of ciliated epithelium.
the views of Mr. Rainey. This excellent
observer* has affirmed the principle that, on
the true breathing portions, or capillary seg-
ments of the lungs, there literally exists no
epithelial lining of any description whatever,
the vessels being as literally naked. To this
" principle," deliberately enunciated and sup-
ported by elaborate "proof" by an acute and
truthful observer, many anatomists have yielded
implicit assent. First, it is here objected that
such a "principle" violates directly all the
lessons of analogy. Analogy ! Is not demon-
stration better than analogy ? In the science
of organised beings, the connected reasoning
founded on analogy cannot be despised. The
closest scrutiny in individual instances may
miss the truth. The manifold illusions of the
microscope may readily mislead. Analogy
supposes a mass of cumulative evidence. The
general law neutralises particular errors. In
no instance whatever, either in the vertebrate
or invertebrate kingdom, has it been proved,
in the course of the present extended inquiry,
that the vessels of a real breathing organ can
exist under a perfectly " naked form." What
is true of invertebrate animals as an organic
law cannot be untrue of the vertebrate. The
* Med. Chir. Trans. 1848.
gills of fishes are furnished with a very
marked epithelial covering. The temporary
branchiae of the amphibia are clothed with
epidermis. The air-bladder is provided with
an epithelial lining, the cells of which admit
of ready and conclusive demonstration. Why
should it not exist in the case of the true
pulmonary structures ? No reason can be
imagined ; but the fact that it does not has
been affirmed by Mr. Rainey. Mr. Rainey's
observations were instituted upon injected
preparations. This is the source of the error
into which he has fallen. If the lung of the
newt be carefully, but quickly, laid open, co-
vered, but not pressed, with a thin slip of glass,
and examined under the microscope, it will
be found that the vibratile cilia have a limited
distribution. Under the favourable oppor-
tunities afforded by such a preparation it is
perfectly easy to follow with the eye the
continuation of the epithelial cells (c, c, fig.
232.) beyond the limits of the ciliary areas
into the true capillary or active breathing
segments. The ciliated portions of the epi-
thelium (over the vessels b, b) exhibit a
flocculeni character, precisely as shown in the
preparations of Mr. Rainey ; while the areas
immediately adjacent appear smooth or naked.
But under the use of a higher power and a
steady gaze the polygonal outlines of the epi-
thelial scales can be distinctly discerned most
readily between the islets of parenchyma (W, d).
If this covering consist of "basement mem-
brane," then basement membrane is composed
of scaliform parts ; but it is not. It is a true
and real and unbroken continuation of the
tracheal and bronchial mucous membrane.
It is only the ciliary appendages to the cells
that cease at a certain limit ; the cells them-
selves continue to invest the whole super-
ficies of the lungs. It is full of interest also
to note that the epithelial scales which cover
the capillary areas of the lung of the newt
(parts which coincide with air-cells of the
mammalian lung) lose not only the external
appendages (cilia), but also their internal
parts (nucleus and granules). This succes-
sive reduction leaves nothing but a hyaline
involucrum enclosing a pellucid fluid. This
is the real structure, supported indeed by a
hypothetical basement membrane, by which
the capillary areas of the pulmonary organs
are invested. It finds a parallel in the trans-
parent scales which cover the cornea.
In these ribless amphibia the operation of
breathing resolves itself into an act of" swal-
lowing" air. The glottidean chink is em-
braced by two minute semilunar pieces of
cartilages and furnished with muscles for
opening and closing the orifice. In the pa-
rietes of the lungs no trace of muscular fibres
can be discovered ; but elastic fibres are
present everywhere among the vessels. It is
by the agency of this elastic tissue, aided by
the abdominal parietes, that the act of expira-
tion is performed. The exterior of the lung
is lined by peritoneum, the scales of which are
much attenuated compared with those of
other parts of the same membrane, as those
284-
RESPIRATION.
of the internal lining. It is a curious fact
that the exterior of the lung should be desti-
tute of cilia, while they should be present on
that of the liver in the newt. They are, how-
ever, on this last organ, limited to the margin 8
Nothing is more easy than to exhibit the living
circulation in the lung of the newt. The
Fig. 233.
