between the cells bears only a single layer of
vessels. The opposed sides of such a layer
must therefore bound two different cells, and
the current of blood by which it is traversed
must be subjected on both its flat sides to the
action of air contained in the cells. If this
plexus were double, only one side could re-
ceive the influence of the atmosphere. This
type is exemplified in the reptilian lung. All
other things being equal the respiration of
the mammal and man must be twice in
amount that of the reptile. But this anato-
mical fact has also an interesting pathological
bearing. When the capillary layer, or rather
the blood borne by it, is the seat of disease,
the products of that disease must be poured
into the two contiguous cells, between which
it is interposed, at the same time and in the



same amount. This accounts for the rapidity
with which pneumonic infiltration occurs.
In the. most injected preparation the diameter
of the capillary vessels of the rete mirabile
exceeds that of the meshes. This, however,
is not an exact expression of their propor-
tions in the living state. The diameter of a
single capillary, measured in the injected state
on the wall of an air-cell, does not exceed
T _^_ (Todd and Bowman.) The human red
blood corpuscle ranges, in the same drop of
blood, from 7 ^o to ^iny f an mcn m dia-
meter, the average being from T3 Vo to ^aVo-
Then two red-corpuscles may traverse any
single capillary abreast ? When allowance is
made for the difference between the internal
and external diameters of the vessel, it will
appear very probable that in man and mam-
mals only a single row of red-corpuscles tra-
verse the pulmonary capillaries at a time. In
the lung of the reptile it is to be proved by
observation of the living circulation, that a
double row of corpuscles really does move
along the vessels. This fact must reduce the
amount of oxygenation which in a given time
a single corpuscle receives.

The pulmonary Veins convey the blood,
arterialised in the plexus just described, to
the left auricle. The distribution of the pul-
monary veins differs strikingly from that of
the artery and bronchia. Each lobule has its
separate arterial and bronchial branch. This
regularity does not obtain with respect to the
veins. The pulmonary veins arise in the form
of minute radicles in the capillary plexus of the
air-cells. Now although, as formerly stated,
the individual air-cells are not furnished with
a separate arteriole and venule, the extreme
branches of the pulmonary artery and the in-
cipient venules are separated in different parts
of the lung by areas of similar dimensions.
It follows, therefore, that the time during
which a globule of blood in different parts of
the lung is exposed to the agency of the air
is equal j in other words, every drop of blood
which enters the lung is arterialised to the
same amount from the equality of the areas
of exposure over which it passes. It should
also be observed that as the capillary web is
spread over several cells, every particle of
blood in transitu from artery to vein traverses
the circumferences of several cells. This is a
beautiful provision for securing certainty to a
vital process. The minute venules unite to
form visible trunks, which course irregularly
over and between the cells and intercellular
passages. They observe a general diagonal
direction, the bronchia and artery occupying
with great constancy the geometrical axes of
the lobule. They emerge out of this lobular
space not at its apex, in company with the
air-tube and artery, but at every or any point
of its circumference. This is more obviously
the case as regards the lobular veins along the
surface of the lung. Those more centrally si-
tuated follow the bronchia more closely. This
explains why Reisseissen, Cruveilhier, and
others have expressed very opposite opinions
upon this point. In the intralobular spaces the



RESPIRATION.



275



veins proceeding from several lobules unite
together into a trunk common to them all.
The larger trunks, resulting from the conflu-
ence of the smaller, converge towards the
roots of the lungs, but by a route different
from that of the bronchia and arteries. Thus
the general mass of the lung may be -regarded
as containing two series of ramified canals;
one transmitting the bronchial tubes, the nerves
and pulmonary artery, the other the pulmo-
nary veins. This interesting fact was well
described by Dr. Addison of Guv's Hospital
in a paper in the Medico Chirurgical Trans-
actions in 1840. At the root of the lungs
four pulmonary veins result, which discharge
their blood into the left auricle. " The cause of
the separate course of the pulmonary arteries
and veins is to be found in the opposite posi-
tion of their radicles in regard to the capillary
net-work of the lobules, it being a convenient
arrangement for the terminal arterial and
venous twigs to hold alternate positions
among the capillary net-work, so that each
arterial twig dispenses its blood in all direc-
tions, and each venous radicle collects it from
all sides." *

The Bronchial System of Vessels consists of
arteries and veins. The bronchial arteries are
commonly described as the nutrient vessels
of the bronchial tubes. They arise from the
front of the descending or thoracic aorta. They
are, however, variable in number as well as in
place of origin. They are commonly described
as the inferior and superior.

