OF SECRETION.
Nature of the function—Why involved in obscurity—Basis of the
apparatus consists of membrane—Arrangement of membrane into elementary
secreting bodies—Cryptæ, follicles, cæca and tubuli—Primary
combinations of elementary bodies to form compound organs—Relation of
the primary secreting organs to the blood-vessels and nerves—Glands
simple and compound—Their structure and office—Development of glands
from their simplest form in the lowest animals to their most complex
form in the highest animals—Development in the embryo—Number and
distribution of the secreting organs—How secreting organs act upon
the blood—Degree in which the products of secretion agree with, and
differ from, the blood—Modes in which modifications of the secreting
apparatus influence the products of secretion—Vital agent by which the
function is controlled—Physical agent by which it is effected.
708. Secretion is the function by which a substance, gaseous, liquid,
or solid, is separated or formed from the nutritive fluid. It is a
function as necessary to the plant as to the animal, and indispensable
alike to the life of both. It is of equal importance to the
preservation of the individual and to the perpetuation of the species.
In all living beings secretions are separated from the nutritive fluid,
and added to the aliment to assist in converting it into nutriment,
and are separated from the nutriment to maintain the composition of
the nutritive mass in a state fit for the continued performance of the
act of nutrition, and to form the germ on the development of which the
continuance of the species depends.
709. The secretions of the plant, varied and abundant, are
indispensable to its nourishment, growth, and fructification. The
secretions of the animal more diversified, and far more constantly
performed, increase in number and elaborateness in proportion to
the range and intensity of the vital endowments and actions. In all
animals high in the scale of organization, and especially in man, the
products of secretion are vast in number, and exceedingly complex in
nature,—membrane, muscle, brain, bone;—the skin, the fat, the nail,
the hair;—water, milk, bile, wax, saliva, gastric juice;—whatever
substances enter as constituents into the corporeal structure;—whatever
substances are specially produced, in order to perform some definite
purpose in the economy;—whatever substances are separated from the
mass, and carried out of the system on account of their useless or
noxious properties:—all are derived from the nutritive fluid, the
blood, and are formed from it by the process of secretion.
710. In this function are included the most secret and subtle processes
of the vital economy,—the ultimate actions of the organic life. Of
the real nature of those actions nothing definite is known; and
they are modified by agencies over which the art and skill of the
experimentalist can exert no adequate control. It is not wonderful
therefore that they should be involved in obscurity: nevertheless,
when all the phenomena are collected and compared, much of the
mysteriousness in which the function appears at first view to be
involved vanishes.
711. The apparatus of secretion is infinity varied in form: when
examined in its complex combinations it appears inextricable in
structure, but the diligence and skill of modern research have unfolded
much of its mechanism, and enabled us to trace the successive steps by
which it passes from its simple to its complex condition.
712. To form an organ of secretion there must be an artery, a vein,
a nerve, an absorbent, and a sufficient quantity of cellular tissue
to allow of the free expansion of these vessels and of their complete
intercommunication. Membrane constitutes such an organ; for membrane
is composed of arteries, veins, nerves, and absorbents sustained and
connected by cellular tissue. Hence membrane constitutes a secreting
organ, in its simplest form. The most important secreting membranes are
the serous (30), the cutaneous (34), and the mucous (33).
713. Serous membrane which lines the great cavities of the body, and
which gives an external covering to the organs contained in them (fig.
LX. a, c), forms an extensive secreting surface. Synovial membrane, or
that which covers the internal surface of joints, and which constitutes
an important portion of the apparatus of locomotion, is essentially the
same in structure and office.
714. Cutaneous membrane, or the skin, which forms the external covering
of the body, is an organ in which manifold secretions are constantly
elaborated; but the skin is only a modification of the membrane
which lines the interior of the body, the mucous. Mucous membrane
forms the basis of the secreting apparatus placed in the mouth,
fauces, esophagus, stomach, and intestines in their whole extent; of
the secreting apparatus auxiliary to that of the alimentary canal,
namely, the pancreas and the liver; probably also of the mesenteric,
or lacteal glands, together with the vast system of lymphatic glands,
and certainly of the glands of the larynx, trachea, bronchi and
air vesicles of the lungs. Hence, while membrane forms the basis
of the secreting apparatus in general, mucous membrane is far more
extensively employed in its construction than any other form of
membrane.
715. 1. In the construction of the secreting apparatus, membrane
disposed in the simplest form, constitutes merely a uniform, smooth,
extended surface. Serous membrane is always disposed in this simple
mode. The costal pleura which lines the internal surface of the walls
of the chest (fig. LX. a); the pulmonary pleura which is continued from
the walls of the chest over the lungs (fig. LX. 5); the peritoneum
which lines the internal surface of the cavity of the abdomen, and
which is reflected over the viscera contained in it (fig. LX. c, and
6, 7, 8, &c.); the synovial membrane which covers all the articular
surfaces; the arachnoid membrane which envelopes the brain, form
simple continuous, serous, secreting surfaces. On the contrary, mucous
membrane is never disposed in this perfectly simple mode; even when it
forms a continuous surface, as in the lining, which it affords to the
alimentary canals, it is more or less plaited into folds or rugæ (fig.
CLXVII. 1).
[Illustration: Fig. CLXXXI.
A portion of the mucous surface of the intestines, showing some of the
mucous glands which present the appearance of fovæ or cryptæ.]
716. 2. The second disposition of membrane in the construction of the
secreting apparatus, is the depression of it into a minute pit or fova,
called a crypt (CLXXXI.), which is sometimes inclosed on all sides,
forming a cell or vesicle (fig. CXXXVIII.).
[Illustration: Fig. CLXXXII.
Portion of the skin and cellular tissue, showing the sebaceous
follicles, as seen under the microscope very highly magnified. 1. The
external surface of the follicles with the blood-vessels ramifying
upon it. 2. Follicles laid open, showing the interior cavity into
which the secreted fluid is poured.]
717. 3. Next, the vesicle, instead of being rounded, is elongated into
a peduncle or neck, not unlike the neck of a bottle (fig. CLXXXII. 1).
This pedunculated vesicle is called a follicle.
718. 4. Then, the follicle is somewhat elongated, without neck and
without terminal expansion (fig. CLXXXVI. 1); and this is called a
cæcum or pouch.
719. 5. And, lastly, the cæcum itself is elongated; so that instead of
presenting the appearance of a pouch, it rather resembles a tube (fig.
CLXXXV. 1), and is accordingly named tubulum.
720. In the construction of the secreting apparatus, membrane, then,
may be said to be disposed into four elementary forms constituting
cryptæ or vesicles, follicles, cæca and tubuli. Membrane, disposed
into these elementary forms, constitutes the simple bodies by the
accumulation and the varied arrangement of which the compound organs
are composed. There is no other known element which enters into the
composition of the most complex secreting organ.