A. small piece coinciding with the true capillary of
respiratory area from the lung of the Newt, viewed
by transmitted light, under a high power. The
blood- corpuscles in streams are seen in the spaces
between the islets a, a, a. Only these parts are
in focus. The hyaline epithelium covering the near
face of the picture is out of sight. The eye looks
beyond it. In this fresh uninjected state the blood-
channels' do not appear to be bounded by separate
and independent parietes. (Original.}
a, a, a, are parenchymatous islets occupying the
meshes of the capillary rete. They are composed
of cells carrying nuclei and granules.
internal bore of the vessel viewed by trans-
mitted light is much greater than the long
diameter of the red corpuscles. The meshes
(flr, a) are mere points. The scene is one
thick, rich, surpassingly beautiful network of
moving blood.
In the frogs and toads the lungs consist of
two large, short, and broad, slightly cancellated
shining bags. They are situated on either
side of the spine, at the roof of the abdominal
cavity. They are remarkably elastic, like
those of the newt. They are capable of
slowly expelling their contents even after the
removal of the abdominal walls, and of draw-
ing themselves up into little hard balls on
either side of the pharynx. They exhibit well
the living circulation. The glottidean aperture
communicates directly with the interior of
the organ. There is, therefore, no trachea.
The orifice of the glottis is surrounded by
rudimentary cartilages somewhat further de-
veloped. The mechanism of breathing is the
same in the frog and toad, in which, like the
newt, the thoracic ribs are wanting, as in the
salamanders. The steps of the process are,
however, better studied in the frog. The
outer surface of the lung in the frog is closely
invested with peritoneum, the epithelium of
which forms a thin pavement coating. This
surface is destitute of cilia. In the frog, as in
234.
Heart, vessels, and lungs of the Frog.
the newt, the edges of the liver are fringed with
motile cilia. The pulmonary artery (see art.
CIRCULATION), derived from the aorta, pro-
ceeds along the outer side of the lung. It lies
immediately underneath the peritoneal epi-
thelium. The very reverse course is taken
by the large venous trunk on the opposite
side. This lies in immediate contact with
the internal or mucous surface. By this ar-
rangement the contact of the blood with the
air is prolonged. The contributory branches
of the vein course along the free internal
edges of the septa bounding the cells. The
branches of the artery occupy the opposed
fixed borders of the same septa. The flat
surfaces, or sides of the cells, being the areas
dividing the arteries and veins, are the scenes
of the capillary segments. To this rule, of
course, the eye, by close scrutiny, may detect
many exceptions. By this distribution of
parts, every spot of the internal superficies
is functionally utilised. The ciliary epithelium
is limited, in its distribution, to the margins
of the cells and the lines of the larger vessels.
The true capillary areas whereon alone respi-
ration actively proceeds are covered only by a
hyaline epithelium, the cells of which can only
be distinguished by their outlines (fg. 232.
c, c). The ciliated tracts, according to the
manner already described in the lung of the
newt, terminate by abrupt borders. The epi-
thelial cell only is continued over the capillary
areas. There prevails an average uniformity in
the dimensions of these areas. Each particle of
blood, therefore, in its transit from the artery
to the vein across this area, is exposed, for
the same period of time, to the influence of
the air. In the lung of the frog and toad the
septa support two layers of reticulate vessels,
RESPIRATION.
285
one on either side of a fibrous partition. A
plane of vessels disposed in such a manner
can only receive the influence of the aerating
element on one side. This fact constitutes
a real anatomical distinction between the
lung of a reptile and that of a mammal. In
this latter case the partitions of the cells are
composed only of a, single stratum, both sides
of which are "exposed to the air. By this
simple mechanical provision the amount of
the respiratory agency is everywhere doubled.
In the structure of the reptilian lung the
elastic fibre forms a predominant element.
It is a substitute for ribs and other accessory
apparatus of breathing. The lungs of the
frog, relatively to the cubic capacity of their
interior, present a much more extensive
active surface than those of the Salaman-
dridae. Thus the purpose of "the cells" is
fulfilled, of multiplying the operative surface.