The superior, two usually in number, arise
either separately or by a common trunk from
the front of the aorta, opposite the third or
the fourth dorsal vertebra, and one directed
to each side adheres to the posterior surface
of the bronchial tube, on which it divides into
branches, and passes into the interior of the
lung.

The inferior, two or more in number, arise
lower down than the preceding, and are distri-
buted, like them, on the bronchus of each
lung : these small arteries give branches to
the oesophagus, bronchial glands and pericar-
dium. The superior bronchial artery of the
left side may arise from the superior inter-
costal artery. Every successful injection ex-
hibits large and long branches from these ves-
sels, leaving the tracks of the bronchi, and
entering into the inter-lobular tissue and
sub-pleural tissue, -j-

The bronchial Veins accompany the arteries,

* Physiol. Anat. by Todd and Bowman, p. 393.
vol. ii.

t In an excellent paper recently read before the
: Society (June ( Jth, 1853), Dr. Heale states this
till more strongly. He denies that the vascu-
lar plexus, distributed over the walls of the bron-
chial tubes, is derived at all from this system of
'-, but from the pulmonary rete. I have re-
peatedly remarked, that the Iron chial plexus cannot
be injected from the aorta. I ascribed the circum-
stance always to some imperfection in the attempt.
It will be seen in the text further on, however, that
my injections prove the presenceof a bronchial plexus
on the exterior of the bronchial tubes, though not
on the interior.



and the branches unite one for each side ;
the right opens into the azygos vein, and the
left into the superior intercostal vein. Many
branches may be traced also to the oesopha-
geal veins, and those of the posterior medias-
tinum. Numerous branches of these veins
may be observed to wander under the pleura,
in the sub-pleural tissue.

Anastomoses between the Bronchial and Pul-
monary Systems of Vessels. Since the days of
Ruysch, Haller, Soemmering, and Reisseissen,
this has proved a vexed question in anatomy.
One point in this controversy has been over-
looked. The bronchial arteries are said to be
the nutrient vessels, not of the lungs, but of
the bronchi. The tissue composing the struc-
ture of the air-cells and intercellular passages
of the lungs is nourished by the blood of the
pulmonary system. In the lungs of Reptiles
there exists no bronchial system of vessels.
The solid walls of these organs are occupied
exclusively by the pulmonary system. It is
upon the latter, therefore, that the function of
nourishing the substance, the parenchyma, of
these organs must devolve in these animals.
Thus is proved the capacity of this blood.
The epithelial particles and elastic fibres; of
the air-cells derive the materials of their nu-
trition from the blood of the plexus (the true
pulmonary) on which they immediately lie.
It is indisputable, therefore, that the afferent
blood of the lungs, like that of every other
gland, discharges a twofold office, that pro-
per to the gland, and that of nourishing its
tissue. Two systems or layers of capillary
plexuses are discoverable on the walls of the
bronchial tubes ; one lies immediately under-
neath the mucous membrane, and exhibits ex-
tended oblong meshes, which run parallel with
the yellow elastic fibres; the other lies on
the outside of the circular muscular layer, so
that the stratum of muscles is interposed be-
tween the two systems of vessels. This outer
layer of vessels, its trunks and capillaries, run
circularly with the fibres of the muscles and
at right angles with those of the submucous
layer. The blood of the former empties itself
into the pulmonary vessels ; that of the latter
(the outer) returns by means of the bronchial
veins. This, in brief, is the result of the au-
thor's investigations. They are confirmatory
of those of Adriani. The mode in which the
bronchial and pulmonary vessels communicate
is stated differently by different authors. Some
suppose that the blood of the bronchial arte-
ries is poured directly into the pulmonary
artery, with the venous blood of which it
admixes, and like which, traversing the respi-
ratory plexus, becomes arterialised before it
reaches the left auricle. On this supposition
the blood entering the left auricle would be
purely and exclusively arterial. By other ana-
tomists of these Rossignol is the most pro-
minent* it is contended that the bronchia



