THE
PHILOSOPHY OF HEALTH;
OR,
AN EXPOSITION
OF THE
PHYSICAL AND MENTAL CONSTITUTION
OF MAN,
WITH A VIEW TO THE PROMOTION OF
HUMAN LONGEVITY AND HAPPINESS.
BY
SOUTHWOOD SMITH, M.D.,
_Physician to the London Fever Hospital, to the Eastern Dispensary,
and to the Jews’ Hospital_.
IN TWO VOLUMES. VOL. II.
_THIRD EDITION._
LONDON:
C. COX, 12, KING WILLIAM STREET, STRAND.
1847.
London: Printed by WILLIAM CLOWES and SONS, Stamford Street.
CONTENTS OF VOL. II.
CHAPTER VIII.
OF THE FUNCTION OF RESPIRATION.
Respiration in the plant; in the animal—Aquatic and aërial
respiration—Apparatus of each traced through the lower to the
higher classes of animals—Apparatus in man—Trachea, Bronchi, Air
Vesicles—Pulmonary artery—Lungs—Respiratory motions: inspiration;
expiration—How in the former air and blood flow to the lungs; how
in the latter air and blood flow from the lungs—Relation between
respiration and circulation—Quantity of air and blood employed in each
respiratory action—Calculations founded on these estimates—Changes
produced by animal respiration on the air: changes produced by
vegetable respiration on the air—Changes produced by respiration on
the blood—Respiratory function of the liver—Uses of respiration Page 1
CHAPTER IX.
OF THE FUNCTION OF GENERATING HEAT.
Of the temperature of living bodies—Temperature of plants—Power
of plants to resist cold and endure heat—Power of generating
heat—Temperature of animals—Warm-blooded and cold-blooded
animals—Temperature of the higher animals—Temperature of the different
parts of the animal body—Temperature of the human body—Power of
maintaining that temperature at a fixed point, whether in intense
cold or intense heat—Experiments which prove that this power is a
vital power— Evidence that the power of generating heat is connected
with the function of respiration—Analogy between respiration and
combustion—Phenomena connected with the functions of the animal body,
which prove that its power of generating heat is proportionate to
the extent of its respiration—Theory of the production of animal
heat—Influence of the nervous system in maintaining and regulating the
process—Means by which cold is generated, and the temperature of the
body kept at its own natural standard during exposure to an elevated
temperature Page 120
CHAPTER X.
OF THE FUNCTION OF DIGESTION.
Process of assimilation in the plant; in the animal—Digestive
apparatus in the lower classes of animals; in the higher
classes; in man—Digestive processes—Prehension, Mastication,
Insalivation, Deglutition, Chymification, Chylification, Absorption,
Fecation—Structure and action of the organs by which these operations
are performed—Ultimate results—Powers by which those results are
accomplished—Two kinds of digestion, a lower and a higher; the former
preparatory to the latter Page 159
CHAPTER XI.
OF THE FUNCTION 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 Page 279
CHAPTER XII.
OF THE FUNCTION OF ABSORPTION.
Evidence of the process in the plant, in the animal—Apparatus
general and special—Experiments which prove the absorbing power of
blood-vessels and membrane—Decomposing and analysing properties
of membrane—Endosmose and exosmose—Absorbing surfaces, pulmonary,
digestive, and cutaneous—Lacteal and lymphatic vessels—Absorbent
glands—Motion of the fluid in the special absorbent vessels—Discovery
of the lacteals and lymphatics—Specific office performed by the
several parts of the apparatus of absorption—Condition of the system
on which the activity of the process depends—Uses of the function
Page 332
CHAPTER XIII.
OF THE FUNCTION OF EXCRETION.
In what excretion differs from secretion—Excretion in the
plant—Quantity excreted by the plant compared with that excreted
by the animal—Organs of excretion in the human body—Organization
of the skin—Excretory processes performed by it—Excretory
processes of the lungs—Analogous processes of the liver—Use of the
deposition of fat—Function of the kidneys—Function of the large
intestines—Compensating and vicarious actions—Reasons why excretory
processes are necessary—Adjustments Page 369
CHAPTER XIV.
OF THE FUNCTION 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 Page 422
THE
PHILOSOPHY OF HEALTH.
CHAPTER VIII.
OF RESPIRATION.
Respiration in the plant; in the animal—Aquatic and aërial
respiration—Apparatus of each traced through the lower to the
higher classes of animals—Apparatus in man—Trachea, Bronchi, Air
Vesicles—Pulmonary artery—Lung—Respiratory motions: inspiration;
expiration—How in the former air and blood flow to the lung; how
in the latter air and blood flow from the lung—Relation between
respiration and circulation—Quantity of air and blood employed in each
respiratory action—Calculations founded on these estimates—Changes
produced by animal respiration on the air: changes produced by
vegetable respiration on the air—Changes produced by respiration on
the blood—Respiratory function of the liver—Uses of respiration.
313. No organized being can live without food and no food can nourish
without air. In all creatures the necessity for air is more urgent than
that for food, for some can live days, and even weeks, without a fresh
supply of food, but none without a constant renewal of the air.
314. The food having undergone the requisite preparation in the
apparatus provided for its assimilation, is brought into contact with
the air, from which it abstracts certain principles, and to which
it gives others in return. By this interchange of principles the
composition of the food is changed: it acquires the qualities necessary
for its combination with the living body. The process by which the air
is brought into contact with the food, and by which the food receives
from the air the qualities which fit it for becoming a constituent part
of the living body, constitutes the function of respiration.
315. In the plant, the air and the food meet in contact and re-act
on each other in the leaf. The crude food of the plant having in its
ascent from the root through the stalk, received successive additions
of organic substances, by which its nature is assimilated to the
chemical condition of the proper nutritive fluid of the plant (320
and 325), undergoes in the leaf a double process; that of Digestion
and that of Respiration. The upper surface of the leaf is a digestive
apparatus, analogous to the stomach of the animal; the under surface
of the leaf is a respiratory apparatus, analogous to the lung of the
animal. For the performance of this double function, incessantly
carried on by the leaf, its organization is admirably adapted.
[Illustration: Fig. CXXII.
View of the net-work which forms the solid structure of the leaf, and
which consists partly of woody fibres, and partly of spiral vessels.
1. Vessels of the upper surface; 2. vessels of the under surface; 3.
distribution of the vessels through the substance of the leaf; 4.
interspaces between the vessels occupied by parenchyma or cellular
tissue.]
316. The solid skeleton of the leaf consists of a net-work composed
partly of woody fibres and partly of spiral vessels which proceed
from the stem, and which are called veins (fig. CXXII. 1, 3). In
the interstices between the veins is disposed a quantity of cellular
tissue, termed the parenchyma of the leaf (fig. CXXII. 4): the whole is
enveloped in a membrane, called the cuticle (fig. CXXIII. 1), which is
furnished with apertures denominated stomata, or stomates (fig. CXXIV.).
[Illustration: Fig. CXXIII.
Vertical section of the leaf as it appears when seen highly magnified
under the microscope. 1. Cells of the cuticle filled with air; 2.
double series of cylindrical cells occupying the upper surface of
the leaf filled with organic particles; 3. irregular cells forming
a reticulated texture occupying the under surface of the leaf; 4.
interspaces between the cells, termed the intercellular passages or
air chambers.]
317. The cuticle consists of a layer of minute cellules, colourless,
transparent, without vessels, without organic particles of any kind,
and probably filled with air (fig. CXXIII. 1). These cellules open
externally, at certain portions of the cuticle, by apertures or
passages which constitute the stomates (fig. CXXIV.), and which present
the appearance of areolæ with a slit in the centre (fig. CXXIV.).
They form a kind of oval sphincters, which are capable of opening
or shutting, according to circumstances, and they are disposed on
both surfaces of the leaf, but most abundantly on the under surface,
excepting in leaves which float on water, in which they are always on
the upper surface only.
[Illustration: Fig. CXXIV.
View of the stomata of a leaf, some of them represented as open and
others as closed.]
318. The cellular tissue or parenchyma, immediately beneath the
cuticle, when examined in thin slices, and viewed under a microscope
with a high magnifying power, presents a regular structure disposed
in perfect order. It consists, on the upper surface, of a layer, and
sometimes of two and even three layers, of vesicles of an oblong or
cylindrical form, placed perpendicularly to the surface of the leaf,
set close to each other (fig. CXXIII. 2), and filled with organic
particles constituting the green matter which determines the colour
of the leaf. On the under surface, on the contrary, the vesicles,
which are larger than the cylindrical, are of an irregular figure,
and are placed in an horizontal direction, at such distances as to
leave wide intervals between each other (fig. CXXIII. 3); yet uniting
and anastomosing together, and thus forming a reticulated tissue,
presenting the appearance of a net with large meshes (fig. CXXIII. 3).
