chiae for a much longer period, affording thus,
an opportunity for the study of the structure

Fig. 227.




Head and branchial appendages of the larva of the
Water-newt (Triton aquaticus vulgaris). (Oriq.)
The branchiae are enveloped in a prolongation of
the general cuticle of the body. The cells of the
epithelium covering the gills are, however, reduced
to a state of great attenuation, compi-essed into
scales, and polygonal in outline. The nearer the
period of transition from the larva to the perfect
reptilian type, the more intimate the resemblance
between the epithelium of the branchiae and that of
the general body. For some time before their obli-
teration, the branchiae cease to be distinguished
by ciliary vibration. This results from the change
which gradually occurs in the anatomical characters
of the component cells. The branchiae in the larva
of the newt consist of a trilobed extension of the
cuticle at either side of the head, the two posterior
lobes, which, in figure, resemble compressed finger-
like processes, presenting on either side secondary
projections, by which the respiratory area is mul-
tiplied. In relation to the size of the bod}', they
are larger than those of the larva of the frog.

and function of these appendages. The ge-
nera syren, protcus, and menobranchus are



RESPIRATION.



279



those only in which the external gills are per-
sistent throughout the whole term of adult
life. Whether temporary, as in the caduci-
branchiate, or persistent, as in the perenni-
branchiate genera, the branchial organs of
amphibia are supported by no skeletal frame-
work analogous to that which sustains the
soft parts of the breathing apparatus of fishes.
They are, essentially, only " productions," un-
der a modified form, of cutaneous structures.
Contemplated only as a mechanical contri-
vance, whether provisional or permanent, up-
on which devolves the most important func-
tion in the animal economy, it demands a
more minute investigation than it has hitherto
received at the hands of anatomists.

The cartilaginous arches erected on the
h void bone do not entirely disappear until the
internal gills have ceased to be distinguish-
able. The circulating system of the decidual
branchiae consists, in its earliest stage, of a
simple artery and vein ; that is, a loop of one
vessel. As the larva grows these two vessels
become separated by an intermediate system
of capillaries. In the latter phase they offer
no remote analogy to the vascular apparatus
of the branchiae of fishes. The cardiac centres
are composed only of a right auricle and one
undivided ventricle ; the left or pulmonary
auricle remains unevolved until the organic
necessities attendant on growth create a ne-
cessity in the system for the exercise of the
pulmonary functions. The left auricle is then
superadded, and the chamber of the ventricle
is partially divided by a median partition, and
the embryonic organism reaches the maximum
limit of development. The pulseless ventral
artery, the resultant of the united afferent
vessels of the branchiae, undergoes oblitera-
tion through disuse. These general observa-
tions form no irrelevant introduction to a
more special examination of the branchial
organs.

Temporary external Gills. The larval
branchiae of the frog and toad are less endur-
ing and less complex than those of the sala-
mandrkfae. From the earliest almost to the
latest moment of their existence they are
furnished with a ciliated epidermis. The gills
are not specially ciliated. The whole cutaneous
surface in the larva of the frog and toad is
similarly endowed. The cilia are in active
play for some time before the larva emerges
out of the egg : an admirable instance of
foresight in the provisions of nature. The
covering of the external gills of the ranidae is
strictly cutaneous. In this situation, as every-
where else, the epidermis betrays its real nature
by the presence of pigmental cells. It is little
less dense than the ordinary covering of the
body. Nor does the vascularity of these tem-
porary branchiae much exceed that of the rest
of the cutaneous surface. These facts pro-
claim their provisional character. At first
they consist of a single minute lobe. This
increases into two and then into several. They
are cylindrical, not flattened, processes. They
bear a single vessel returning upon itself. In
this particular of ultimate structure they dif-



fer from the branchiae of the salamandridae.
In these a capillary net-work is constructed
between the artery and vein. This greater

Fig. 228.




One of the gifts of the Newt viewed transparently.