The "septa" project from the sides into the
interior of the organ. In this respect they
may be likened to the gills of fishes ; for,
like the latter, " they penetrate the surround-
ing medium." The lungs of ophidian rep-
tiles are generally composed of true unsym-
metrical, long cylindrical or fusiform sacs,
extending from the pharynx far into the
cavity of the abdomen, above the other
viscera, and surrounded with the serous
lining of that cavity. They are capable of con-
taining a considerable quantity of air, which,
when driven out with force, produces the
"hiss" peculiar to the serpent. In some ge-
nera, as the coluber, typhlops and vipera, the
lung of one side only is developed ; in others,
as the boa and python, the two lungs are sym-
metrically or equally developed. The lungs,
in these families, communicate, by means
of a long and narrow trachea, surrounded
by incomplete cartilaginous "rings," with
the back part of the tongue. In all ophidia,
the lungs display internally, but only on the
anterior and upper parts, an elaborate system
of alveoli or cells, more like secondary lungs
than air-cells. The posterior two thirds
of the internal superficies are almost plane,
or devoid of" cells," like the lung of the newt.
The alveoli, traced from before backwards,
become shallower and shallower, until at
length they disappear. It thus appears that
the serpent may store up in its lung a con-
siderable volume of air which, slowly passing
out over the vascular air-cells, prevents the
carbonic acid, the effete product of the pro-
cess, from contaminating the whole contents
of the organ. Each " alveolus," separately
examined by vertical section, is found to
communicate by a single opening with the
general chamber of the lung. Traced inwards,
it divides and subdivides into secondary and
tertiary tiers of "alveoli," each cell being
isolated by dissepiments of which the struc-
ture is identical with those of the frog's lung
already described. Each cell is a separate
cavity. It does not communicate with those
adjacent by openings in the septa. These
septa are utilised in the outspreading of the
vascular rete. Each septum, aa in the frog,
carries two layers of capillary blood-vessels,
separated from each other and supported by
an intermediate stratum of elastic tissue.
In the mechanism of breathing, this tissue
enacts an important office. Over the interior
of the ophidian lung, like the batrachian, the
ciliated epithelium is limitedly distributed.
The true capillary areas which chiefly co-
incide with the flat sides and bottoms of the
cells are clothed only with 'hyaline epithelium.'
Everywhere throughout the interior of the
lung, along the courses of the larger vessels,
the borders of cells, or along lines of thick-
ened tissue, the phenomenon of ciliary vibra-
tion may be readily detected. It is thus evi-
dent that the office of cilia is mechanical, if
not to cause determinate currents in the air,
at least in the halitus and fluid which, by
accumulating, may obstruct the respiratory
function of the capillary areas.
In the boa and python the length of the
left lung is generally less by a third or half
than that of the opposite side ; but in coluber,
crotalus and others, it is much smaller and
quite rudimentary, appearing as little more
than an obliterated appendage. The genera
CcBcilia and Amphi&bcEna have the left pulmo-
nary organ developed, and the right short-
ened : this arrangement probably varies ac-
cording to the species. Vipers and other
serpents possess only a single lung, which
on that account is very long. The lungs
of the saurian reptiles conform in character
to those of the ranidce and salamandridcs.
They are elongated sacs, cellulated internally.
They extend far back along the roof of the
abdominal cavity. Like those of the ophi-
dians, they are divisible into a cellulated and
smooth or non-cellulated portion. The
former is limited to the upper and anterior
half of the organ ; the latter to the inferior
wall and posterior half. There may be a
mechanical reason in this peculiar distribu-
tion of parts. The cells in the lungs of the
saurians exhibit none of the regularity so
characteristic of those of the ophidians. They
are larger and more irregular. The partitions
of the cells are more slender and more de-
licately membranous.
The whole interior of the lungs in the
higher saurians is multiplied into cells. An
axis without definite walls, like a trachea,
runs from one end of the organ to the other,
as is the case in the lung of the turtle.
From either side of this axis, large orifices
lead to the more subdivided portions, or
secondary and tertiary air-chambers. On the
contrary, each lung in Scincus ojficinalis forms
a single continuous cavity ; but the entire
surface of the parietes is cellulated by small
projecting reticulate septa. The internal
dorsal and anterior half of the lung of the
chameleon is, as usual in the sauria y minutely
cellulated. Further back the cells become
larger, and the septa smaller, until at the
posterior part the walls consist only of plane
membrane both less vascular and less can-
cellated than the anterior. The coscal ex-
tremity of the organ is drawn out into an
286
RESPIRATION.
appendage-like process, which reaches the
farthest boundary along the roof of the abdo-
minal cavity. These appendages may be
aptly compared to the abdominal air-cells of
birds which communicate with open ex-
tremities of the bronchial tubes. The " dia-
phragm" in the mammals precludes this
interblending of the thoracic and abdominal
organs, or the diffusion of air into any of the
cavities of the body.