319. A leaf, then, consists of a double congeries of vesicles
containing organic particles, penetrated by woody fibre and air vessels
(which is probably the true nature of the spiral vessels), the whole
being enclosed within a hollow stratum of air-cells.
320. The crude sap, composed principally of water, holding in solution
carbonic acid, acetic acid, sugar, and a matter analogous to gum,
is transmitted through the leaf-stalk to the cylindrical vesicles
of the upper surface of the leaf (fig. CXXIII. 2). These vesicles
exhale a large proportion of the water; the evaporation of which is
so powerfully assisted by the action of the sun’s rays, that it would
probably become excessive, were it not for the perpendicular direction
of the cylindrical vesicles (fig. CXXIII. 2); but in consequence of
their being disposed perpendicularly to the surface of the leaf, their
ends only are presented towards the heavens (fig. CXXIII. 2), and thus
the main part of their surface is protected from the direct influence
of the solar rays. The primary effect of the evaporation carried on in
the cylindrical vesicles, is the condensation of the organic matters
contained in the sap.
321. At the same time that the cylindrical vesicles pour the
superfluous water of the sap into the surrounding atmosphere, they
abstract from the atmosphere in return carbonic acid, which, together
with that already contained in the sap, is decomposed. The oxygen is
evolved; the carbon is retained. The physical agent by which this
chemical change, which constitutes the digestive process of the plant,
is effected, is the solar ray; hence the vesicles which contain the
fluid to be decomposed, are placed on the upper surface of the leaf,
where their contents are fully exposed to the action of the sun;
and hence also this process takes place only during the day, and
most powerfully under the direct solar ray: but although the direct
influence of the sun be highly conducive to the process, yet it is
not indispensable to it; for it goes on in daylight although there be
no sunshine. Light, then, would appear to be the physical agent which
effects on the crude food of the plant a change analogous to that
produced on the crude food of the animal by the juices of the stomach.
322. After the sap has been elaborated in the cylindrical vesicles,
by the exhalation of its watery particles, by the condensation of its
organic matter, by the retention of carbon and the evolution of oxygen,
it is transmitted to the reticulated vesicles of the under surface of
the leaf (fig. CXXIII. 3), These vesicles, large, loose, and expanded,
as they have an opposite function to perform, are arranged in a mode
the very reverse of the cylindrical: in such a manner as to present
the greatest possible extent of surface to the surrounding air (fig.
CXXIII. 3): at the same time the broad interspaces between them (fig.
CXXIII. 4) are so many cavernous air-chambers into which the air is
admitted through the stomates (fig. CXXIV.). The cylindrical vesicles,
exposed to the direct rays of the sun, are protected by the closeness
with which they are packed; and by the small extent of surface they
present to the heavens: the reticulated vesicles, whose function
requires that they should have the freest possible exposure to the
surrounding air, are protected from the solar ray, first by their
position on the under surface of the leaf; and, secondly, by the dense
and thick barrier formed by the stratum of cylindrical vesicles (fig.
CXXIII. 2).
323. In the cylindrical vesicles carbonic acid is decomposed; in the
reticulated vesicles, on the contrary, carbonic acid is re-formed. The
oxygen required for this generation of carbonic acid is abstracted
partly from the surrounding air; the carbon is derived partly,
perhaps, from the air, but chiefly from the digested sap, and the
carbonic acid, formed by the union of these elements, is evolved into
the surrounding atmosphere.
324. This operation, which is strictly analogous to that of respiration
in the animal, in which carbonic acid is always generated and expired,
is carried on chiefly in the night. In this manner, under the influence
of the solar light, the leaf decomposes carbonic acid; retains the
carbon and returns the greater part of the oxygen to the air in a
gaseous form. At night, in the absence of the solar ray, the leaf
absorbs oxygen, combines this oxygen with the materials of the sap to
produce carbonic acid, which, as soon as formed, is evolved into the
surrounding air. The carbonic acid gas exhaled during the night is
re-absorbed during the day and oxygen is evolved; and this alternate
action goes on without ceasing; whence the plant deteriorates the
air by night, by the abstraction of its oxygen and the exhalation of
carbonic acid; and purifies it by day by the evolution of oxygen and
the abstraction of carbonic acid.
325. The result of these chemical actions is the conversion of
the crude sap into the proper nutritive juice of the plant. When
it reaches the cylindrical vesicles, the sap is colourless, not
coagulable, without globules, composed chiefly of water holding in
solution carbonic and acetic acids, sugar, gum, and several salts;
when it leaves the reticulated vesicles it is a greenish fluid,
partly coagulable and abounding with organic particles under the
form of globules. Its chemical composition is now wholly changed; it
consists of resinous matter, starch, gluten, and vegetable albumen. It
is now thoroughly elaborated nutritive fluid; the proper food of the
plant (cambium); rich in all the principles which are fitted to form
vegetable secretions: it is to the plant what arterial blood is to
the animal, and like the vital fluid formed in the lung, the cambium
elaborated in the leaf, is transmitted to the different parts and
organs of the plant to serve for their nutrition and development.
326. The formation of this nutritive fluid by the plant is a vital
process, as necessary to the continuance of its existence, as the
process of sanguification is necessary to the maintenance of the life
of the animal. If the plant be deprived of its leaves, if the cold
destroy, or the insect devour them, the nutrition of the plant is
arrested; the development of the flowers, the maturation of the fruit,
the fecundation of the seeds, all are stopped at once, and the plant
itself perishes.
327. The proper nutritive juice of the plant, completed by the process
of respiration, is formed by the elaboration of organic combinations
of a higher nature than those afforded by the sap. Acid, sugar, gum
(325) are converted into the higher organic compounds, resin, gluten,
starch, albumen, probably by chemical processes, the result of which
is the inversion of the relative proportions of oxygen and carbon. In
the organic matters contained in the sap, the proportion of oxygen,
compared with that of carbon, is in excess; on the contrary, in the
higher compounds contained in the cambium, the carbon preponderates: by
the inversion of the relative proportions of these two elements, the
organic compounds of a lower nature, appear to be changed into those of
a higher; to be brought into a chemical condition nearer to that of the
proper substance of the plant; a condition in which they receive the
last degree of elaboration preparatory to their conversion into that
substance.
328. In the process of respiration in the animal, as in the plant,
parts of the digested aliment mix with the air; parts of the air mix
with the digested aliment; and by this interchange of principles, the
chemical composition of the aliment acquires the closest affinity to
that of the animal body; is rendered fit to combine with it; fit to
become a constituent part of it.
329. The extent and complexity of the respiratory apparatus in the
animal, is in the direct ratio of the elevation of its structure and
the activity of its function, to which the quantity of air consumed by
it is always strictly proportionate.
330. The process of respiration in the animal is effected by two
media, air and water; but the only real agent is the air; for the
water contributes to the function only by the air contained in it.
Respiration by water is termed aquatic, that by the atmosphere,
atmospheric or aërial respiration.
331. The quantity of air contained in water being small, aquatic
is proportionally less energetic than aërial respiration; and,
accordingly, the creatures placed at the bottom of the animal scale,
having the simplest structure and the narrowest range of function, are
all aquatic.
332. Whatever the medium breathed, respiration in the animal is
energetic in proportion to the extent of the respiratory surface
exposed to the surrounding element. As the water-breathing animals
successively rise in organization, their respiratory surface becomes
more and more extended, and a proportionally larger quantity of water
is made to flow over it. It is the same in aërial respiration: the
higher the animal, the greater the extent of its respiratory surface;
and the larger the bulk of air that acts upon it.
333. Whatever the medium breathed, respiration is effected by the
contact of fresh strata of the surrounding element with the respiratory
surface. The mode in which this constant renewal of the strata is
effected, is either by the motion of the body to and fro in the
element; or by the creation of currents in it, which flow to the
respiratory surface. A main part of the apparatus of respiration
consists of the expedients necessary to accomplish these two objects;
and that apparatus is simple, or complex, chiefly according to the
extent of the mechanism requisite to effect them.
334. Whatever the medium breathed, the organic tissue which constitutes
the essential part of the immediate organ of respiration is the skin.
The primary tissue of which the skin is composed is the cellular (23
et seq.), which, organized into mucous membrane (33 et seq.), forms
the essential constituent of the skin (34). In all animals the skin
covers both the external and the internal surfaces of the body (34).