( Original.)

a indicates the right auricle, which with v, the
ventricle, constitutes the heart of the true fish ; a'
shows the left or pulmonary auricle, which, being
superadded to the two former parts, raises the car-
diac organ to the reptilian standard, marked by the
presence of two auricles and one incompletely par-
titioned ventricle ; b, b f , denote the circuit of the
branchial system in conformity with the pisciform
type. This system of vessels being obliterated
during the metamorphosis of the larva, the pul-
monary vessels (c c') enlarge, the rudimentary lungs
at the same time expand, the associated auricle
grows in muscularity and dimensions, and the fish
rises to the grade of 'an air-breathing reptile, d, is
an enlarged view of the gill of the larva of the newt
soon after its escape from the ovum. Secondary
processes (d, d, d, d) are extended backwards, which
materially multiply the surface. The whole gill is
clothed with ciliated epithelium, the cells of which
lose their cilia, and become non-vibr-atile for some
time before the cessation of the branchial breathing,
and the oblong-cilia-bearing cells (e) are transmuted
into epidermal scales ( /) entirely destitute of cilia.

elaboration coincides with their longer dura-
tion. What is ephemeral in purpose is tem-
porarily formed. This is nature's workman-
ship. They consist literally of small pro-
longations of 'the skin, which is everywhere,
as here, ciliated. At the moment of their
fullest development, the larval branchiae of
the frog consist of four filamentary lobes.
These are sessile upon tne body or stem of
the branchiae ; they are somewhat granular
on the surface, and slightly irregular in form.
There is also frequently a short additional
branch at the base of the posterior one. In
these interesting organs the movement of the
blood is readily demonstrated. It is a beauti-
ful spectacle. It advances in a single current
along one side and returns along the other.
No sooner have these exquisite organs at-
tained their greatest development than they
begin to diminish in size. They become ob-
tuse, and are gradually so reduced as to be
withdrawn within the branchial cavity, and
concealed by a little operculum of the integu-
ment. The nature of this change of structure,
which attends the transition of the branchiae
from the external to the internal condition,
has never yet been defined by anatomists. It
will be immediately described.

The external Gills of the Salamandridce ex-
ceed the former in size, in the number of the
appended lobules and in the complexity of

T 4



280



RESPIRATION.



their vascular system {figs. 227, 228.) Like
those of the ranidae they are clothed in a
vibratile epidermis, numerously starred by
pigmental cells, in common with the rest of
the body. For some time before the deca-
dence of these organs in the larvae of the triton
they cease to exhibit the phenomenon of ciliary
vibration. The vibratile epidermis undergoes
a change by which the ciliated cell becomes
succedeed by the simple. This event foretells
the approaching extinction of the parts. In
their earliest condition the branchiae of the
newt discover only four minute simple cylindri-
cal filaments. Each grows in length and thick-
ness, and throws out from the inferior surface
a double row of pectinated processes. These
are more complexly constructed than the pri-
mitive filaments. They carry not only an
afferent and efferent trunk, but an elaborate
plexus of capillary vessels. The pigment cells
are limited in their distribution to the larger
lobes, and to the line of the larger vessels.
The epidermis of the secondary processes of
the branchiae is reduced to extreme tenuity.
Through it the eye readily tracks the move-
ments of the individual blood corpuscles on
the branchial capillaries. These elliptical bodies
move like a boat, their long axes coinciding
with that of the channel in which they are
travelling* Sometimes several proceed abreast.
The diameter of the vessels of the temporary
branchiae is greater than those of the lungs.
In general terms it can be confidently stated
that the quantity of blood circulating in the
temporary branchiae of the amphibia, at the
period of their maximum development, is far
less in relation to the amount contained in the
whole body than that which the lungs, when
fully formed, are capable of carrying. This
inferior amount of blood is physiologically ex-
pressive of an inferior functional power in the
case of the temporary organs. Their respira-
tory function is really only supplemental to
that of the whole body. The whole cutaneous
surface, as in the Naematoid annelids, is richly
Ciliated. It is organised like the branchiae.
On these parts, however, the epidermal layer
is not so attenuated as that with which the
branchiae are invested. On these latter there
is, however, a very perceptible epidermal co-
vering. Its scales exhibit the ordinary hexa-
gonal figure.