With reference to the minute structure of
the lungs in the saurians, it coincides pre-
cisely with the account given of those of the
ranidcc. Each septum consists of a central
basis or framework of elastic fibrous tissue
lined on either side by a reticulate layer of
vessels. This plexus is again overspread by
a " hyaline pavement epithelium." Rich tracts
of ciliary epithelium may be discovered along
the margins of cells, the course of vessels,
and the lines of condensed structures. The
double layer of vessels borne by each septum
may be noted as a point of structure dis-
tinctive of the reptilian lung. The lungs of
the chelonian reptiles are very voluminous.
They extend over the whole dorsal part of
the trunk as far as the pelvis. They are
fixed by the pleura to the ribs, which also
separate them from the cavity containing the
digestive and generative organs. They are
symmetrically developed on the two sides.
Through the centre of each lung longitudi-
nally an unwa\\ed axis extends from the an-
terior to the posterior extremities. This is
the main road for the air-currents. From
this axis, secondary passages, parietal ly cel-
lulated, radiate towards every point of the
circumferences of the organ. The ultimate
cells are very capacious. They communicate
little with each other. Each group has its
common outlet, thus resembling a lobule.
In the reptilian lung, however, there exist
no lobules ; an anatomical particular in which
they are distinguished from that of all mam-
malia. It is a criterion of lower organisation.
The vibratile cilia which line the nasal and
buccal passages, the pharynx and oesophagus,
the larynx and trachea of all reptiles are
most remarkable for tenacity of life in the
lungs of the chelonia. In the trachea of the
turtle, along certain tracts of the lungs, the
motion of cilia may be detected several months
after death. The physiological value of the
breathing process in any given animal cor-
responds, not with the volume of air inspired
per any unit of time, but with the measure of
the blood-surface exposed to its agency, the
rate at which the blood-current moves, the
numerical proportion of its red corpuscles,and
the frequency of the respiratory movements.
The small, but minutely, subdivided lung of
the mammal presents a much more extensive
surface for the outspreading of the rete mira-
bile than the very voluminous, but spacious-
chambered lung, of the chelonian. The total
volume of air inhaled by the mammal is less
than that which the lung of the turtle is
capable of containing ; but in the former case
it is more minutely distributed and divided ;
it is more effectually employed ; the contact
between it and the blood-web is far more
extensive and intimate ; while it acquires a
higher temperature than in the latter. In
these several particulars, cold differ from
warm-blooded animals.
Respiratory Organs of Fishes.
The aquatic type, distinctive almost univer-
sally of the breathing organs of invertebrate
animals, obtains also in the lowest order of the
vertebrata. Fishes and the lower amphibia
respire on the branchial plan. The difference
between a gill and a lung rests more on ap-
parent than real and ultimate grounds. In
the last anatomical analysis this difference
vanishes, and the eye is arrested only by the
close structural affinities which reduce the
two varieties to an essential unity of type. In
both, the blood is exposed to the agency of the
aerating element by means of reticulated ves-
sels, furnished with distinct parietes, and pre-
senting a diameter little in excess of that of
the corpuscles of the blood ; so that these
latter must travel through the true respiratory
capillary in a single series. This fact denotes
the extreme measure to which the subdivision
of the blood-stream is carried. It is a funda-
mental requirement of the breathing organ,
that all structures interposed between the
blood and the surrounding element should be
reduced to the utmost degree of attenuation.
Accordingly, it is found that the epithelium
overlying the rete mirabile consists of a single
layer of attenuated scales, perfectly destitute
of those contained parts which give bulk and
density. In no instance whatever within the
limits of the vertebrtita (excepting, as stated
already, the branchire of the amphibia) are
the true respiratory capillaries covered by a
ciliated epithelium. This rule applies also
to the branchiae even of the higher inverte-
brata, such as the Crustacea and cephalopoda.