When forming the external envelop, this organ commonly retains the
name of skin; when forming the internal lining, it is generally called
mucous membrane; and in all animals, from the monad to man, either in
the form of an external envelop, or an internal lining, or by both in
conjunction, or by some localization and modification of both, the
skin constitutes the immediate organ of respiration. In different
classes of animals it is variously arranged, assumes various forms,
and is placed in various situations, according to the medium breathed,
and the facility of bringing its entire surface into contact with the
surrounding element; but in all, the organ and its office are the
same: it is the modification only—that modification being invariably
and strictly adaptation, which constitutes the whole diversity of the
immediate organ of respiration.
335. At the commencement of the animal scale, in the countless tribes
of the polygastrica (vol. i. p. 34, et seq.), respiration is effected
through the delicate membrane which envelops the soft substance of
which their body is composed. The air contained in the water in which
they live, penetrating the porous external envelop, permeates every
part of their body; aërates their nutritive juices; and converts them
immediately into the very substance of their body. They are not yet
covered with solid shells, nor with dense impervious scales, nor with
any hard material which would exclude the general respiratory influence
of water, or render necessary any special expedient to bring their
respiratory surface into contact with the element.
336. But in some tribes even of these simple creatures there is visible
by the microscope an afflux of their nutritive juices to the delicate
pellicle that envelops them, in the form of a vascular net-work, in
which there appears to be a motion of fluids, probably the nutritive
juices flowing in the only position of the body in which they could
come into direct contact with the surrounding element. In some more
highly advanced tribes, as in wheel animalcules, there is an obvious
circulating system in vessels near the surface of the skin. In other
tribes, the internal surface constituting the alimentary canal, is of
great extent and width, and forms numerous cavities which are often
distended with water. In this manner a portion of the internal, as
well as the external surface is made contributary to the function
of respiration, and this extended respiration is conducive to their
great and continued activity, to their rapid development, and to the
extraordinary fertility of their races.
[Illustration: Fig. CXXV.—_Medusa._
1. The mouth; 2. the stomach; 3. large canals going from the stomach;
4. smaller canals which form; 5. a plexus of vessels at the margin of
the disc serving for respiration; 6. margin of the disc.]
337. In creatures somewhat higher in the scale, a portion of the
external surface is reflected inwards in the form of a sac, with an
external opening (fig. CXXV. 1). In some medusæ there are numerous
sacs of this kind, which pass inwards until they are separated only
by thin septa from the cavities of the stomach. The water permeating
and filling these sacs comes into contact with an interior portion
of the body, not to be reached through the external surface. At the
margin of the disk (fig. CXXV. 6) there is spread out a delicate
net-work of vessels (fig. CXXV. 5); these vessels communicate with
small canals (fig. CXXV. 4) which open into larger canals (fig. CXXV.
3) that proceed directly from the stomach (fig. CXXV. 2). As the
aliment is prepared by the stomach, it is transmitted thence by these
communicating canals to the exterior net-work of vessels where it is
aërated.
338. As organization advances, as the component tissues of the body
become more dense, and are moulded into more complex structures, when,
moreover, these structures are placed deep in the interior of the body,
far from the external envelop, and proportionally distant from the
surrounding element, the respiratory apparatus necessarily increases
in complexity. The first complication consists in the formation of
minute, delicate, transparent tubes (fig. CXXVI. 5), which communicate
with the external surface by a special organ (fig. CXXVI. 4) that
conveys water into the interior of the body (fig. CXXVI. 5). By means
of these ramifying water-tubes, upon the delicate walls of which the
blood-vessels are spread out in minute and beautiful capillaries, the
water is brought into immediate contact with the vascular system.
[Illustration: Fig. CXXVI.—_Holothuria._
1. Mouth; 2. salivary sacs; 3. intestine; 4. cloaca; 5. ramified
tubes, conveying water for respiration into the interior of the body.]
339. Next, in the ascending scale, the external envelop of the body is
extended into a distinct additional or supplemental organ, by which
the function of the skin is assisted. This additional organ is called
branchia or gill. The simplest form of branchia consists of folds
or duplicatures of skin, forming ramified tufts (fig. CXXVII. 1),
which in general have a regular and often a symmetrical disposition
on the external surface (fig. CXXVII. 1). Sometimes, as in the water
breathing annelides, these tufts form a fan-like expansion around the
head; but at other times they are disposed in regular series along the
whole extent of the body.
[Illustration: Fig. CXXVII.—_Lumbricus Marinus._
1. Respiratory tufts. 2. Artery and vein, supplying the respiratory
apparatus. 3. Dorsal vessel.]
340. Instead of branchiæ in the form of ramified tufts, the ascending
series of animals, namely, the higher crustacea, possess branchiæ
composed of numerous, delicate, thin laminæ or leaves, divided from
each other, yet placed in close proximity, like the teeth of a
fine comb, whence this arrangement is termed pectinated. Over the
blood-vessels of the system spread out on these delicate, fringed,
pectinated leaves, the water is driven in constant streams.
341. Still higher in the scale, as in molluscous animals, an internal
sac is formed to which are sometimes attached numerous tufts; but which
at other times is itself plaited into beautifully disposed regular
folds, crowded with blood-vessels and constantly bathed with fresh
currents of water.
[Illustration: Fig. CXXVIII.
Trichoda showing the form and a frequent arrangement of Cilia.]
342. In all these water-breathing creatures, respiration is effected,
either by the progressive motion of the body through the water, or by
the creation of currents which bring fresh strata of the fluid into
contact with the respiratory surfaces. Both objects are effected by the
same instruments, namely, minute fibres having the appearance of fine
hairs or bristles. These fibres which are called cilia, have in general
an elongated, flattened, thin, and tapering form (fig. CXXVIII). Their
number, position, and arrangement, are infinitely various. Sometimes,
as in the poriferous animals, they are so minute that they cannot
be rendered visible to the eye even by the microscope, although the
evidence of their existence and action is indubitable. Sometimes they
are of great size and strength, attached by distinct ligaments to the
body and moved by powerful muscles, as in wheel animalcules. Sometimes,
as in polypiferous animals, they are disposed around the orifice of the
polypes or upon the sides of the tentacula, the instruments by which
the animal seizes its prey. Sometimes they are symmetrically disposed
in longitudinal series along the surface of the body, as in the Beroe
pileus; at other times they are arranged in circles; whenever there
are branchiæ, they are disposed around the margin of the branchial
apertures, and always on the margins of the minute meshes which compose
the branchiæ themselves.
343. In some cases the number of these cilia is immense. Each polype,
for example, has usually twenty-two tentacula, and there are about
fifty cilia on each side of a tentaculum, making two thousand two
hundred cilia on each polype. As there are about one thousand eight
hundred cells in each square inch of surface, and the branches of an
ordinary specimen present about ten square inches of surface, we may
estimate that an ordinary specimen of this zoophite presents more
than eighteen thousand polypes, three hundred and ninety-six thousand
tentacula, and thirty-nine million six hundred thousand cilia. But
other species contain more than ten times these numbers. Dr. Grant has
calculated that there are about four hundred million cilia on a single
Flustra foliacea.
344. The motions of these cilia are regular, incessant, and when in
full activity far too rapid to be distinguished by the eye even when
assisted by the microscope. They are generally to be perceived only
when their motions are comparatively feeble. They produce two effects.
In animals capable of progressive motion, they transport the body
through the water, while they constantly bring new strata of water into
contact with the respiratory surface. In this case they are partly
organs of locomotion, and partly organs subservient to respiration.
On the other hand, in animals which are not capable of moving from
place to place, they create currents by which the respiratory surface
is constantly bathed with fresh streams of water. These currents
are regular, constant, unceasing. Like some physical phenomena not
depending on vitality, it is a continued stream as regular as the
motions of rivers from their source to the ocean, or any other
movements depending on the established order of things. Dr. Grant, to
whom we are indebted for our knowledge of the true nature of these
currents, as well as of the instruments by which they are effected,
gives the following account of the observation which led to the
discovery:—“I put,” says he, “a small branch of the spongia coalita,
with some sea water into a watch-glass, under the microscope, and on
reflecting the light of a candle through the fluid, I soon perceived
that there was some intestine motion in the opaque particles floating
through the water. On moving the watch-glass, so as to bring one of the
apertures on the side of the sponge fully into view, I beheld, for the
first time, the splendid spectacle of this living fountain, vomiting
forth from a circular cavity an impetuous torrent of liquid matter,
and hurling along in rapid succession opaque masses which it strewed
everywhere around. The beauty and novelty of such a scene in the animal
kingdom long arrested my attention, but after twenty-five minutes of
constant observation, I was obliged to withdraw my eye from fatigue,
without having seen the torrent for one instant change its direction,
or diminish in the slightest degree the rapidity of its course. I
continued to watch the same orifice, at short intervals, for five
hours, sometimes observing it for a quarter of an hour at a time, but
still the stream rolled on with a constant and equal velocity.”