721. One of these elementary bodies may exist as a simple organ, or
many may be collected into a mass to form a compound organ. When single
they are called solitary: when collected into a mass, aggregated. Each
elementary body has a mode of aggregation peculiar to itself. Vesicles
aggregate by clustering together (fig. CXXXVIII.), and adhering as
if by a common stem (fig. CXXXVIII.); follicles by uniting at their
orifices (fig. CLXXXIII.), and forming masses which are disposed either
in a linear direction (fig. CLXXXIII.) or in fasciculi (fig. CLXXXIV.);
cæca by forming bundles, parallel or branched (fig. CLXXXVI.);
and tubuli by forming masses straight (fig. CLXXXV.), tortuous or
convoluted (figs. CLXXXV. and CLXXXIX.).
[Illustration: Fig. CLXXXIII.
Aggregated follicles disposed in a linear direction, here represented
of their natural size, as seen near the mouth in the goose.]
[Illustration: Fig. CLXXXIV.
Conglomerated follicles.]
722. When a single elementary body, as a vesicle or follicle, forms
a distinct secreting organ, the matter secreted is elaborated at the
inner surface of the organ (fig. CLXXXII. 2), and is contained within
its cavity. When needed it quits this cavity through the walls of
the vesicle, or at the orifice of the follicle, on the application
of the appropriate stimulus. When a number of cryptæ or vesicles are
aggregated into clusters, the individual vesicles sometimes open by
distinct orifices into a common receptacle or sac (fig. CLXXXIV.). When
follicles are aggregated into a mass, and the mass is disposed in a
linear direction (fig. CLXXXIII.), each follicle pours out its secreted
matter by its own orifice (fig. CLXXXIII.); but if conglomerated, into
a common mass by a common orifice (fig. CLXXXIV.).
[Illustration: Fig. CLXXXV.
1. Parallel tubuli, opening by distinct orifices into—2. A common
cavity.]
[Illustration: Fig. CLXXXVI.
Branched cæca, showing—1. The cæca terminating in—2. Excretory ducts
which unite to form—3. A common trunk.]
723. In like manner, in some very simple arrangements of cæca and
tubuli, each body opens by its own distinct orifice (fig. CLXXXV.
2). But in the more complex arrangements of these bodies, it is
indispensably necessary to modify this mode of parting with their
contents. When the elementary bodies are aggregated into dense,
thick masses (fig. CLXXXIX.), when layer after layer of these masses
containing myriads of myriads of follicles, cæca, or tubuli, are
superimposed one upon another, (fig. CLXXXIX.), it is impossible that
each individual body can have a separate orifice. In this case a
minute tube springs from each body (fig. CLXXXVI. 2); and a complete
connexion is established between all the individuals composing the
mass by the free intercommunication of these tubes (fig. CLXXXVI. 2).
Of these tubes the minutest unite together, and form larger branches
(fig. CLXXXVI. 2); these larger branches again uniting form still
larger branches (fig. CLXXXVI. 2), until, by their successive union,
the branches form at length a single trunk (fig. CLXXXVI. 3), with
which all the individual branches, whether great or small, communicate,
and into which they all pour their contents (fig. CLXXXII. 2, 3).
The bodies from which these tubes take their origin, and the minute
tubes themselves, are called secreting canals (fig. CLXXXII. 1, 2);
the common trunk formed by their union is termed the excretory duct
(fig. CLXXXII. 3). The secreting canals contain the secreted matter;
the excretory duct collects this matter, and conveys it to the part of
the body in which it is appropriated to the specific purpose which it
serves in the economy.
724. The basis of the secreting canals consists, then, of membrane
disposed in one or other of the elementary forms described (712, _et
seq._), These secreting canals constitute a peculiar system of organs
wholly different from all the other organs of the body. The form of
these organs, their structure and their relation to the blood-vessels
and nerves, have formed subjects of laborious investigation and of
keen controversy during several centuries. The honour of discovering
the exact truth on these points is due to very recent researches.
725. Malpighi, an Italian, who flourished at Bologna in the middle of
the 17th century, was the first to establish a special inquiry into
the intimate structure of the secreting apparatus. After many years
of laborious examination he arrived at the conclusion that a minute
sac or follicle is invariably interposed between the termination of
the capillary artery and the commencement of the excretory duct.
According to him, the capillary artery conveys the blood to the
follicle, separates from the blood the substance secreted, and the
excretory duct arising from one extremity of the follicle conveys the
secreted fluid, when duly prepared, to its destined situation. By
injection, by dissection, by the microscope, by experiment on living
animals, and by the phenomena of disease, he conceived that he had
demonstrated that this is the true structure of the secreting apparatus
in its most complex form. This view was generally acquiesced in by his
contemporaries and by succeeding anatomists and physiologists; and in
the time when Ruysh wrote was the received opinion.
726. Ruysh, who flourished at Amsterdam, and was contemporary with
Malpighi, but a younger man, and who published on the glands about
twenty years after Malpighi, according to the account of Haller,
“employed wonderful patience, with the assistance of his daughters,
in rendering all his preparations elegant and beautiful, being
equally skilled in the methods of softening, hardening, filling, and
drying.” Of Ruysh it was said that while others, in their anatomical
preparations, merely exhibited the horrid features of death, he
preserved the human body in all the freshness of life, even to the
expression of the features. The fineness of his injections, the
dexterity with which he unfolded the minute vessels, nerves, and
absorbents, and exhibited their combinations and relations in the most
delicate structures, the skill with which he preserved his preparations
in transparent fluids, and the elegance with which he displayed them
in their natural forms and folds, excited universal admiration; and
philosophers, statesmen, princes, kings, all the learned and noble of
the day, crowded to his museum.
727. By his superior method of injecting, Ruysh conceived that he was
able completely to disprove Malpighi’s doctrine. He maintained that
the bodies which Malpighi mistook for sacs or follicles are in reality
convoluted vessels; that these vessels are capable of being completely
unravelled; that, when unfolded, their continuity with the excretory
duct is perfectly demonstrated; that secretion is performed by the
capillary artery itself, without the intervention of any other organ;
and that when the secreted substance is duly prepared, it is poured by
the capillary directly into the excretory duct.
728. Modern research has demonstrated that the opinion of Malpighi
approaches nearer the truth than that of Ruysh, who appears to have
mistaken the secreting canals for the ultimate division of the
arterial vessels. Malpighi, indeed, did not succeed in discovering
the elementary bodies of which the secreting apparatus is composed;
but he arrived at the very verge of the truth. Profiting by the art
which Ruysh brought to so much perfection, by the facts which Malpighi
disclosed, and, above all, by the improved structure of the microscope,
and the increased skill which has been acquired in the manipulation
of the instrument, the modern physiologist is enabled to see what
was formerly beyond the cognizance of sense, and to demonstrate what
before could only be matter of conjecture. Availing himself of these
advantages with consummate skill, and applying himself to the task with
indefatigable industry, Professor Müller, of Berlin, has investigated
the structure of the secreting apparatus in the whole animal kingdom,
and has traced the progressive development of the several secreting
organs through the entire animal series, from their simplest form in
the lowest animal, to their most complex in the highest.