This demonstration, which dispels all doubt,
establishes the physiological principle, that
the presence of epithelium is compatible with
the respiratory office of the part which it
clothes. This law prevails throughout the
class of fishes ; it has also been reduced to
actual fact by the author throughout the whole
sub-kingdom of the invertebrata. But it must
not be forgotten that its office on the breath-
ing organs is almost exclusively mechanical.
In no known example among vertebrated
animals does the epithelial investment of
a respiratory surface develop itself into any
of the forms of a secreting organ. No
" follicles " are, at any time or under any
circumstances, discovered on these localities,



in this class of animals, but in the inver-
tebrata follicular glandules are constant on
the surfaces of the respiratory organs. The
constituent uales are therefore function-
ally passive. Nuclei and a granular proto-
plasm would find no purpose to subserve if
they were present in a highly developed form.
Thus is exemplified the law of " demand"
and "supply:" disuse entails attenuation on
all living structures. Either the gases by
which the epidermis of respiratory localities
is traversed suppress the glandular office of
the scales, or these latter, from the first, re-
ceive a special organisation. The scales of
the branchial epithelium contain nothing but
a pellucid fluid. This fluid conde?ises, fluidi-
fies, the respiratory gases in transitu. This is
the office of the refined covering under study.
The "principle" that the epithelium of the
breathing organs is required by the physical
conditions of its office to be reduced to the
state of the utmost thinness receives new
proofs from the study of the internal branchiae.

From all that is known it is probable that
in minute structure the branchiae of the per-
enni-branchiates conform to the plan of the
temporary organs just described. The ge-
neral arrangement of the primary branchial
vessels and the structure of the heart are
identical.

The internal temporary Branchia of the
Amphibia, The process by which these or-
gans are withdrawn into the interior of the
branchial chamber is not simply that of short-
ening. It is the labour of a new organisation.
The internal gills of the tadpole differ in type

Fig* 229.




The internal branchiae of the Tadpole of the Frog,
b, c, are the primary trunks supported by the
cartilaginous arch c, which give off the looped
processes a; d, is one of the vascular loops viewed
transparently, and showing the arrangement of
vessels in them, and the epithelium by which they
are covered.

of structure from the external. The ultimate
vessels of the latter are differently looped.



RESPIRATION.



281



In both they are simple loops, but a distinc-
tion is obvious ; so evident as to render it
impossible that the transition from the ex-
posed to the concealed state of the branchiae
can consist in a bodily retractation. On the
external organs, preparatorily to their disap-
pearance, the vibratile cilia first cease, the
epidermis then increases in density, the meshes
between the blood-capillaries enlarge, and the
vessels become obliterated. These declining
changes are not limited to the extreme distal
ends of the branchial lobules. They occur
simultaneously on every part of the surface.
Temporary arches (c) of delicate cartilage now
arise within the branchial chamber. It is from
the convexities of these arches after the manner
of the pisciform type that the new vessels (/z)
of the internal temporary gills proceed. They
are appended under the character of de-
licate flocculi. Enlarged, they appear as
minute digitations. Each carries a looped
vessel, and is loosely invested with a de*
b'cate membrane (rf). This membrane belongs
to the mucous, not to the epidermal, class j
and yet it differs in a striking manner from
that which lines the rest of the branchial
chamber. Xo where is it ciliated. That co-
vering the branchial vessels is remarkably
thinner than the parietal portion. The former,
however, is true epithelium. Its constituent
scales are distinctly traceable by their out-
lines, though they are as structureless as a
basement membrane. It is not often that it
happens that the epithelium of a breathing
organ overlies, as in this instance, perfectly
homogeneous parts. Nothing but the proper
coats of the vessel lie underneath. They are
literally structureless and hyaline. The cells
of the superficial epithelium, therefore, admit
of indisputable definition. It is not " base-
ment membrane," but epithelium, though
attenuated, that here invests the respiratory
vessels. By this demonstration a principle is
established. Epithelium is not supplanted by
any other structure on the organs dedicated
to respiration* No other instance, however,
is known within the limits of the vertebrate
kingdom in which this epithelium is ciliated,
than that afforded in the case of the tem-
porary external gills of the Amphibia. On
those of fishes these motar organisms do not
exist. Wherefore this distinction? Why
should they exist on the external and not on
the internal gills? It is not a law of the
mucous membrane that they should not exist,
for they occur in other tracts of this same
structure. These are questions of ultimate
design which it is not given to science to
answer.