[Illustration: Fig. CXXIX.—_Diagram of the Apparatus of the Circulation
and Respiration in the Fish._
1. Auricle (Single) of the heart. 2. Ventricle (single) of the heart.
3. Trunk of the branchial artery. 4. Division of the branchial
artery going to the branchiæ or gills. 5. Leaves of the branchiæ. 6.
Branchial veins, which return the blood from the branchiæ, and unite
to form. 7. the aorta, by the division of which the aërated blood is
carried out to the system.]
345. The simple expedients which have been described suffice for
carrying on the function of respiration in the water-breathing
invertebrata; but in creatures that possess a vertebral column, and the
more perfect skeleton of which it forms a part, there is a prodigious
advancement in the organization of the whole body, of the nervous and
muscular systems especially, the organs of the animal, as well as in
all the organs of the organic life. A corresponding development of
the function of respiration is indispensable. Accordingly, a sudden
and great development in the apparatus of this function is strikingly
apparent in fishes, the lowest order of the vertebrata, in which the
branchiæ, though still preserving the same form as in the animals
below them, are large and complex organs. The branchiæ of fishes
still consist of fringed folds of membrane disposed, as in the
preceding classes, in laminæ or leaves (fig. CXXIX. 5); but there are
now commonly four series of these leaves, on each side of the body,
placed in close approximation to each other, the several leaves being
divided into minute fibres, which are set close like the barbs of a
feather, or the teeth of a fine comb (fig. CXXIX. 5). Each leaf rests
either on a cartilaginous or a bony arch, which exactly resembles the
rib of the more perfect skeleton, and performs a strictly analogous
function; for these arches are capable of alternately separating from,
and of approximating to, each other, and these alternate motions are
effected by appropriate muscles. As these movements of separation or
approximation take place, the branchiæ are either opened or closed,
and their surface proportionally expanded or contracted. Upon these
leaves (fig. CXXIX. 5) the veins (347) of the system (fig. CXXIX. 4)
are spread out in a state of capillary division of extreme minuteness,
forming a net-work of vessels of extreme tenuity and delicacy. So
prodigiously is the surface increased for the expansion of these
vessels by the leaf-like disposition of the branchiæ, that it is
computed that the branchial surface of the skate is at least equal to
the surface of the whole human body.
346. Through this extended surface the whole blood of the system
must circulate, and every point of it must be unceasingly bathed
with fresh streams of water. To generate the force necessary for
the accomplishment of these objects, an increase of power must be
communicated both to the circulating and to the respiratory apparatus.
Neither the contractile power of the vessels by which in some of
the simpler animals the nutritive fluid is put in motion, nor the
contraction of the rudimentary heart by which in creatures somewhat
higher in the scale a more decided impulse is given to the blood, are
sufficient. A muscular heart, capable of acting with great power, is
now constructed, which is placed in such a position as to enable it to
propel with velocity the whole blood of the body through the myriads
of capillary vessels that crowd every point of the surface of the
branchial leaflets. To bring the water with the requisite degree of
force into contact with this flowing stream, the apparatus of cilia
is wholly inadequate. The water entering by the mouth, is driven with
force, by the powerful muscles of the thorax, through apertures that
lead to the branchial cavities. At the instant that the branchial
leaves receive the currents of water through the appropriate apertures,
the cartilaginous or bony arches which sustain the leaves, separate to
some distance from each other, and to that extent expand the leaves and
proportionally increase the surface exposed to the water: at the same
time, the rush of water through the leaves unfolds and separates each
of the thousand minute filaments of which they are composed, so that
they all receive the full action of the fluid as it flows over them.
347. After the venous blood of the system has been thus exposed to the
action of the respiratory medium, it is taken up by the vessels called
the branchial veins (fig. CXXIX. 6), which for the reason assigned
(372) are functionally arteries, as the branchial artery (fig. CXXIX.
4) is functionally a vein. The branchial veins uniting together form
the great arterial trunk of the system, (fig. CXXIX. 7) by which the
aërated blood is carried out to every part of the body.
348. But as if even this extent of apparatus were insufficient to
afford the amount of respiration required by the system of the fish,
the entire surface of its body, which in general is naked and highly
vascular, respires like the branchiæ. Moreover, many fishes swallow
large draughts of air, by which they aërate the mucous surface of
their alimentary canal, which also is highly vascular; and still
further, numerous tribes of these animals are provided with a distinct
additional organ, a bag placed along the middle of the back filled
with air. Commonly this air bag communicates with some part of the
alimentary canal near the stomach, by means of a short wide canal
termed the ductus pneumaticus, but sometimes it forms a simple shut
sac without any manifest opening; at other times it is divided and
subdivided in a perfectly regular manner, forming extended ramified
tubes; while at other times its ramifications present the appearance of
so many pulmonary cells. It is the rudiment of the complex lung of the
higher vertebrata, and it assists respiration; although since in some
tribes it contains not atmospheric air but azote, it is without doubt
subservient to other uses in the economy of the animal.
349. In water-breathing animals, from the lowest to the highest, it
is then manifest that a special apparatus is provided for, constantly
renewing the streams of water that are brought into contact with their
respiratory surface.
[Illustration: Fig. CXXX.—_Tracheæ._
1. Integument or skin of the body. 2. Spiracula opening on the
external surface of the skin. 3. Tracheæ, or air tubes, proceeding in
form of radii from the spiracles to 4. the alimentary canal.]
350. It is the same in aërial respiration. In the simplest form of
aërial respiration the apparatus consists of minute bags or sacs,
placed commonly in pairs along the back, which open for the admission
of the air on the external surface, by small orifices called spiracula
or spiracles (fig. CXXX. 2), at the sides of the body. In the common
earth-worm there are no less than one hundred and twenty of these
minute air vesicles, each of which is provided with an external opening
placed between the segments of the body. In the leech, the number is
reduced to sixteen on each side, which open externally by the same
number of minute orifices. Over the internal surface of these air
vesicles the blood of the system is distributed in minute and delicate
capillaries; and is capable of being aërated by whichever medium may
pass through the external orifices, whether water or air.
351. In this simple apparatus is apparent the rudiment of the more
perfect aërial respiration by the organs termed tracheæ, minute air
tubes which ramify like blood-vessels through the body (fig. CXXX. 3).
These air tubes open on the external surface by distinct apertures
termed _spiracula_ or _spiracles_ (fig. CXXX. 2), which are commonly
placed in rows on each side of the body (fig. CXXX. 2), with distinct
prominent edges (fig. CXXX. 2), often surrounded with hairs; sometimes
guarded by valves to prevent the entrance of extraneous bodies, and
capable of being opened and closed by muscles specially provided for
that purpose. These tubes, as they proceed from the spiracles to
be distributed to the different organs of the body, often present
the appearance of radii (fig. CXXX. 3), and when traced to their
terminations are found to end in vesicles of various sizes and
figures, but commonly of an elongated and oblong form. These minute
vesicles, when examined by the microscope, are seen to afford still
minuter ramifications, which are ultimately lost in the tissues of the
body.
352. The tracheæ are composed of three tunics, the external dense,
white and shining; the internal soft and mucous, between which is
placed a middle tunic, dense, firm, elastic, and coiled into a
spiral. By this arrangement the tube is constantly kept in a state of
expansion, and is therefore always open to the access of air. A great
part of the blood of the body, in the extensive class of creatures
provided with this form of respiratory apparatus, including the almost
countless tribes of insects, is not contained in distinct vessels, but
is diffused by transudation through the several organs and tissues of
the body. All the creatures of this class live in air, and possess
great activity; they therefore require a high degree of respiration;
yet they are commonly small in size, and often some portions of
their body consist of exceedingly dense and firm textures; hence to
have localized the function of respiration, by placing the seat of
it in a single organ, would have been impossible, on account of the
disproportionate magnitude which such an organ must have possessed; in
this case it was easier to carry the air to the blood, than the blood
to the air, and accordingly the air is carried to the blood, and, like
the blood in creatures of higher organization, is diffused through
every part of the system.
[Illustration: Fig. CXXXI.—_Respiratory Organs of the Scorpion._
1. Spiracles. 2. Integument of one half of the body turned back.
3. Branchial organs. 4. Cells or pouches in which they are lodged.
_a._ One of the respiratory organs removed and magnified, showing
its resemblance to the branchial leaflets, and presenting the
pectinated appearance described in the text.]