729. From the researches of this physiologist, and from the labours of
others, his countrymen and contemporaries, who have engaged in the
investigation with an ardour second only to his own, it is demonstrated
that the secreting apparatus of the animal body is disposed in one
or other of the elementary forms which have been described. The
blood-vessels are distributed upon the walls of these elementary
bodies, whether simple cryptæ follicles, cæca, or tubuli, or whether
these bodies are accumulated and combined into the largest and most
complex series of secreting canals, just as the branches of the
pulmonary artery are distributed upon the walls of the air-vesicles
in the rete mirabile of the lungs. The air-vesicles of the lungs are
secreting organs, and afford an excellent example of the mode in which
the blood-vessels are distributed upon the walls of the elementary
secreting bodies. The arteries do not form continuous tubes with the
secreting bodies or their excretory ducts, as was maintained by Ruysh;
neither is the secreting body interposed between the termination of
the artery and the commencement of the excretory duct, as was thought
by Malpighi; but the ultimate divisions of the arteries are spread
out upon the walls of the secreting bodies, where they terminate in
veins by a delicate vascular net-work (fig. CLXXXVII. 2). The minutest
branch of the artery is always smaller than the minutest secreting
body on the walls of which it is distributed. According to Müller, the
arteries, spread out upon the walls of the secreting bodies, form a
distinct and peculiar system of vessels visible under the microscope.
In the more complex secreting organs, immediately before reaching their
distribution upon the walls of the secreting canals, the ultimate
divisions of the arteries form an intricate and delicate net-work
(fig. CLXXXVII. 2). When at length they reach the secreting canals the
arteries no longer divide and subdivide, but are always of the same
uniform size in the same secreting organ, though their magnitude is
different in every different kind of secreting organ. These ultimate
divisions of the arteries are the proper capillary arteries. It is in
these arteries that the changes are wrought upon the blood which it
is the object of the various processes of secretion to effect. In the
walls of these arteries there are visible no pores, no apertures, no
open extremities by which the secreted fluid, when formed from the
blood, is conveyed into the cavity of the secreting canals; it probably
passes through the walls of the vessels into the secreting canals by
the process of endosmose (804).
[Illustration: Fig. CLXXXVII.
A thin portion of the surface of the kidney taken from the scianus,
showing—1. The termination of the cæca forming the uriniferous
duct; and—2. A delicate vascular net-work, consisting of capillary
blood-vessels about to be distributed on the walls of the cæca.]
730. Secreting organs are very abundantly supplied with nerves,
which are derived for the most part from the organic portion of the
nervous system; although for the reasons assigned (vol. i. p. 77, _et
seq._) sentient nerves are mixed with the organic. The more important
secreting organs have each a distinct net-work or plexus of organic
nerves, which surround the blood-vessels distributed to the organ,
(fig. CLXX. 3), and which envelopes more especially the arterial trunks
and their larger branches (fig. CLXX. 3). From these plexuses nervous
filaments spring in countless numbers (fig. CLXX. 3), which are spread
out upon the walls of the arteries, just as the arteries are spread
out upon the walls of the secreting canals. The nerves never quit
the arteries; are never spent upon the membranous matter which forms
the basis of the secreting organ, but are lost upon the walls of the
capillary arteries. The nerves uniformly increase in number and size as
the arteries diminish in magnitude and as their capillary terminations
become thinner and thinner.
731. When the secreting apparatus consists of simply extended membrane,
a close net-work of capillary arteries with their accompanying nerves
is spread out over the whole extent of the secreting surface. This
simple arrangement is sufficient to separate from the blood the simple
secretion in this case required.
732. When the secreting apparatus consists of simple cryptæ, follicles,
cæca, or tubuli, a similar net-work of capillary arteries and nerves
is spread out on the sides of this more extended surface. The more
elaborate secretion now formed is received into the interior of these
organs, where it remains for some time, and whence it is ultimately
conveyed as it is needed by the actions of the system.
733. But when the secreting apparatus consists of aggregates of cryptæ,
follicles, cæca, and tubuli, with their net-works of arteries and
nerves, a much more complex structure is built up, which is destined to
perform a proportionably elaborate function. An aggregation of these
secreting bodies into a large mass, enveloped in a common membrane,
so as to form a distinct body of a solid consistence, constitutes the
organ termed a gland. Simply extended membrane, with its apparatus
of arteries and nerves does not constitute a gland. Simple cryptæ,
follicles, cæca, and tubuli, with their larger apparatus of arteries
and nerves, do not constitute a gland. The first is simply secreting
surface; the second are simply secreting cryptæ, follicles, cæca or
tubuli; but when these bodies are aggregated into dense and solid
masses with an extended system of excretory ducts, and when the whole
of this apparatus is inclosed in a proper membrane so as to form a
distinct body, such a body is termed a gland.
734. Primary aggregations of these secreting bodies constitute what is
termed a conglobate, that is, a simple gland; such are all the glands
connected with the absorbent or lymphatic system. Secondary aggregates,
or aggregates composed of simple glands, constitute what is termed
a conglomerate, that is, a compound gland; such are all the organs
commonly termed viscera, as the liver, the spleen, the pancreas, the
kidney, and so on.
735. The conglobate, or simple gland, being formed by the aggregation
of cryptæ, follicles, cæca, or tubuli, inclosed in a proper membrane,
presents the appearance of a simple solid body, commonly of a rounded
or oblong form (fig. CLXXVI. 516). On the contrary, the conglomerate
or compound gland, being formed by the aggregation of conglobate or
simple glands, presents the appearance of a compound body composed of
a congeries of masses (fig. CLXV. 1). The larger masses enveloped in
their own proper membrane are termed lobes (fig. CXCI.); the smaller
masses, also enveloped in their own proper membrane, are termed lobules
(fig. CXCI.); the lobules, when carefully examined, are seen to be
composed of still smaller masses, and these of masses yet more minute,
until at length patient, laborious, and skilful dissection brings into
view the ultimate constituent elements, which are invariably found to
consist of simple cryptæ, follicles, cæca, or tubuli.
736. Thus membrane having a specific arrangement of blood-vessels and
nerves, from being simply extended, is folded into a few elementary
forms; the bodies which result constitute simple secreting organs;
these bodies collected together form, by their aggregation, compound
organs; the compound organs, uniting, form aggregates still more
compound, until at length a structure is built up highly elaborate and
complex. But this complexity of combination and arrangement does not
alter the constitution of the organs; their form varies, but their
nature remains essentially the same. All consist alike of membrane
organized in a similar mode. The complex contains no element not
possessed by the simple gland, and the gland contains no element not
possessed by the secreting surface. But there is this difference in
the complex organs. Every kind and degree of change in the form of the
secreting apparatus, from membrane simply extended, to membrane coiled
up into the most complex gland, is attended with an accumulation and
concentration of secreting surface. The crypt contains a larger extent
of secreting surface than the simple membrane; the follicle than the
crypt; the cæcum than the follicle; and the tubulum than the cæcum. A
certain amount of secreting surface is gained by the disposition of the
simple membrane into the form of the crypt. The collection of a number
of crypts into a cluster doubles the extent of the secreting surface by
the extent of every crypt that is added to the cluster. The addition
of every cluster doubles the whole extent of surface acquired by a
single cluster. But when stems spring as if from a common trunk; when
branches spring from a stem; when small branches spring from the large
branches, and yet smaller branches from the small in a series, which
the eye, assisted by the most powerful microscope, is wholly unable to
trace; when all the clusters thus formed are collected, and combined
into a compact mass, the intricacy of which no art can completely
unravel, the extent of surface obtained is altogether immeasurable. How
immense must be the extent of surface thus acquired in such an organ as
the human lungs, in such a gland as the human liver!