Air-bladder of Jishes. This organ repre*
sents the prototypal form of " the lung" in
the animal kingdom. It is present in nearly
all osseous fishes. It is always tensely filled
with gas. In that of marine fishes, oxygen
predominates ; in that of fresh water, nitrogen.
Humboldt found the gas in the air-bladder
of the electric gymnotusto consist of 96 parts
of nitrogen, and 4 of oxygen. Biot found 87
parts of oxygen, nitrogen, and carbonic acid in



the deep Mediterranean fishes. No hydrogen
has ever been detected in this organ. It
occupies the roof of the abdomen, between
the kidneys and chylopoaetic viscera, and
sometimes (gymnotus ophiocephalus, coius),
beneath the caudal vertebrae to nearly the
end of the tail. In some species of diodon,
telradon, dactyhpterus, pemelodus, and poia-
notus, it is bifurcated. In arius gngora, poly-
pterus and lepidosiren t it is divided lengthwise
into two bladders. In the cyprinidte and
characlnid(B it is divided transversly into two
communicating compartments. Many other
varieties of form occur. (Vide art. PISCES.)
The proper walls of the air-bladder consist of
a shining silvery fibrous tunic, the fibres being
arranged for the most part transversely and
circularly, and in two layers. They are con-
tractile and elastic. This coat yields the
finest gelatine. Its fibres belong to the white
variety: they "swell" under the action of
acetic acid. A stratum of vessels is inter-
posed between the mucous membrane and
the fibrous layer. The meshes formed by
these vessels are considerably larger and more
oblong than those of the pulmonary capil-
laries. In the latter instance the meshes
exceed the vessels in diameter. The arteries
of this organ are derived sometimes from the
abdominal aorta, sometimes from the casliac
artery, sometimes from the last branchial vein ;
and in thelepidosiren they are continued from
the aortic termination of the two non-rami-
fied branchial arteries, and therefore convey
venous blood to the cellular, lung-like, double
air-bladder (Owen). The veins of the air-
bladder return, in some fishes, to the portal
vein ; in some to the hepatic vein ; in some
to the great cardinal vein ; and in the lepido-
siren, they penetrate by a common trunk the
great portal vein formed by the confluence of
the visceral and vertebral veins of the trunk.
In the protopherus and ganoid fishes the
vessels of this organ form no retia mirabilia
and vaso-ganglions, but rather a diffused
capillary network, more close and rich in the
anterior than the posterior part. In the
osseous fishes, several varieties of the vascular
system of this organ occur. That of the carp
forms tufts of capillaries throughout the
whole interior of the organ, a variety of
which tufts occurs in the pike. The perch
and cod exhibit a vaso-ganglion, a body pe-
culiar to the air-bladder of fishes. In the
cod-fish, a large artery, a branch of the coeliac,
and a still larger vein, which empties itself
into the mesenteric, perforate together the
fibrous tunic of the bladder. Before they
reach the inner surface, they divide into
some branches which then radiate and sub-
divide upon the mucous membrane. The
arterioles frequently anastomose with each
other. Both are inextricably interwoven, and
form the basis of the so-called "air-gland,"
which is essentially a larger "bipolar rete
mirabile" (Miiller), of vaso-ganglion. In the
cod the ultimate vessels of this gland have a
loop-like arrangement, their free surface (a a),
being covered over with another layer of



282



RESPIRATION.



vessels and epithelium This organ, however,
is further composed of a number of pecu-

Fig. 230.




Plan of blood-vessels in the gland (aerogenic?^) of the
air-bladder of the Cod-fish, showing the simply
looped character of the vessels. ( Original.)
a, a, indicate a stratum of fibres, vessels, and
epithelium lining the internal surface of the gland
in common with the whole interior of the air-
bladder.

liarly arranged, elongated corpuscles, which
descend in two rows from each vascular
branch, and are bound together by a loose
cellular tissue : the corpuscles are beset
with fine villiform processes* Thus it should
be noticed that the veins as well as the
arteries concur to form the vaso-ganglions.
The vaso-ganglions of the eel and conger
are placed at the sides of the opening of the
air-duct, are "bipolar," and consist of arteries
and veins ; their efferent trunks do not ramify

Fig. 231.




Plan of the blood-vessels in the glands of the air-
bladder of the Eel. They consist of straight pa-
rallel ultimate vessels of uniform diameters, of
arteries, and veins, carrying streams of blood moving
in opposite directions. (OriginaL)

in the immediate margin of the vaso-ganglion
from which they issue, as in the vaso-gan-
glion of the cod, turbot, acerine, and perch,
but run for some distance before they again
ramify to form the common capillary system
of the lining membrane of the air-bladder.
In the parasitic and suctorial dermopteri and
pleuronectida3 and ray-tribe the air-bladder
does not exist.