[Illustration: Fig. CXXXII.—_Apparatus of Respiration in the Frog._
1. Trachea. 2. Vesicular lungs. 3. Stomach.]
353. The next advancement in the ascending scale is, by a step which
obviously connects this higher class with the classes below and above
it. It consists of distinct cells, termed pulmonic cavities (fig.
CXXXI. 4), which communicate externally by spiracula (fig. CXXXI.
1), like tracheæ (351), but which are lined internally by a soft and
delicate membrane plaited into folds, disposed like the teeth of a
comb (pectinated) (fig. CXXXI. _a_), presenting a striking analogy
to the structure of gills (345), and therefore called by the French
writers pneumo-branchiæ. These cavities have the internal form of an
aquatic organ, but they perform the function of air-breathing sacs. In
scorpions (fig. CXXXI. 1) and spiders, this form of the apparatus is
seen in its simplest condition; in the slug and snail it is more highly
developed: for in these latter animals a rounded aperture, placed
near the head, and guarded by a sphincter muscle, that alternately
dilates and contracts, leads to a single cavity, which is lined with a
membrane delicately folded, and overspread with a beautiful net-work of
pulmonary blood-vessels.
354. Passing from this to the lowest order of the air-breathing
vertebrata (fig. CXXXII.), the apparatus is perfectly analogous,
but more developed. In the reptile, this air-breathing sac, which
now constitutes a true and proper lung, instead of being simple
and undivided, is formed by numerous septa, which traverse each
other in all directions, into vesicles or cells (fig. CXXXII. 2),
which proportionally enlarge the surface for the distribution of
blood-vessels. In the Batrachian reptile, as the frog, salamander,
newt, &c. (fig. CXXXII.), the vesicles, comparatively few in number,
are of large size, and as thin and delicate as soap-bubbles. In the
ophidian reptile, as the serpent, the sac is large and elongated, but
divided only in the upper and back part into vesicles; while in the
Saurian reptiles, as the crocodile, lizard, chamelion, &c., the sac is
comparatively small, but subdivided into very minute vesicles, bearing
a close analogy to the more perfectly organized lung of the higher
animals.
[Illustration: Fig. CXXXIII.—_Respiratory Apparatus of the Bird, as
seen in the Swan._
1. The Trachea. 2. The lungs. 3. Apertures through which air passes
into, 4. Air cells of the body. 5. A bristle passed from one of the
air cells of the body, to the cavity containing the lungs. 6. A
bristle passed from the cavity of the thigh-bone into another air cell
of the body.]
355. In birds, the next order of vertebrata (fig. CXXXIII.), as in
insects, the class of invertebrated animals which are formed for flight
(352), the respiratory organs extend through the greater part of
the body (fig. CXXXIII. 4). The lungs (fig. CXXXIII. 2), which still
consist of a single pulmonic sac on each side (fig. CXXXIII. 2), are
divided into cells, minute compared with those of the reptile, yet
large compared with those of the quadruped; at the same time numerous
air sacs, similar in structure to those of the lungs, but of larger
size, are distributed over different parts of the body (fig. CXXXIII.
4), which communicate with the air cells of the lungs (fig. CXXXIII.
3); while of these larger sacs, several communicate also with the bones
(fig. CXXXIII. 6), so as to fill with air those cavities which in other
animals are occupied with marrow.
356. In the mammalia, the highest order of the vertebrata, respiration
is less extended through the system, and is concentrated in a single
organ, the lung, which, though comparatively smaller in bulk than in
some of the lower classes, is far more developed in structure. The lung
in this class consists of a membranous bag, divided into an immense
number of distinct vesicles or cells, in the closest possible proximity
with each other, yet not communicating, and presenting, from their
minuteness, a vast extent of internal surface. This bag is confined to
a distinct cavity of the trunk, the thorax (fig. CXXXIV.), completely
separated from the abdomen by the muscular partition, the diaphragm
(fig. CXXXIV. 10). This organ no longer sends down cells into the
abdomen, nor membranous tubes into the bones; but is concentrated
within the thorax along with the heart (fig. CXXXIV. 2, 3, 8). In
all the orders of this class, the development and concentration of
the organ are in strict proportion to the perfection of the general
organization.
[Illustration: Fig. CXXXIV.—_View of the Respiratory Apparatus in Man._
1. The Trachea. 2. The right lung. 3. The left lung. 4. Fissures,
dividing each lung into, 5. Large portions termed lobes. 6. Smaller
divisions termed lobules. 7. Pericardium. 8. Heart. 9. Aorta. 10.
Diaphragm separating the cavity of the thorax from that of the
abdomen.]
357. In man there are two pulmonary bags (fig. CXXXIV. 2, 3), of nearly
equal size, which, together with the heart, completely fill the large
cavity of the thorax (fig. CXXXIV.), their external surface being
everywhere in immediate contact with the thoracic walls. One of these
bags is placed on the right side of the body, constituting the right
lung (fig. CXXXIV. 2), and the other on the left, constituting the left
lung (fig. CXXXIV. 3). Each lung is divided by deep fissures, into
large portions called lobes (figs. CXXXIV. 4, and CXXXV. 6), of which
there are three belonging to the right, and two to the left lung. Each
lobe is subdivided into innumerable smaller parts termed lobules (figs.
CXXXIV. 6, and CXXXV. 6), while the lobules successively diminish in
size until they terminate in minute vesicles that constitute the great
bulk of the organ (fig. CXXXV. 8).
358. The complete centralization of the respiratory function which thus
takes place in man, renders the apparatus exceedingly complex both on
account of the expedients which are necessary to obtain the requisite
extent of surface, in the small allotted space, and to bring into
contact within that space the fluids that are to act on each other.
[Illustration: Fig. CXXXV.—_View of the Air Tubes and Lung._
1. The larynx. 2. Trachea. 3. Right bronchus. 4. Left bronchus. 5.
Left lung; the fissures denoted by the two lines which meet at 6,
dividing it into three lobes, and the smaller lines on its surface
marking the division of the lobes into lobules. 7. Large bronchial
tubes. 8. Minute bronchial tubes terminating in the air cells or
vesicles.]
359. The apparatus consists of a vessel to carry the air to the blood;
a vessel to carry the blood to the air; an organ in which the air and
the blood meet; and an organization by which both fluids are put in
motion. The vessel that carries the air to the blood is the windpipe
(fig. CXXXV. 1, 2); the vessel that carries the blood to the air is
the pulmonary artery (fig. CXL. 7); the organ in which the blood and
the air meet is the lung (fig. CXXXV. 5); the organization which puts
the air in motion, is the structure of bones, cartilage and muscles,
called the thorax (figs. CXLI. and CXLVI.), and the engine that
communicates motion to the blood is the right ventricle of the heart
(fig. CXL. 5).
360. The windpipe is a tube which extends from the mouth and nostrils
to the lung (figs. CLIII. 1, 9, and CXXXV. 2, 5). It is attached to
the back part of the tongue (fig. CLII. 2, 9), and passes down the
neck immediately before the esophagus, or the tube which leads to the
stomach (fig. CLIII. 9, 12).
361. In the different parts of its course the windpipe is differently
constructed, performs different offices, and receives different names
according to the diversity of its structure and function. The first
division of it is called the larynx (fig. CXXXV. 1.), the second the
trachea (fig. CXXXV. 2), the third the bronchi (figs. CXXXV. 3, 4, 7,
and CXXXVII.), and the fourth the air vesicles or cells (figs. CXXXV.
8, and CXXXVIII. 2).
[Illustration: Fig. CXXXVI.—_Posterior View of the Larynx and Trachea._
1. The os hyoides. 2. Thyroid cartilage. 3. Cricoid cartilage. 4.
Arytenoid cartilages, separated from each other. 5. Epiglottis. 6.
Opening of the glottis. 7. Termination of the cartilaginous rings of
the trachea. 8. The ligamentous portion of the trachea. 9. Trachea
laid open, showing its internal mucous surface and follicles, with the
anterior portion of the cartilaginous rings appearing through it.]
362. The first portion of the windpipe called the larynx (figs. CXXXV.
and CXXXVI), constitutes the organ of the voice. It is situated at
the upper and fore part of the neck (fig. CLIII. 7, 9), immediately
under the bone to which the root of the tongue, called the os hyoides
(figs. CLIII. 6, and CXXXVI. 1), is attached. The larynx forms a
very complex structure, and is composed of a variety of cartilages,
muscles, ligaments, membranes, and mucous glands (fig. CXXXVI. 2, 3,
4, 5). At its upper part is a narrow opening of a triangular figure
called the glottis (fig. CXXXVI. 6), by which air is admitted to and
from the lung. Immediately above this opening is placed the cartilage,
which obtains its name from its situation, _epiglottis_ (fig. CXXXVI.