737. In such an aggregation the concentration is also equal to the
accumulation; the maximum of surface is comprised in the minimum of
space, and the energy and elaborateness of the function of a secreting
organ is uniformly proportionate to such a concentration of its
secreting substance.
[Illustration: Fig. CLXXXVIII.
Aggregated and clustered cæca opening into the alimentary canal,
performing the function of the liver.]
738. Hence the complexity of the compound gland in the higher animals
would appear to arise solely from the intricate arrangement of the
immense mass of secreting matter concentrated in a small compass;
hence also the progressively increased complication indicated in the
successive development of the glandular system in the animal series.
Thus, for example, among the distinct organs formed for the purpose
of elaborating a specific secretion, being intimately connected with
the process of digestion, one of the first is the salivary gland. Low
down in the scale, in the animal in which the first rudiment of a
salivary gland is traceable, it consists of a single follicle, which
appears to serve the office of a gland. In an animal a little higher
in structure, two, three, or four follicles combine to form a somewhat
less simple organ. In an animal still higher in the series, a number of
follicles are clustered together and form a much more complex organ;
and in this manner, as the organization of the animal becomes higher
and higher, the complexity of the gland increases, until at length it
is composed of a countless number of follicles collected into clusters,
the clusters disposed into lobes, the lobes subdivided into lobules,
and the lobules into still smaller particles, the ultimate elements
of the glandular apparatus. In like manner, when the first rudiment
of the liver is discoverable, it consists of a single pouch or cæcum;
somewhat higher in the series, the organ is composed of two or more
cæca distinct and free; and then, as its complexity increases with
the perfection of the organization, cæca are accumulated upon cæca;
the aggregates so formed are closely compacted, disposed into lobes,
divided into lobules, and subdivided into the ultimate particles of the
glandular apparatus. So in a gland composed of tubuli, as the kidney,
the organ in its rudimentary state consists of a few straight tubuli:
as its structure advances more tubuli are added: next, the increasing
tubuli superimposed one upon another become tortuous; then the tubuli
still accumulating, become not merely tortuous, but convoluted; and
last of all, countless numbers of tubuli are closely compacted into
exceedingly convoluted masses. Uniformly, the lower the animal and the
simpler the organ, the larger and the more manifest are the elementary
parts of the gland; but in the higher animals these elementary bodies
are so minute as to be altogether microscopical and their arrangement
is so complex that it can be unravelled only with extreme difficulty.
[Illustration: Fig. CLXXXIX.
Portions of the kidney taken from the ophidian reptile, as seen
under the microscope, highly magnified. A one portion of the kidney,
showing—1. The trunk of the artery passing to be distributed to—2. The
diverging tubuli, forming the uriniferous ducts which terminate in—3.
The common excretory duct called ureter.—B another portion of the same
kidney, showing the extremely convoluted course of—4. The uriniferous
ducts. 5. The smaller excretory ducts, or secreting canals, converging
and uniting to form—6. The common excretory duct called the ureter.]
739. It is a striking confirmation of the correctness of this view
of the structure of the glandular apparatus, that whenever in the
ascending series a gland appears for the first time in any class, the
elementary bodies are so large, and are disposed in so simple a mode,
that a slight examination is sufficient to demonstrate their primitive
form, and to render it manifest that they consist either of vesicles,
follicles, cæca, or tubuli, more or less aggregated. This is seen
in the obvious structure presented by the liver, the pancreas, the
salivary glands, and the mammæ, in the simple animals in which these
organs first appear. Thus the liver in animals low down in the scale
is manifestly composed of simple clustering follicles: in the fish the
pancreas is composed of simple branched follicles: in the bird, the
salivary glands are composed of simple parallel tubuli; and in the
cetacea the breasts are composed of simple branched tubuli.
[Illustration: Fig. CXC.
A lobule of a gland in the progress of development in the ovum of
the bird, as seen under the microscope, showing the origin of the
excretory ducts in the semipellucid gelatinous blastema, and the
branching and foliated arrangement of the follicles in which the
excretory ducts terminate.]
740. But the microscope, by bringing the successive development of the
compound gland in the embryo of the higher animal under the cognizance
of sense, perfectly discloses the nature of its composition. In
the development of the incubated egg every step of the progressive
formation of the compound gland is rendered visible to the eye.
When this process is carefully watched, it is seen that the part of
the gland first formed is the excretory duct, which springs from
the blastema, the common mass of matter out of which all the organs
are formed. From this duct the elementary parts of the gland bud
just as bunches of grapes bud from the stalk. The buds, at first
at considerable distances from each other, approach nearer as they
increase by new growths, until at length they come into actual
contact. The growth continuing, and the compactness of the substance
of the gland proportionally increasing, the primitive form of the
elementary bodies which compose it is ultimately lost. The substance
of the gland now appears to consist of compact solid matter, which
is commonly termed parenchyma. The component particles of this
parenchymatous and apparently solid substance present a clustered or
grape-like appearance, from which they early obtained the name of
acini, from the Latin word acinus, a berry. This term, originally
employed merely to express the clustered and branching appearance
of the elementary parts of the gland, has since been used in widely
different senses. By some it has been employed to express solid
glandular grains constituting a supposed distinct parenchymatous
substance, differing in every different gland. It is now proved that no
such solid granular particles enter into the composition of any gland
in the animal kingdom. By others the term acini has been employed to
express granular bodies composed of blood-vessels, directly continuous
with the excretory ducts, and from which the excretory ducts derive
their origin. Recent investigation has demonstrated that there is no
continuity of the blood-vessels into the excretory duct either in the
acini or in any other part of the gland. It is established that the
blood-vessels are spread out upon the walls of the secreting canals
and do not form with them continuous tubes. The bodies which have been
mistaken for granular particles, constituting the so called solid
acini, are really the shut extremities of hollow follicles, cæca, or
tubuli, which appear solid only from the closeness with which they are
compacted. When carefully dissected and examined under the microscope,
their real nature becomes apparent, and this is also sometimes capable
of being demonstrated by injection; for some of these elementary bodies
are vesicular, and can be filled with mercury, when they present a
beautiful appearance like clusters of diamonds; or they may be inflated
with air, just as the air vesicles of the lungs.