The gill of the fish differs from that of the
crustacean in the extreme minuteness with
which the blood current is subdivided, and in
the existence of specially parieted vessels ;
conditions which denote an ' intensified
measure in the function of breathing in the
instance of the vertebrate animal. In the
blood of the vertebrata the floating cells are
infinitely more numerous, relatively to the
bulk of the fluid, than in that of the inverte-
brata ; a fact more expressive than the
former of the greater activity of the respira-
tory process in the vertebrate than in the in-
vertebrate animal.
It is an axiom in physics, that no gas is
capable of passing through an organic septum
without first assuming the fluid form. This
axiom destroys the apparent difference be-
tween a gill and a lung. In contact with the
gill the aerating medium is already fluid: in the
case of the lung, it takes this condition only in
the act of passing through the partition dividing
the blood from the external medium. Between
the gill of the fish and the true lung ofthe ver-
tebrate animal there is discernible, however,
this differential character, that in the former
RESPIRATION.
287
the epithelium clothing the active capillary
segments forms a thicker layer (than in the
latter. This is the only true and ultimate
anatomical distinction between a gill and a
lung.
The preceding general facts will form an
appropriate introduction to the study of
special details, on which it is proposed now to
enter.
The Lancelet (Branchiostoma) occupies the
first grade in the vertebrate series. It exhi-
bits the branchial organs under the least com-
plex terms. A capacious branchial sac, the
dilated oesophagus, occupies the mid-portion
of the body. It communicates with the ex-
terior in front by means of a large oesophageal
opening, and behind by a branchial outlet and
a short intestinal canal. The parietes of the
stomach display special provisions for breath-
ing under the character of membranous du-
plications of the internal surface. These folds
are invested with a vibratile epithelium. In
this particular the branchiae of this fish ap-
proach those of the lower molluscs, and de-
part from those of all other fishes. A con-
venient arrangement of this subject will
consist in first studying the mucous membrane
of the branchiae of fishes ; 2nd. the blood-
system ; and 3rd. the supporting frame-work.
^Mucous membrane of the branchice. The
gills of fishes are lubricated and defended by
a glary viscid secretion : this is the product of
the epithelium. This latter, therefore, even
on these parts enacts a secernent office. On
the plane superficies of the leaflets, the epi-
thelium constitutes a single stratum, resting
immediately on the expanse of the rete mira-
bile. In this situation, as in all others, the
epithelial layer is supported by a limitary mem-
brane. From its extreme attenuation, how-
ever, it does not easily admit of separate defini-
tion. Neither cvtoblasts, granules, or any other
immature particles, mingle with or underlie
this layer of adult epithelium. It is difficult
to conceive the mechanism of their renewal.
In the interior of its component cells, how-
ever, the eye clearly distinguishes a nucleus
and a few pellucid granules. It has already
been proved that these latter parts are almost
suppressed in the epithelium which lines the
active capillary segments of the true lung.
Of the branchial epithelium of fishes it may
be said that it unites the glandular to the me-
chanical function of the pulmonary epithelium,
that its office is exclusively mechanical. The
halitus of the air-cells is not a secreted pro-
duct. It is a transpiration.
Viewed with reference to the principles of
exosmoses, a layer of amorphous granules,
the cytoblasts of future epithelia, interposed
between the blood vessels and the super-
ficial strata of adult cells, would obviously
render the partition to be traversed by the
gases engaged in respiration very inconve-
niently dense. The structure presented by the
epithelium of the branchiae implies, that in the
organs of aquatic respiration the blood is brought
less directly into relation with the external
medium than in those of atmospheric breathing.
The cells investing the branchial capillaries
are not structureless, hyaline, flattened scales,
devoid of nucleus and granules, as though
the principle aimed at were merely the me-
chanical one of thinning to the extremest
practicable limit all structures between the
blood and the outer medium. They constitute
irregularly oval bodies, carrying a nucleus,
and provided with a few pellucid molecules.
No cilia exist in the gills of fishes. In their
fixed condition, as in the cyprinoid families,
in their concealed situation, as in the shark
tribe, or their exposed and free position, as
exhibited in the higher osseous orders, forms
enough diversified, these organs are charac-
terised alike by the complete and uniform