5), which is attached to the root of the tongue (fig. CLIII. 6, 7), and
which may be distinctly seen in the living body by pressing down the
tongue.
363. The Epiglottis is highly elastic, and is an agent of no
inconsiderable importance in respiration, deglutition, and speaking.
In respiration it breaks the current of air which rushes to the lungs
through the mouth and nostrils, and prevents it from flowing to the
delicate air cells with too great a degree of force. During the
action of deglutition the epiglottis is carried completely over the
glottis (fig. CLIII. 6, 7, 8), partly because it is necessarily forced
backwards, when the tongue passes backwards in delivering the food to
the pharynx (fig. CLIII. 6, 7, 8, 10), partly because it is carried
backwards by certain minute muscles which act directly upon it, and
perhaps also partly in consequence of its own peculiar irritability.
The moment the action of deglutition has been performed the epiglottis
springs from the aperture of the glottis, partly by its own elasticity,
and partly by the return of the tongue to its former position. During
the act of speaking the column of air which is expelled from the lung,
which rushes through the glottis, and which thus forms the voice,
strikes against the epiglottis, and the voice becomes thereby in some
degree modified.
[Illustration: Fig. CXXXVII.
View of the trachea, showing, first, the division of the tube into
the right and left bronchus, and the subdivision of the bronchi into
the bronchial tubes; and secondly, the membranous and cartilaginous
tissues of which the organ is composed.]
364. The second portion of the windpipe termed the trachea (fig. CXXXV.
2), commences at the under part of the larynx (fig. CXXXV. 1), and
extends as far as the third dorsal vertebra, opposite to which it
divides into two branches which are termed the bronchi (fig. CXXXV. 3,
4, and CXXXVII.). One of these branches, called the right bronchus,
goes to the right lung; the other branch, called the left bronchus,
goes to the left lung (fig. CXXXV. 3, 4).
365. The trachea of man, like the tracheæ of the air-breathing insect
(351), is composed of three tissues. These tissues differ essentially
from each other in nature, and are widely different in form and
arrangement. They consist of membrane, muscle, and cartilage.
366. The membranous portion of the human trachea consists of
three coats, the cellular (fig. CXXXVII.), the ligamentous (fig.
CXXXVI. 8), and the mucous (fig. CXXXVI. 9). From the cellular and
ligamentous coats the tube receives its strength, and in some degree
its elasticity; and the mucous coat constitutes the chief seat of
the respiratory function. Between the ligamentous and mucous coats
are placed two sets of muscular fibres; the first, the external set,
passes in a circular direction around the tube; the second set,
placed immediately beneath the circular, is disposed longitudinally,
and collected into bundles. The office of the circular fibres is to
diminish the calibre of the tube, and that of the longitudinal is to
diminish its length.
367. As the tracheæ of the insect are kept constantly open for the
free admission of air by their middle membranous tunic, dense, firm,
elastic, and coiled into a spiral (351), so, for the accomplishment
of the same purpose, there are placed between the membranous coats
of the human trachea delicate rings of the more highly organized
substance, cartilage (35). These cartilaginous rings amount in the
entire course of the tube to sixteen or eighteen in number (fig. CXXXV.
2); each cartilage being about a line in breadth, and the fourth of a
line in thickness. They never form complete circles, but only a large
segment of a circle (fig. CXXXVI. 7); the circle is incomplete behind
(fig. CXXXVI. 7, 9), because there the esophagus is in direct contact
with the trachea (fig. CLIII. 9, 12), and instead of dense and firm
cartilage, a soft and yielding substance is placed in this situation,
in order that there may be no impediment to the free dilatation of the
esophagus during the passage of the food.
368. The point at which the bronchi enter the substance of the lung is
called the root of the lung (fig. CXXXV. 3, 4). As soon as the bronchi
begin to divide and ramify within the lung each cartilage, instead of
preserving its crescent shape, is divided into two or three separate
pieces, which nevertheless are still so disposed as to keep the tube
open. With the progressive diminution in the size of the bronchial
branches, their cartilages become less numerous, and are placed at
greater distances from each other, until at length as the bronchi
terminate in the vesicles, the cartilages wholly disappear; and with
the decreasing number and size of the cartilages, the thickness of the
cellular, ligamentous, and muscular coats of the bronchi also lessens,
until at the points where the cartilages disappear, the muscular and
mucous tunics, now reduced to a state of extreme tenuity, alone remain.
The essential constituent of the air vesicles, then, is the mucous
membrane; but there is reason to suppose that the muscular tunic is
likewise continued over these vesicles.
369. It has been stated that the tracheæ of the insect terminate in the
different tissues of its body by minute vesicles of an oblong form.
The termination of the bronchi in the human lung presents a strikingly
analogous appearance. Malpighi, who with extraordinary talent and
success devoted his life to the investigation of the minute structures
of the various organs of the human body, represents the mucous membrane
of the bronchial tubes as terminating in minute vesicles of unequal
size: and Reisseissen, who has more recently resumed the inquiry and
examined this structure with extreme care, agrees with Malpighi in
stating that the bronchial tubes at their terminal points expand into
minute, delicate, membranous vesicles of a cylindrical and somewhat
rounded figure (fig. CXXXVIII. 2). The bronchial tubes do not divide
to any great degree of minuteness (fig. CXXXVIII. 1), but terminate
somewhat abruptly in the vesicles (fig. CXXXVIII. 2), which though
minute are large enough to be visible to the naked eye (fig. CXXXVIII.
2). Viewed in connexion with the bronchial tubes at their terminal
points, the vesicles present a clustered appearance, not unlike
clusters of currants attached to their stem (fig. CXXXVIII. 2).
[Illustration: Fig. CXXXVIII.—_View of the Bronchial Tubes terminating
in Air vesicles._
Fig. 138.
Fig. 139.
External view.—1. Bronchial tube. 2. Air vesicles. Fig. 139. The same
laid open.]
370. In the insect, for the reason assigned (351), these vesicles are
diffused over the system, aërating every point of the body; in man
they are concentrated in the lung; yet by their minuteness, and by
the mode in which they are arranged, they present in the small space
occupied by this organ, so extended a surface that Hales, representing
the size of each vesicle at the 100dth part of an inch in diameter,
estimates the amount of surface furnished by them collectively at
20,000 square inches. Keil estimating the number of the vesicles at
174,000,000, calculates the surface they present, at 21,906 square
inches. Leiberkuhn at 150 cubic feet; and, according to Monro, it is
thirty times the surface of the human body.
[Illustration: Fig. CXL.
1. The trachea. 2. The right and left bronchus; the left bronchus
showing its division into smaller and smaller branches in the lung,
and the ultimate termination of the branches in the air vesicles. 3.
Right auricle of the heart. 4. Left auricle. 5. Right ventricle. 6.
The aorta arising from the left ventricle, the left ventricle being in
this diagram concealed by the right. 7. Pulmonary artery arising from
the right ventricle and dividing into, 8. The right, and 9. The left
branch. The latter is seen dividing into smaller and smaller branches,
and ultimately terminating on the air vesicles. 10. Branches of one of
the pulmonary veins proceeding from the terminations of the pulmonary
artery on the air vesicles, where together they form the net-work of
vessels termed the Rete Mirabile. 11. Trunk of the vein on its way to
the left auricle of the heart. 12. Superior vena cava. 13. Inferior
vena cava. 14. Air vesicles magnified. 15. Blood-vessels distributed
upon them.]
371. Such is the structure of the vessel that carries the air to the
blood, and such is the mode of its distribution.
The vessel that conveys the blood to the air is the pulmonary artery,
the great vessel which springs from the right ventricle of the heart
(fig. CXL. 5).
The pulmonary artery soon after it issues from the right ventricle of
the heart divides into two branches (fig. CXL. 7, 8, 9), one for each
lung (fig. CXL. 8, 9). Each branch of the pulmonary artery as soon as
it enters its corresponding lung (fig. CXL. 9) divides and ramifies
through the organ in a manner precisely similar to the bronchial
tubes. Every branch of the artery is in contact with a corresponding
branch of the bronchus (fig. CXL. 2), divides as it divides, and
accurately tracks its course throughout (fig. CXL. 2), until the
ultimate divisions of the artery at length reach the ultimate vesicles
of the bronchus (fig. CXL. 2, 10), upon the delicate walls of which
the capillary arteries rest, expand, and ramify, forming a net-work of
vessels, so complex that the anatomist who first observed it, named it
the _Rete Mirabile_, the wonderful net-work, and it is still called
the _Rete Mirabile Malpighi_, or the _Rete Vasculosum Malpighi_ (fig.