[Illustration: Fig. CXCI.
Section of the liver in the lower animal in the progress of
development, as seen under the microscope, showing the rudimentary
division into lobes and lobules, and the elongated terminations of
the biliferous ducts, or cylindrical acini variously disposed in a
branching and foliated manner.]
741. On watching the formation of the gland in the development of
the embryo, it would appear that at first free streams of blood, or
blood not contained in proper vessels, pass around the acini, the shut
extremities of the excretory ducts, or the secreting canals. “So it
would seem,” says Müller, “when we examine the evolution of the liver
and kidney in the embryo of the lower animal; for the interstices of
the canals appear bloody, without the slightest trace of the walls of
blood-vessels. I conceive that in the beginning new streams arise in an
amorphous mass (a mass without form), not bounded by proper parieties;
but that soon walls are formed, which present definite boundaries
to the streams, the density of the substance around the streams
gradually increasing.” It is in this manner that the connexion is first
established between the system of capillary blood-vessels and that of
the secreting organs.
742. In its embryo state the compound gland of the highest animal
consists of mere excretory ducts, wonderfully similar to the simple
secreting bodies of the lowest classes. But in the higher animal this
simple form of the gland is transient: gradually, with the progressive
evolution of the embryo, it passes into a more complex structure; while
in the lower animal the simple form of the gland remains permanently
the same through the whole term of life.
743. Such are the main points which have been ascertained relative to
the structure of the secreting apparatus, which enters in one or other
of its forms, as a constituent element, into almost every part of
the animal body. Wherever there is nutrition there is secretion, and
wherever there is secretion there is one or other of these secreting
bodies. How immense the number of these organs in the human body! Every
point in the interior of the walls that bound the great cavities is a
secreting surface. Every point of the secreting surface that lines the
alimentary canal, from its commencement to its termination, is studded
with distinct secreting organs. Every point of the skin is still more
thickly studded with distinct secreting organs. By the naked eye, and
still more distinctly with a lens, may be seen the pores through which
the vapour that constitutes the insensible perspiration incessantly
exudes. Next are the open mouths of myriads of sebacious follicles that
pour out upon the skin the oily matter which gives it its suppleness
and softness; and besides all these, are the hairs, each the product
of a secreting organ placed immediately beneath the skin. An attempt
to count the number of pores and hairs visible to the eye within the
compass of an inch, and thence to compute the number on the whole
surface of the skin, may convey some conception of the amount of these
organs; yet these form but a small part of the secreting apparatus.
The great viscera of the body, the brain, the lungs, the liver, the
pancreas, the spleen, are portions of it; all the organs of the senses,
the eyes, the ears, the nose, the tongue; all the organs of locomotion;
every point of the surface of every muscle, and a great part of the
surface and substance of the very bones are crowded with secreting
organs.
744. Since every secreting organ is copiously supplied with blood,
it follows that a great part of the blood of the body is always
circulating in secreting organs; and, indeed, it is to afford materials
for the action of these organs that the blood itself is formed.
745. How do these organs act upon the blood? All that is known of the
course of that portion of the blood which flows through an organ of
secretion is, that it passes into arteries of extreme minuteness, which
are spread out upon the external walls of the elementary secreting
bodies, and which, as far as they can be traced, pass into capillary
veins,—nowhere terminating by open mouths—nowhere presenting visible
outlets or pores; their contents probably transuding through their thin
and tender coats by the process of endosmose.
746. As it is flowing through these capillary arteries, the blood
undergoes the transformations effected by secretion, forming—1. The
fluids, which are added to the aliment, and which accomplish its
solution, and change it into chyme. 2. The fluids, which are added to
the chyme to convert it into chyle, and both to chyle and lymph, to
assist in their assimilation. 3. The fluids which, poured into the
cavities, facilitate automatic or voluntary movements. 4. The fluids,
which serve as the media to the organs of the senses by which external
objects are conveyed to the sentient extremities of the nerves for
their excitement. 5. The fluids which, deposited at different points
of the cellular tissue, when more aliment is received than is needed,
serve as reservoirs of nutriment to be absorbed when more aliment is
required than can be afforded by the digestive organs. 6. The fluids
which are subsequently to be converted into solids. 7. The fluids which
are eliminated from the common mass, whether of fluids or solids, to be
carried out of the system as excrementitious substances. 8. In addition
to all these substances, which are indispensable to the preservation of
the individual, those which are necessary to the perpetuation of the
species.
747. In order to form any conception of the mode in which the secreting
organs act upon the blood, so as to elaborate from it such diversified
substances, it is necessary to consider the chemical composition of the
different products of secretion, and the degrees in which they really
differ from each other, and form the common mass of blood out of which
they are eliminated.
748. By chemical analysis, it is established that all the substances
which are formed from the blood by the process of secretion are either
water, albumen, mucus, jelly, fibrin, oil, resin, or salts; and,
consequently, that all the secretions are either aqueous, albuminous,
mucous, gelatinous, fibrinous, resinous, oleaginous, or saline.
749. 1. AQUEOUS SECRETIONS.—From the entire surface of the skin, and
also from that of the lungs, there is constantly poured a quantity of
water, derived from the blood, mixed with some animal matters, which,
however, are so minute in quantity, that they do not communicate to the
aqueous fluid any specific character.
750. 2. ALBUMINOUS SECRETIONS.—All the close cavities, as the thorax,
the abdomen, the pericardium, the ventricles of the brain, and even
the interstices of the cellular tissue, are constantly moistened by a
fluid which is termed serous, because it is derived from the serum of
the blood. This serous fluid consists of albumen in a fluid form, and
it differs from the serum of the blood chiefly in containing in equal
volumes a smaller proportion of albumen. Membranes of all kinds consist
essentially of coagulated albumen; and the albumen, as constituting
these tissues, differs from albumen as existing in the serum of the
blood only in being unmixed with extraneous matter, and in being in a
solid form.
751. 3. MUCOUS SECRETIONS.—As all the close cavities, or those which
are protected from the external air, are moistened with a serous
fluid, so all the surfaces which are exposed to the external air, as
the mouth, the nostrils, the air-passages, and the whole extent of the
alimentary canal, are moistened with a mucous fluid. Mucus does not
exist already formed in the blood. It is always the product of a gland.
Some of the mucous glands are among the most elaborate of the body;
still the main action of the gland seems to be to coagulate the albumen
of the blood, for the basis of mucous is coagulated albumen. The fluid
that lubricates the mucous surfaces in their whole extent, the saliva,
the gastric juice, the tears, the essential part of the fluid formed
in the testes and in the ovaria, are mucous secretions. Hence the most
complex and elaborate functions of the body, respiration, digestion,
reproduction, are intimately connected with the mucous secretions:
nevertheless, as far as regards their chemical nature, the mucous
differ but slightly from the albuminous secretions; and it is probable
that a slight change in the secreting organ is sufficient to convert
the one into the other. By the irritation of mercury on the salivary
glands, the saliva, properly of a mucous, is sometimes converted into a
substance of an albuminous nature; and irritation in some of the serous
membranes occasionally causes them to secrete a mucous fluid.