The ductus pneumaticus exists in the eel,
sturgeon, amia, erythrinus, lepidosteus, lepido-
siren, polypterus. It is remarkable that in
these fishes the vaso-ganglion is not deve-
loped. " Under all diversities of structure
and function the homology of the swim-
bladder with the lungs is clearly traceable ;
and finally, in those orders of fishes which
lead more directly to the reptilia as, for
example, the salamandroid ganoidei and pro-



topteri those further modifications are super-
induced by which it becomes also analogous
in function to the lungs of the air-breathing
amphibia." *

The Lungs in the Batrachia. In the ichthioid
amphibia there exist two long membranous pul-
monary sacs, extending, like the air-bladder of
fishes, far backwards into the cavity of the
abdomen, above the other viscera, but freely
moveable in the cavity of the peritoneum, and
invested with this serous membrane. They
consist of smooth plane-walled sacs, and
communicate with the pharynx by means of
their membranous ducti pneumatici or tra-
cheas. This simple condition of the lungs
occurs in a permanent form in the salaman-
dridae. Each sac is provided with a pulmo-
nary artery, which runs in a straight course
along the outer side of the organ. From
this vessel, branches proceed with great regu-
larity at right angles and at definite distances.
From the midpoint of the space between the
arterioles, a venule arises to run round the
opposite semi-cylinder of the organ into the
chief trunk of the pulmonary vein. In con-
sequence of this regularity in the distribution
of the arteries and veins, the true capillary
interspaces present a regularity of area, ft
follows from this arrangement that each drop
of blood, in its passage from the extreme
artery to the extreme vein, undergoes in
every part of the lung the same quantum of
aeration. It is commonly supposed, by com-
parative anatomists, that the simple lungs of
the salamandridcB present a perfectly smooth
and uniformly plane surface internally, such
that every spot participates with equal acti-
vity in the office of aerating the blood. This,
however, is not the case. The septa which,
in the case of the frogs and toads, divide the
internal superficies into cells, exist in a rudi-
mentary state, but unquestionably in the lung
of the newt. They are indicated by inter"
seeling lines of vibratile cilia. They coincide
chiefly with the principal branches of blood-
vessels. Bundles of elastic fibres also run
parallel with the vascular trunks, which confer
upon these delicate organs an uncommon
amount of elasticity. To the next point in
the minute structure of the lungs especial

* R. Owen, Cat. of Phys. Ser. of Coll. Sur. 4to,
1832-40; Mttller, J., Vergleichende Anatomic der
Myxinoiden : Abhand. Akad der Wissenschaften
zu Berlin, 1834 ; Agassiz, Hist, des Poissons Fos-
siles, 1833-45 ; Cuvier et Valenciennes, Histoire
Nat. des Poissons, 1845 ; De Blainville, Annales des
Sciences Naturelles, 1837 ; Bojanus, Versuch einer
Deutung der Knochen in Kopfe der Fische, in
Oken'slsis, 1818; Yarrell on British Fishes, 8vo,
1836; Paley, Nat. Theol. 8vo, ed. 10. 1805; Bre-
scher, Recherches sur 1'Organe de 1'Ouie des Pois
sons, 1838 ; Monro, The Structure and Physiology
of Fishes explained and compared with those of
Man and other Animals, fol. 1785; Scarpa, De Au-
ditu et Olfactu, 1789 ; Hunter, Obs. on the Ani-
mal Economy, Palmer's ed., 1837; Ratke, in
Die Physiologic von Burdach, 8vo, i. 1826 ; Allen
Thompson, Jameson's Journal, 1830-31 ; Duvernoy,
Sur le Me'canisme de la Respiration dans les Pois-
sons, in Ann. des Sc. Nat. 1839; De la Roche, Obs.
sur la Vessie Aerienne des Poissons, Ann. du Mu-
seum, t. xiv. 1809. f



RESPIRATION.



283



attention is invited. The conclusions here-
after to be drawn will be found opposed to

Fig. 232.




A small portion of the lung of the Newt laid open and
examined by transmitted light, under a high power,
such that only the surface (internal or mucous) is
in focus. ( Origin al. )

a, a leading branch of the pulmonary artery,
giving off at very regular intervals which break at
once into the true capillaries e, c, c. ; d, d, d, denote
the parenchymous islets which fill up the meshes of
the capillary plexuses. (They are the true pul-
monary parenchyma.} g, marks the abrupt line which
abruptly limits the distribution of the ciliated epi-
thelium, which follows the larger vessels in tracts ;
d, c, c, c, coinciding with the true respiratory or
capillary areas of the lung, are seen to be destitute