CXL. 2, 9, 10).
372. The blood which has finished its circulation through the system,
returned by the great systemic veins (fig. CXL. 12, 13), to the right
side of the heart (fig. CXL. 3), is driven by the right ventricle (fig.
CXL. 5), into the pulmonary artery (fig. CXL. 7); by the branches of
which (fig. CXL. 8, 9) it is distributed to the air vesicles of the
lungs: consequently the right heart of man bears precisely the same
relation to the lungs, that the single heart of the fish bears to the
branchiæ; the former is a pulmonic, as the latter is a branchial heart;
one half of the double heart of the more highly organized creature is
employed to circulate the venous blood of the system through the lungs,
as the whole of the single heart of the less highly organized animal,
is employed to propel the blood through the branchiæ (368). From the
capillary branches of the pulmonary artery in the Rete Mirabile (fig.
CXL. 9), arises another set of vessels termed the pulmonary veins
(fig. CXL. 10), which receive the blood from the venous vessels spread
out on the air vesicles: for the pulmonary artery is functionally a
vein, since it contains venous blood, though it is nominally an artery
because it carries blood from the heart (269); and in like manner the
pulmonary veins are functionally arteries since they contain arterial
blood, though they are nominally veins because they carry blood to the
heart (272). The branches of the pulmonary arteries are larger in size
and greater in number than those of the pulmonary veins, the reverse of
what is observed in any other part of the body; because the pulmonary
artery contains the blood which is to be acted upon by the air, while
the pulmonary veins merely receive the blood which has been acted upon
by the air, and the former ramifies more minutely than the latter, in
order that the air may act on a larger surface of blood.
373. In the Rete Mirabile the junction of the air-vessel with the
blood-vessel is accomplished. The combination of these two sets of
vessels constitutes the lung; for the lung is composed of air-vessels
and blood-vessels united, and sustained by cellular tissue, and
inclosed in the thin but firm membrane called the pleura (104 and 105).
374. Such is the arrangement of that part of the respiratory apparatus
which contains the fluids that are to act on each other. The object
of the remaining portion of it is to produce the movements which are
necessary to bring the fluids into contact. This is accomplished by
the mechanism and action of the thorax and diaphragm (figs. CXLI. and
CXXXIV. 10).
375. These organs, which invariably act in concert, are so constructed
and disposed, that when in action they give to the chest two alternate
motions, one that by which its capacity is enlarged; and the other that
by which it is diminished. These alternate movements are called the
motions of respiration. The motion by which the capacity of the chest
is enlarged is termed the action of inspiration, and that by which it
is diminished the action of expiration.
376. The action of inspiration, or that by which the capacity of the
chest is enlarged, is effected by the combined movements of the thorax
and diaphragm; by the ascent of the thorax and by the descent of the
diaphragm.
377. The osseous portion of the thorax, which has been fully described
(69 _et seq._), consists of the spinal column (fig. CXLI. 1), the
ribs with their cartilages (fig. CXLI. 2, 3), and the sternum (fig.
CXLI. 4). The soft portion of the thorax consists of muscles and
membrane (figs. CXLII., CXLVI., and CXLVII.), together with the common
integuments of the body. The chief boundaries of the cavity of the
thorax before, behind, and at the sides, are osseous, being formed
before by the sternum and the cartilages of the ribs (fig. CXLI. 4,
3); behind by the spinal column and the necks of the ribs (fig. CXLI.
1,2); and at the sides by the bodies of the ribs. Below the boundary is
muscular, being formed by the diaphragm (fig. CXLIII. 3).
378. Externally the thorax is convex and enveloped by muscle and skin;
internally it is concave (fig. CXLIII. 1), and lined by a continuation
of the same membrane which envelops the lungs, the pleura (104). But
that portion of the pleura which lines the internal wall of the thorax
is called the costal pleura (pleura costalis), in contradistinction to
that which envelops the lungs, which is termed the pulmonary pleura,
or pleura pulmonalis (104). By the costal pleura, a thin but firm and
strong membrane, smooth, polished, and like all the membranes of its
class (serous membrane 30, _et seq._), kept in a state of perpetual
moisture and suppleness, by a fluid secreted at its surface, the
movements of the thorax are facilitated, at the same time that they are
prevented from injuring the delicate organs contained in it.
379. The moveable parts of the osseous portion of the thorax are the
ribs and sternum. The ribs, though by one extremity tied with exceeding
firmness to the spinal column by ligaments specially constructed, and
admirably adapted for that purpose (figs. LVI. 1, and LVII. 1), and
though attached at their other extremity by their cartilages to the
sternum (fig. LVIII.), are capable of three motions, an upward, an
outward, and a downward motion.
[Illustration: Fig. CXLI.—_View of the osseous portion of the Thorax._
1. Spinal column. 2. Ribs. 3. Cartilages of ribs. 4. Sternum.]
380. The ribs form a series of moveable arches, the convexity of the
arches being outwards, and the whole being disposed in an oblique
direction (fig. CXLI. 2). The first rib springs from the vertebral
column at nearly a right angle (fig. CXLI. 2); the acuteness of this
angle increases in succession as the ribs descend from the first to
the last (fig. CXLI. 2); in this manner each rib is inclined obliquely
outwards and downwards, and the obliquity thus given to the general
direction of the ribs augments progressively from above downwards (fig.
CXLI. 2).
381. In consequence of this conformation and arrangement of the ribs,
every degree of motion which is communicated to them, necessarily
influences the capacity of the space they enclose. If they are moved
upwards they must enlarge that space at the sides, because the
intervals between each other will be increased (fig. CXLI. 2); and from
behind forwards, because the distance between the spinal column and the
sternum (the sternum being protruded forwards with their cartilaginous
extremities) (fig. CXLI. 3, 4), will be increased. If, on the other
hand, they are moved downwards, the capacity of the thorax will be
proportionally diminished in every direction (fig. CXLI.).
[Illustration: Fig. CXLII.
View of the intercostal muscles which fill up the interspaces between
the ribs. These muscles consist of a double layer of fibres, the
external and the internal, which cross or intersect each other.]
382. One part of the action of inspiration consists, then, of this
ascent of the ribs. The ascent of the ribs is effected by the
contraction of a double layer of muscles called the intercostal (fig.
CXLII.), placed in succession between each rib; and which communicate
this motion in the following mode. The first rib is fixed; the second
rib is moveable, but less moveable than the third, the third than the
fourth, and so on through the series: consequently the contraction of
the intercostal muscles (figs. CXLII. and CXLVI. 2) must elevate the
whole series, because the upper ribs afford fixed points for the action
of the muscles; and so, when all these muscles contract together, they
necessarily pull the more moveable arches upwards towards the more
fixed (figs. CXLI. and CXLVI. 2).
383. But from the oblique direction of the ribs, they cannot ascend
without at the same time protruding forwards their anterior extremities
(fig. CXLI.). Those extremities being attached to the sternum, which
forms the anterior wall of the thorax, they cannot be protruded
forwards without at the same time carrying the sternum forwards with
them (fig. CXLI.). Thus, by this two-fold motion of the ribs, an upward
and consequently an outward motion, the capacity of the thorax is
increased from behind forwards, that is, in its small diameter.
384. Such is the part of the action, in inspiration, performed by the
motion of the ribs. The remaining part of that action, by far the most
important, consists of the enlargement of the capacity of the thorax
from above downwards, or in its long diameter. This is effected by the
descent of the diaphragm (fig. CXLIII.).
385. The diaphragm is a circular muscle, forming a complete but
moveable partition between the thorax and the abdomen (figs. CXXXIV.
10, and CXLIII. 3). When not in action its upper surface forms an
arch (figs. CXLIII. 4, and CXLV. 1), the convexity of which is towards
the thorax (figs. CXLIII. 4, and CXLV. 1), and reaches as high as
the fourth rib (fig. CXLV. 1); its under surface, or that towards
the abdomen, is concave (figs. CXXXIV. 10, and CXLV. 1). Its central
portion is tendinous (fig. CXLIII. 4). This central tendinous portion
of the diaphragm, which is in apposition with the heart (fig. CXXXIV.
8), and firmly attached to the pericardium (fig. CXXXIV. 7), is nearly
if not quite immoveable: it is only the lateral or muscular portions
(fig. CXLIII. 4) that are capable of motion. Its central portion is
constructed of dense and firm tendon, and is immoveable, primarily, in
order to afford one of the two fixed points (the ribs affording the
other fixed point), essential to the action of the muscular fibres that
constitute its lateral or moveable portions; and secondarily, in order
to afford a support to the heart, which rests upon this central tendon.