752. 4. GELATINOUS SECRETIONS.—The proximate principle termed jelly
abounds plentifully in several of the solids of the body, and more
especially in the skin; but jelly does not exist already formed in
the blood. Yet it is not the product of a gland, neither is there any
known organ by which it is formed. Out of the body albumen is capable
of being converted into jelly by digestion in dilute nitric acid: this
conversion is probably effected by the addition of a portion of oxygen
to the albumen. Albumen contains more carbon and less oxygen than
jelly; the proportions of hydrogen and nitrogen in both being nearly
the same. According to MM. Gay Lussac and Thénard, the elements of
albumen and jelly are,
Carbon. Oxygen. Hydrogen. Nitrogen.
Albumen 52.883 23.872 7.54 15.765
Jelly 47.881 27.207 7.914 16.988
The conversion of albumen into jelly is incessantly going on in the
system; and the process accomplishes most extended and important uses.
In the lungs at the moment of inspiration oxygen enters into the blood
in a state of loose combination; but in the system, at every point
where the conversion of albumen into jelly takes place, oxygen probably
enters into a state of chemical combination with albumen; and the new
proximate principle, jelly, is the result. The agent by which this
conversion is effected appears to be the capillary artery: the primary
object of the action is the production of a material necessary for
the formation of the tissues of which jelly constitutes the basis, as
the skin; but a secondary and most important object is the production
of animal heat; the carbon that furnishes one material of the fire
being given off by the albumen at the moment of its transition into
jelly; and the oxygen that furnishes the other material of the fire
being afforded to the blood at the moment of inspiration. This view
affords a beautiful exposition of the reason why jelly forms so large a
constituent of the skin in all animals. The great combustion of oxygen
and carbon, the main fire that supports the temperature of the body, is
placed where it is most needed, at the external surface.
753. 5. FIBRINOUS SECRETIONS.—The pure muscular fibre, or the basis
of the flesh, is identical with the fibrin of the blood. It contains
a larger proportion of nitrogen, the peculiar animal principle, and
is consequently more highly animalized than the preceding substances.
It appears to be simply discharged from the circulating blood by the
capillary arteries, and deposited in its appropriate situation; no
material change in its constitution being, it would seem, necessary to
fit it for its office.
754. 6. OLEAGENOUS SECRETIONS.—Fat of all kinds, which is found so
extensively connected with the muscles, and with many of the viscera,
and which is more or less diffused through the whole extent of the
cellular tissue, marrow, milk, and nervous and cerebral matter, are
essentially of the same nature. The basis of them all is oil; and oil
exists already formed both in the chyle and in the blood.
755. 7. RESINOUS SECRETIONS.—The peculiar substance forming the basis
of bile, picromel; the peculiar substance forming the basis of urine,
urea; the peculiar substance connected with the muscular fibre, and
forming a component part of almost all the solids and fluids of the
body, osmazome, consists of a common principle—a resin, which exists
already formed in the blood, and more especially in the serosity of the
blood.
756. 8. SALINE SECRETIONS.—The substances termed saline, namely, the
acids, the alkalis, and the neutral and earthy salts, are disposed
over every part of the system: they enter more or less into all the
constituents both of the solids and fluids; they form more especially
the phosphate of lime, the earthy matter of which bones are composed;
and they all exist already formed in the blood.
757. From this account, then, it appears, that by chemical analysis,
the blood is ascertained to contain water, albumen, fibrin, oil, resin,
and various saline and earthy substances: it follows, that, with the
exception of the absence of jelly, the constituents of the body and the
constituents of the blood are nearly identical; and it is probable that
they will be found to be perfectly identical when their analysis shall
have become complete.
758. It is also manifest that in by far the greater number of cases the
various substances of which the body is composed are simply separated
from the nutritive fluid at the parts of the body at which they are
deposited; and that, existing already formed in the blood, they are
merely deposited there, and not generated. Still, however, since it is
certain that gelatin cannot be recognized in the blood, and since it
is doubtful whether some other substances found in different textures
and secretions really exist in the blood, it is necessary, in the
present state of our knowledge, to suppose, that although most of the
constituents of the living tissues are contained in the blood, yet that
in some instances a material change is effected in their nature at
the time and place of their escape from the circulation; and that in
these cases the secreted substances are not simple extracts from, but
products of, the blood.
759. It is by the apparatus of secretion that this separation,
evolution, or re-formation, is effected. Out of a fluid which contains,
blended together, almost all the heterogeneous substances of which the
body is built up, particular substances are selected from the common
mass, and are deposited in certain parts, and only in certain parts.
Although by the most careful examination of the structure of the
apparatus, it is not possible to form a precise conception of the mode
in which this separation is effected, yet we are enabled to perceive a
number of contrivances which we can readily understand must conduce to
the accomplishment of the object.
760. 1. Of these, the most obvious is mechanical arrangement.
761. In its passage to different organs the blood is propelled
through canals of extreme minuteness: in every different case these
canals differ from each other in size; pass off from their respective
trunks at different angles; possess different degrees of density; are
variously contorted, and are of various lengths. In some they are
straight, in others convoluted; at one time branching, at another
pencillated, and at another starry. The veins, too, in some cases, are
almost straight, in others exceedingly tortuous, in others reticulated;
and the freedom of their communication with the arteries varies so
much, that in some cases fine injections pass from the one set of
vessels to the other with the greatest facility, while, in others
they pass with extreme difficulty. The consequence of these divers
arrangements of the capillary blood-vessels is, that the current of
the blood must necessarily flow in them with different degrees of
velocity; its particles must be placed at different distances from each
other, and must be presented to each other in different positions and
in widely different proportions. In no two secreting organs are any
two of these conditions exactly alike. In the lower orders of animals,
in which secretion is seen in its simplest condition, the general
nutritive fluid, elaborated and contained in a single internal cavity,
appears to furnish a variety of products very different from itself, by
a process hardly more complex than mere transudation through a living
membrane. In the higher animals the different secreting organs may be
considered, in part at least, as mechanical contrivances adapted to
carry on analogous transudations—fine sieves or strainers diversly
constructed. A fluid containing such heterogeneous matters as the
blood, held in combination by so slight an affinity, slowly transuding
through series of tubes, the mechanical arrangement of which is so
varied, must yield a different substance in every different case.
Thus by simply filtering the blood a vast variety of products may be
obtained, merely in consequence of a varied disposition of the minute
tubes of which the filters are composed.
