is effected in the constituents of the vital fluid at the time
and place of their escape from the circulation.
983. How the constituents of the blood escape from the circulation and
incorporate themselves with the substance of the tissues there can
be no difficulty in conceiving, wherever the capillaries terminate
in membraneless canals, channels worked out for the reception of the
nutrient stream by the force of the current itself; and in every case
in which the capillaries, retaining their membranous tunics, remain
true and proper vessels, their contents escape through their delicate
walls by the process of endosmose (803), for which their structure
appears to be admirably adapted.
984. But in the capillary vessels there exists only blood. Universally
and invariably before the blood passes from under the influence of the
capillary vessels it has ceased to be blood. Arterial blood is conveyed
by the carotid artery to the brain; but the cerebral arteries do not
deposit blood, but brain. Arterial blood is conveyed by the capillary
arteries to bone; but the osseous capillaries do not deposit blood, but
bone. Arterial blood is conveyed by the muscular arteries to muscle,
but the muscular capillaries do not deposit blood but muscle. The blood
conveyed by the capillaries of brain, bone, and muscle is the same;
all comes alike from the systemic heart, and is alike conveyed to all
tissues; yet in the one it becomes brain, in the other bone, and in the
third muscle. Out of one and the same fluid are manufactured cuticle,
and membrane, and muscle, and brain, and bone; the tears, the wax, the
fat, the saliva, the gastric juice, the milk, the bile, all the fluids,
and all the solids of the body (310).
985. These phenomena are wholly inexplicable on any known mechanical
principles. It is equally impossible to refer them to mere chemical
agency, or to any properties of dead matter. We are therefore under
the necessity of referring them to a principle which, for the sake of
distinguishing it from anything mechanical or chemical, we term vital.
As the actions which take place between the integrant particles of
bodies, giving rise to chemical phenomena, are referred to one general
principle, termed chemical affinity, so the actions which take place in
living bodies, giving rise to vital phenomena, may be referred to one
general principal, termed vital affinity. The term explains nothing,
it is true, it merely expresses the general fact; but still it is
convenient to have a term for the expression of the fact. The property
itself will ever remain an ultimate fact in physiology, however exactly
the limits of its agency, and the laws according to which it modifies
the mechanical and chemical relations of the substances subjected to
its influence, may hereafter be ascertained; just as chemical affinity
will ever be an ultimate fact in physics, whatever discoveries may yet
be made of the extent of its agency and of the conditions on which its
action depends.
986. It is then an ascertained fact, that there exists between the
blood and the tissues a mutual reaction, not of a physical, but of a
vital nature, in which the blood takes as active a part as the tissue,
and the tissue as the blood; the blood exerting a vital attraction on
the tissue, and the tissue on the blood. We only express this ultimate
fact when we say (and this is all we can do) that in every part of the
body, by virtue of a vital affinity, the tissue attracts from the blood
the molecules of matter appropriate to its chemical composition, and
the blood attracts from the tissue the particles which, having served
their purpose there, are destined to other uses in the economy; or, if
wholly useless, are absorbed into the current of the circulation to be
expelled from the system.
987. We can see how the particles of matter which are attracted by the
tissue from the blood are so deposited and disposed that the tissue
always preserves its own shape, bulk, and relation to the surrounding
tissues. This definite arrangement is the result of an action which has
been already stated to be proper to the absorbent vessels. Previously
to the deposition of a new particle of matter by a capillary, an old
particle is removed by an absorbent, either a lymphatic or a vein. In
removing the old matter, the absorbent forms a mould into which the
capillary deposits the new molecules; and the form of every tissue and
organ depends on the kind of mould formed for the reception of its
nutrient matter by the absorbent vessel. The absorbents are thus the
architects of the system; and the capillaries are both chemists which
form the rough material employed in the structure, and masons which
deposit and arrange it. The conjoint action of both sets of vessels is
necessary to the formation of the simplest tissue; and it is by their
united labour that the compound organs are built up out of the simple
tissues.
988. It is conjectured that the immediate living agents by which this
vital attraction is exerted between the blood and the tissues are
the organic nerves. These nerves consist of two sets, those which
enter as constituents into the tissues and those which accompany the
capillaries. It has been shown (304), that while the membranous tunics
of the capillaries diminish, the nervous filaments distributed to them
increase; that the smaller and thinner the capillaries the greater
the proportionate quantity of their nervous matter; and that this is
most remarkably the case in organs of the greatest irritability. It is
conceived that the capillaries, in consequence of the nervous structure
which thus envelops them, exert upon the fluid which is flowing through
them an influence perfectly analogous to that of the secreting organ,
in consequence of which similar particles are abstracted from the blood
as those which compose the tissue in which the operation takes place.
989. It is further conjectured that the physical agent by which this
action upon the blood is effected is the galvanic fluid. Dutrochet
believes that he has actually formed muscular fibre from albumen by
galvanism. He considers the red particles of the blood as pairs of
electrical plates, and thinks that the nucleus is electronegative,
and the capsule electropositive. Müller has repeated and critically
examined the interesting experiments of Dutrochet; and while he arrives
in many essential points at different results, expresses the highest
admiration of the ingenious manner in which this philosopher has sought
to solve a great problem. “If,” says Müller, “a drop of an aqueous
solution of the yolk of egg (in which very small microscopic globules
are suspended) be galvanised, the currents discovered by Dutrochet will
be observed. The wave, proceeding from the copper or negative pole, in
which the alkali of the decomposed salt accumulates, is transparent,
from the solution of albumen by the alkali. The wave, proceeding from
the positive or zinc pole, particularly in its circumference, is
opaque, and white from the acid it contains. Both waves encounter,
and exactly in the line of contact a linear coagulum is immediately
produced, which assumes the form of the line of contact, and is curled
at times as the edges of the waves are meeting. The meeting of both
waves takes place with a lively motion, in the line of contact, when
the deposition of coagulum takes place; but as soon as the deposition
of coagulum has occurred, all is tranquil, and not the least trace of
motion is observed. It is therefore inconceivable how an observer of
the first rank, like Dutrochet, can pronounce this coagulated albumen
contractile muscular fibre, generated by galvanism; it is nothing but
coagulated albumen. This coagulum, besides, like the albumen which
is deposited by galvanism round the zinc pole, has no consistence,
but is composed of globules easily separated by stirring, and only
precipitated in the line where the two waves meet without cohesion.”
990. But though science has not yet succeeded in ascertaining with
certainty the physical agency to which the ultimate changes that
take place in organized matter are to be referred, there cannot be
a question that they are dependent on physical agents; and the
legitimate object of scientific inquiry is to discover what those
agents are, and to ascertain the modifications they undergo by those
vital affinities to the influence of which they are subjected.
991. The discoveries which science has already made relative to the
influence of certain physical agents on particular organs, and to the
influence of the whole circle of physical agents on the whole living
economy, have added not a little to human power over human health
and disease. But these agents also exert an influence scarcely less
momentous on the entire apparatus and action of the animal life, so
inseparably linked with the organic. An account will therefore be
next given of the structure and function of the nervous and muscular
systems. The exposition of these systems, which will be as brief as
possible, will be followed by a full account of the action of physical
agents on the whole of this complex and wonderful organization. The
detail of the ascertained phenomena will have a strict reference to
the development of the physical and mental powers of the human being,
and thereby a close and practical application will be attempted of
physiology to the production and preservation of health.
THE END.