Thus, in consequence of this tendon being rendered absolutely fixed,
the motions of the diaphragm are completely prevented from incommoding
the motions of the heart; the function of respiration from interfering
with the function of the circulation.
[Illustration: Fig. CXLIII.—_View of the Diaphragm._
1. Cavities of the thorax. 2. Portion of cavity of the abdomen. 3.
Lateral or muscular and moveable portions of the diaphragm. 4. Central
or tendinous and fixed portion of the diaphragm.]
386. During the action of inspiration the muscular or lateral portions
of the diaphragm contract (fig. CXLIII. 3); its muscular fibres
shorten themselves, and are approximated towards the central tendon
(fig. CXLIII. 2); the consequence is that the whole muscle descends
(fig. CXLIV. 1); passes from the fourth to below the seventh rib
(fig. CXLIV.), loses its arched form and presents the appearance of
an oblique plane (fig. CXLIV.). At the same time the muscles of the
abdomen are protruded forwards (fig. CXLIV. 2), and the viscera
contained in its cavity are pushed downwards. The result of these
movements is, that the capacity of the thorax is enlarged by all the
space that intervenes between the fourth rib (fig. CXLV. 1), and the
lowest point of the oblique plane formed by the diaphragm (fig. CXLIV.
1), together with all that gained by the protrusion of the walls of the
abdomen and the descent of its viscera (fig. CXLIV. 2).
[Illustration: _Views of the Diaphragm in the different states of
Respiration._
Fig. CXLIV.
Fig. CXLV.
Fig. 144.—1. Diaphragm in its state of greatest descent in
inspiration. 2. Muscles of the abdomen, showing the extent of their
protrusion in the action of inspiration. Fig. 145.—1. Diaphragm in the
state of its greatest ascent in expiration. 2. Muscles of the abdomen
in action forcing the viscera and diaphragm upwards.]
387. By the action of the intercostal muscles, then, the capacity of
the thorax is enlarged at the sides and from behind forward, or in its
short diameter; by the action of the diaphragm, the capacity of the
thorax is enlarged from above downwards, or in its long diameter; by
the combined action of both, the capacity of the thorax is enlarged in
every direction, and thus the motion of inspiration is completed.
388. Expiration, the respiratory motion which alternates with that of
inspiration, consists of the diminution of the capacity of the thorax,
which is effected by the converse motions of the same organs; that is,
by the descent of the ribs and the ascent of the diaphragm.
389. By the descent of the ribs, the capacity of the thorax is
diminished in its short diameter, because by this motion, the oblique
arches of the ribs are approximated to each other and to the spinal
column, and the sternum is also approximated to the spinal column.
The descent of the ribs is effected first by the elasticity of their
cartilages (fig. CXLI. 2). When the intercostal muscles relax, the
force which raised the ribs ceases to be applied, and that moment the
elasticity of the cartilages comes into play, and carries the ribs down
wards. Secondly, by the contraction of the abdominal muscles (figs.
CXLV. 2, and CXLVI. 6, 7, 8), the direct effect of which is to pull the
ribs downwards (fig. CXLVI. 6, 7, 8).
390. By the ascent of the diaphragm the capacity of the thorax is
diminished in its long diameter (fig. CXLV. 1). When the diaphragm
ascends, it changes from the figure of an oblique plane (fig. CXLIV.
1), re-assumes its arched form (fig. CXLV. 1), and reaches as high as
the fourth rib (fig. CXLV. 1). At the same time the abdominal muscles
contract (fig. CXLV. 2), and are carried inwards towards the spinal
column (fig. CXLV. 2). The result of these movements is, that the
capacity of the thorax is diminished by all the space that intervenes
between the lowest point of the oblique plane formed by the diaphragm
and the fourth rib (fig. CXLV. 1), and by all the abdominal space lost
by the contraction of the muscles of the abdomen (fig. CXLV. 2).
[Illustration: Fig. CXLVI.—_View of the principal external Muscles of
Respiration._
1. The muscle called the Scalenus. 2. The muscles called the
Intercostals. 3. Subclavius. 4. The bone called the Clavicle. 5. The
muscle called the Serratus Magnus Anticus. 6. Obliquius Externus. 7.
Rectus. 8. Obliquius Internus.]
391. The first step necessary to the ascent of the diaphragm is the
relaxation of its muscular fibres. As soon as these fibres are in a
state of relaxation, that is, when the organ has changed from an active
to a completely passive state, the powerful muscles of the abdomen
(fig. CXLVI. 6, 7, 8) contract, and push the abdominal viscera and
the diaphragm with them upwards towards the cavity of the chest (fig.
CXLV. 2); and thus, by the descent of the ribs and the ascent of the
diaphragm, the capacity of the thorax is diminished in every direction,
and the motion of expiration is completed.
392. Such is the mechanism by which the capacity of the thorax is
alternately enlarged and diminished in the two alternate states of
inspiration and expiration, and the mechanism thus adjusted works in
the following mode.
393. Expiration succeeding to the state of inspiration, the ribs
descend, the diaphragm ascends, the capacity of the thorax lessens,
and the compressed lungs are forced within the smallest possible
space. Then, inspiration, succeeding to the state of expiration, the
ribs ascend and the diaphragm descends; the capacity of the thorax is
enlarged, and the lungs freed from their pressure expand and fill the
greater space obtained. In about a second and a half after the state
of inspiration has been induced, that of expiration recommences; the
motion of inspiration occupying about double the time of the motion
of expiration, and these alternate conditions succeed each other in a
regular and uniform course, day and night, during our sleeping and our
waking hours to the end of life.
394. As long as the function is performed in a perfectly natural
manner, a given number of these alternate movements takes place
in a certain time, constituting what is termed the rhythm of the
respiratory motions. These motions perfectly regular in number and
time, are likewise, in the natural state of the function, performed
only with a certain degree of energy; but they are variously modified
at the command of the will; in obedience to numerous sensations and
emotions; in the performance of a great variety of complex actions,
and in different states of disease. These modifying circumstances may
cause the action of inspiration to be more full and deep, and that of
expiration to be more forcible and complete than natural; or they may
cause both movements to be shorter and quicker than common: hence the
distinction of respiration into ordinary and extraordinary.
395. In ordinary respiration, that is, when the respiratory motions
are perfectly calm and easy, the ascent and descent of the ribs are
scarcely perceptible; the action is confined almost exclusively to the
ascent and descent of the diaphragm. In this condition the only action
of the intercostal muscles is to fix the ribs, and thus to afford one
of the two fixed points which have been shown (385) to be essential
to the action of the diaphragm. But in extraordinary respiration,
that is, when circumstances happen in the economy which require that
those motions should be extended, auxiliary sources can be put in
requisition. There are many powerful muscles situated about the breast,
shoulder and back (fig. CXLVI. and CXLVII.); which are capable of
elevating the ribs and protruding the sternum to a very considerable
extent (figs. CXLVI. 1, 2, 3, 5; and CXLVII. 1, 2, 3). Where, for
example, the fullest inspiration which it is possible to take is
required, the bones of the shoulder and shoulder-joint are firmly
fixed by resting the hands upon the knees, and then every muscle which
has the slightest connexion with the thorax, either before or behind,
capable of raising the ribs, is added to the inspiratory apparatus
(figs. CXLIV. and CXLVII.); at the same time that the abdominal muscles
are relaxed to the utmost degree, in order to facilitate the ascent of
the ribs and the descent of the diaphragm (figs. CXLIV. 2, and CXLVI.
6, 7, 8). If, on the contrary, the fullest possible expiration is
required, the abdominal muscles contract most forcibly (fig. CXLV. 2),
and every other muscle which is capable of still farther depressing the
ribs and of elevating the diaphragm (fig. CXLVI. 6, 7, 8) is called
into intense action. By these forcible and extraordinary efforts the
thorax may be enlarged or diminished double its ordinary capacity.
[Illustration: Fig. CXLVII.—_View of Muscles which are capable of
assisting in elevating the Ribs and protruding the Sternum, in states
of extraordinary respiration._
1. The muscle called the Great Pectoral. 2. The Small Pectoral. 3. The
Serratus Magnus.]
396. Such are the mechanism and action of the powers which communicate
to the thorax, the motions by which its capacity is alternately
enlarged and diminished, and by which the requisite impulse is
communicated to the fluids which flow to and from the lungs in the
different states of respiration; that is, by which air and blood flow
to the lungs in the action of inspiration, and from the lungs in the
action of expiration.