762. 2. But in the second place, this diversity of mechanical
arrangement is calculated in a high degree to promote and to modify
chemical action. The contact or proximity of the particles of bodies,
the extent of surface which those particles present to each other, the
space of time in which they continue in contact, the degree of force
with which they impinge against each other, the degree of temperature
to which they are exposed,—these, and circumstances such as these,
are conditions which exert the most powerful influence over chemical
decomposition and re-combination. In the different secreting organs,
as has been shown, the blood must necessarily pass through vessels
having every conceivable diversity of diameter: in those vessels it
must consequently flow with corresponding differences of velocity. Some
of these diameters will admit one constituent of the blood, as one of
the red particles; others may be large enough to admit two or more of
the red particles abreast; others may be so small as to be incapable of
admitting a single red particle, receiving only the more fluid portions
of the blood; in some vessels these different constituents will be in
one degree of proximity, in others in another; in some they will remain
long in contact, in others only for an instant: it is obvious that
from such different conditions the chemical products may be infinitely
varied.
763. Such is the composition of chemical bodies, that a great diversity
of substances is obtainable merely by changing one condition, the
proportions in which the elementary particles combine.
764. Oxygen and nitrogen combined in one proportion form atmospheric
air; in another proportion, nitrous oxide; in another, nitric oxide;
in a fourth, nitrous acid; and in a fifth, nitric acid. Few secretions
formed from the blood differ more widely from each other than the
products thus formed from these two elementary bodies.
765. Urea consists of two prime equivalents of hydrogen, one of
carbon, one of oxygen, and one of nitrogen. Remove one of the atoms
of hydrogen, and take away the atom of nitrogen, urea is converted
into sugar; combine with urea an additional atom of carbon, it is
changed into lithic acid. In like manner add a small quantity of water
to farina, it is converted into sugar; to fibrin, it is changed into
adipocere. From a reservoir containing a quantity of substances in
the state of vinous fermentation, draw off portions of the liquor at
different stages of the process, and cause these to pass through tubes
of various diameters and with various degrees of velocity, there will
be obtained at one time an unfermented syrup, at another, a fermenting
fluid, at another, wine, at another, vinegar. Out of the body place the
blood in a state of rest, it will spontaneously separate into serum and
crassamentum, and the crassamentum will further separate into fibrin
and red particles. Add to the serum a certain portion of acid, it will
be coagulated into solid albumen; add to this solid albumen another
portion of acid, it will be converted into jelly. Add a certain portion
of acid to fibrin, it will be changed into adipose matter; bring the
acid into contact with the red particles, they will be converted into a
substance closely resembling bile. If by the rough chemistry which the
art of man can conduct so great a variety of substances may be obtained
out of a single compound, is it not wonderful that a far greater
variety should be produced by the delicate and subtle chemistry of life.
766. 3. But a third most important agent in the process of secretion is
some influence derived from the nervous system.
1. It is proved, by direct experiment, that the destruction of the
nervous apparatus, or of any considerable portion of it, stops the
process of secretion. By experiments performed by Mr. Brodie, it is
ascertained that the secretion of the urine is suspended by the removal
or destruction of the brain, though the circulation be maintained in
its full vigour by artificial respiration.
2. The section, and still more the removal, of a portion of the
sentient nerves of the stomach (the par vagum, or eighth pair),
according to some experimentalists, deranges and impedes; according to
others, totally arrests the process of digestion.
3. Other classes of phenomena illustrate in a striking manner the
influence of the nervous system over the process of secretion.
The sight, nay, even the thought of agreeable food, increases the
secretions of the mouth. Pleasurable ideas excite, painful ideas
destroy, the appetite for food; probably, in the one case, by
increasing, and, in the other, by suspending the secretion of the
gastric juice: the emotion of grief instantly causes a flow of tears;
that of fear, of urine; the sight or thought of her child fills the
maternal breasts with milk, while the removal of the child from the
mother diminishes and ultimately stops the secretion.
767. Even the imagination is capable of exerting a powerful influence
over the process. A female who had a great aversion to calomel was
taking that medicine in very small doses for some disease under which
she was labouring. Some one told her that she was taking mercury:
immediately she began to complain of soreness in the mouth; salivated
profusely, and even put on the expression of countenance peculiar to a
salivating person. On being persuaded that she had been misinformed,
the discharge instantly began to diminish, and ceased altogether
in a single night. Two days afterwards she was again told, on good
authority, that calomel was contained in her medicines, upon which
the salivation immediately began again, and was profuse. That this
salivation was not produced by the calomel, but was the effect solely
of the influence of imagination on the salivary glands, was proved
by the absence of redness of the gums, which always takes place in
mercurial salivation, and also by the absence of the peculiar fætor,
which is characteristic of the action of this metal on the system.
768. The same influence is apparent even in the lower animals: exhibit
food to a hungry dog, the saliva will pour from its mouth. Rob the nest
of the bird of its eggs as soon as they are laid, the bird may be made
to deposit eggs almost without end, though if the eggs are allowed to
remain undisturbed, it will lay only a certain number. The bird is led
by instinct to continue to deposit eggs in the nest until a certain
number is accumulated; that is, a mental operation acts upon the
ovarium, the secreting organ in which the eggs are formed, maintaining
it in a state of active secretion for an indefinite period; whereas
without that mental operation the secretion would be limited to a
definite number.
769. In all these cases it is probable that the vital agent by which
the effect is produced on the secreting organs is the organic nerve.
Though the sentient part of the nervous system may in many cases be
the part primarily acted on, yet there is reason to believe that
the ultimate effect is invariably produced on the organic part, the
sentient nerves in this case acting on the organic, as in other cases
the organic act on the sentient, in consequence of that intimate
connexion which, for the reason assigned (vol. i. p. 79), is
established between both parts of this system. For,
770. 1. The true object of the sentient part of the nervous system is
to establish a relation between the body and the external world; the
object of the organic part is to preside over the functions by which
the body is sustained and nourished, that is, over the processes of
secretion.
771. 2. The nerves which are distributed to the secreting arteries, and
which increase in number and size as the arteries become capillary,
are, for the most part, derived from the organic portion of the nervous
system (fig. CLXX. 3). This anatomical arrangement clearly points to
some physiological purpose, and indicates the closeness of the relation
between the function of the organic nerve and the ultimate action of
the capillary artery.
772. 3. It is demonstrated that the sentient part of the nervous
system, though occasionally influencing and modifying secretion, is not
indispensable to it. In tracing the normal or regular development of
the human fœtus, it is found that the heart is constructed and is in
full action before the brain and spinal cord, the central masses of the
sentient part of the nervous system, are in existence; and that these
masses are themselves built up by processes to which the action of the
heart is indispensable; consequently, innumerable acts of secretion
must have taken place, those, for example, which have been necessary
to form the different substances which enter into the composition of
the heart, before the brain and spinal cord exist. In like manner in
the anormal or irregular development of the fœtus, as in the production
of monsters, there may be not a vestige of head, neck, brain or spinal
cord, while there may be a perfect heart, perfect lungs, perfect
intestines, and various portions even of the osseous system.
773. However in the perfect animal secretion may be under the influence
of the brain and spinal cord, it is clear that, since the process can
go on without them, it must be independent of them. It is a false
induction from these facts drawn by some physiologists that secretion
is independent of the nervous system. They do prove that it is
independent of one part of the nervous system, the sentient; but it
does not follow that it is independent of the other part, the organic.
774. 4. It is demonstrated that the organic part of the nervous
system is not only independent of the sentient part, but that it
is even pre-existent to it. Researches into the development of the
nervous system, as shown in the progressive growth of the fœtus of
different animals, have proved that the existence of the organic
nerves is manifest long before that of the sentient; that nerves are
discoverable in the tissues, before the brain and the spinal cord are
formed; that as these masses become visible and grow, nerves springing
from the tissues advance towards the central nervous masses, and
at length unite with them; but that this union does not take place
until the development of the nervous system is considerably advanced.
These curious and most instructive facts show that in the fœtus,
though the brain and spinal cord may have been destroyed or have
been non-existent, yet that the organic nerves may have been in full
action. After a communication has been once established between the two
parts of the system, indeed, the destruction of the brain or spinal
cord may stop secretion, not because these organs are indispensable
to secretion; but because the destruction of one part of the system
involves the death of the other, just as the organic life itself
perishes soon after the destruction of the animal.
775. The existence of the organic nerve is probably simultaneous
with that of the secreting artery: from the first to the last moment
of life the nerve regulates the artery; the influence of the one is
indispensable to the operation of the other; and, by their conjoint
action, the sentient nerve itself, as well as every other organ, is
constructed.
776. There is reason to believe that the physical agent by which the
organic nerve influences secretion is electricity. The nerve appears to
be the medium by which electrical fluid is conveyed to the secreting
organs, and the nerve probably influences secretion by influencing
chemical combination, through the intervention of this most powerful
chemical agent. This is rendered probable by the observation of various
phenomena, and by the result of direct experiment.
777. 1. It is proved that galvanic phenomena may be excited by
the contact of the nerve and muscle in an animal recently dead. A
galvanic pile may be constructed of alternate layers of nervous and
muscular substance, or of nervous substance and other animal tissues.
A secreting organ liberally supplied with organic nerve is probably
then in its physical structure nothing but a galvanic apparatus. It
is certain that some animals, as the raia torpedo, possess a special
electrical apparatus composed essentially of nervous matter; that
the nerves which compose this apparatus correspond strictly with the
organic nerves of the human body; that they are distributed principally
to the organs of digestion and secretion, and that they exert a
powerful influence over these processes; for, when the animal is
frequently excited to give shocks, digestion appears to be completely
arrested; so that, after the animal’s death, food swallowed some time
previously is found wholly unchanged.
778. 2. It is universally admitted that the nerves in all animals
possess an extreme sensibility to the stimulus of electricity, and more
especially to that form of it which is termed galvanism.
779. 3. Direct experiment proves that the stimulus of galvanism may
be made to produce in the living-body precisely the same effect as
the nervous influence. It has been stated, that the division of the
par vagum, in the neck of a living animal, suspends the digestion of
the food probably by stopping indirectly the secretion of the gastric
juice. If after the division of the nerves, their lower ends, that
is, that portion of the nerves which is still in communication with
the stomach, but no longer in communication with the brain, be made
to conduct galvanic fluid to the stomach, secretion goes on as fast
as when the nerves are entire and conduct nervous influence. Dr.
Wilson Philip having divided the par vagum in the neck of a living
animal, coated a portion of the lower end of the nerves with tin foil,
placed a silver plate over the stomach of the animal, and connected
respectively the tin and silver with the opposite extremities of a
galvanic apparatus. The result was that the animal remained entirely
free from the distressing symptoms which had always before attended the
division of the nerves, and that the process of digestion, which had
been invariably suspended by this operation, now went on just as in the
natural state of the stomach. On examining the stomach after death, the
food was found perfectly digested, and afforded a striking contrast to
the state of the food contained in the stomach of a similar animal, in
whom the nerves had been divided, but which had not been subjected to
the galvanic influence.
780. 4. On applying a low galvanic power to a saline solution contained
in an organic membrane, Dr. Wollaston found that the galvanic fluid
decomposed the saline solution, and that the component parts of the
solution transuded through the membrane; each constituent being
separately attracted to the corresponding wire of the interrupted
circuit. This experiment, says this acute and philosophical
physiologist, illustrates in a very striking manner the agency of
galvanism on the animal fluids. Thus the quality of the secreted fluid
may probably enable us to judge of the electrical state of the organ
which produces it; as for example, the general redundance of acid
in urine, though secreted from blood that is known to be alkaline,
appears to indicate in the kidney a state of positive electricity; and
since the proportion of alkali in bile seems to be greater than is
contained in the blood of the same animal, it is not improbable that
the secretory vessels in the liver may be comparatively negative.
781. We may imagine, says Dr. Young, that at the division of a minute
artery a nervous filament pierces it on one side, and affords a pole
positively electrical, and another opposite filament a negative pole.
Then the particles of oxygen and nitrogen contained in the blood, being
most attracted by the positive point, tend towards the branch which is
nearest to it; while those of the hydrogen and carbon take the opposite
channel; and that both these portions may be again subdivided, if it
be required; and the fluid thus analysed may be recombined into new
forms by the reunion of a certain number of each of the kinds of minute
ramifications. In some cases the apparatus may be somewhat more simple
than this; in others, perhaps, much more complicated; but we cannot
expect to trace the processes of Nature through every particular step;
we can only inquire into the general direction of the path she follows.
782. Considerations such as these afford us a glimpse into the mode in
which Nature conducts some of her most secret and subtile operations;
or rather into the immediate agency by which she effects them; for,
properly speaking, of the mode in which she works, we do not obtain
the slightest insight, and even of her immediate agency our view, at
least in the present state of our knowledge, is indistinct and vague.
By the study of the apparatus which she builds up, we can trace back
her operations a step or two; but in every case, at a certain point,
the apparatus itself becomes so delicate as to elude our senses, and
then of course we are necessarily at a stand. So, the rough materials
with which she carries on her great work of secretion, by careful
analysis we can separate into divers parts, and ascertain that each
part possesses peculiar properties. The main channels by which she
conveys these varied constituents to the different parts of the system
we can trace; the delicate organs by which she produces on these rude
materials her wonderful transformations we can see; but beyond the
threshold of these organs we cannot go. Why from one common mass of
fluid the same variety of peculiar substances are constantly separated,
and each in its respective place: why the kidney never secretes milk,
nor the liver urine, nor the breast bile: why membrane, and muscle, and
bone, and fat, and brain, are uniformly deposited in the same precise
situation: why these depositions go on with uniformity, constancy and
regularity; and by what laws each process is controlled and modified,
we do not know. But though with whatever diligence we investigate these
operations, the great problem remains, and probably ever will remain
unresolved, still it is both a pleasurable and a profitable labour to
follow Nature in her path, to the extreme point to which it is possible
to trace her footstep; for the phenomena themselves are often in the
highest degree curious and interesting; while their order and relation
can seldom be so considered as to be understood, without the suggestion
of practical applications of great and permanent usefulness.