* " Dans les injections faites paries arteres bron-
chiques, le liquide revenait en abondance par les
veines pulmonaires, en bien moindre quantite par
tubes, though not les veines bronchiques, et on n'en retrouvait aucune
trace dans les rameaux de Partere pulmonaire,**

T 2



276



RESPIRATION.



blood is poured into the pulmonary system at
the left side of the respiratory rete. The cur-
rent, therefore, entering the left auricle is not
pure arterial blood : it is alloyed by the venous
rivulet received from the bronchial system,
a reptilian characteristic traceable in human
organisation.

By a third class of observers it is said, that
the capillaries of the pulmonary and those of
the bronchial system of vessels intimately
inosculate. The precise solution of this
question is difficult, in consequence of the
readiness with which an injection thrown into
one vessel will pass into another by extrava-
sation. Other anatomists suppose that the
three above-described modes of communica-
tion actually exist. It is certain that these
two systems do communicate, and that only a
part of the blood of the bronchial arteries re-
turns by the bronchial veins. More recently, a
new aspect has been given to this controversy
by the statements of Dr. Heale, to the effect
that the bronchial and the pulmonary systems
of vessels do not in any manner or degree
communicate. He maintains, on the evidence
afforded by his injections, that the vascular
web of the air-cells extends, and is prolonged
over the internal surfaces of the bronchial
tubes. Dr. Heale assigns to this extension of
the rete mirabile the power of prolonging the
aeration of the blood. This is impossible.
The bronchial tubes, the minutest, are inter-
nally lined by a dense ciliated epithelium.
Such epithelium does not exist on the true
capillary parts of the lungs of any vertebrated
animal. Where there is ciliated epithelium, a
universal principle of structure requires in the
higher vertebrated animals that the function of
breathing should be suppressed. This prin-
ciple, however, does not obtain in respiratory
organs of the invertebrata, and in the bron-
chial organs of lower vertebrata.

Respiratory Organs of Birds.

The lungs of birds are two in number,
symmetrically developed, flattened, and ir-
regularly triangular in figure. They are fixed,
by means of areolar tissue, to the ribs and
vertebral column, from the inequalities of
which they receive deep impressions. They
extend from the second dorsal vertebra as far
as the kidneys, and laterally to the junction
of the vertebral with the sternal ribs. In
their fixed position under the back and near
the centre of gravity, they contrast strikingly
with the lungs of mammals, which float
loosely in the thoracic chamber. In colour
they are blood-red, and in general texture
they are more fragile than the lungs of
mammals. They are not divided by deep

" Par les arteres pulmonaires, Pinjection revenait
en entier par les veines correspondantes et jamais
par les arteres bronchiques."

" Enfin, Pinjection pousse'e par leg veines pulmo-
naires remplissait tous les autres vaisseaux san-
guins du poumon, c'est-a-dire, Partere pulmonaire,
les arteres et les veines bronchiques." Rossignol,
Op. cit. p. 64.



fissures into lobes, like the mammalian lung ;
lobuii, however, exist, although more length-
ened in form than those of the mammal
lung. In the former, as in the latter, a

Fig. 225.




A. Lobule of the lung of a Bird represented in ideal

longitudinal section. ( Original. )
a, a, a, primary bronchi maintaining a uni-
formity of diameter and terminating ccecally ; b, b, b,
second'ary bronchi, maintaining also a regularity
of diameter and opening into a dense cubic laby-
rinth of blood-vessels c, c.

B. A small piece of the ultimate portion of the lung,
representing the arrangement of the ultimate re-
spiratory capillaries.

lobule is a smaller lung. All its parts are
complete. The lobuii are embraced and
isolated by membranes of areolar elastic
tissue. A pleural investment embraces their
sternal aspects, and an aponeurosis, proceed-
ing from the diaphragmatic muscles below,
blends its fibres with those of this covering.
The trachea, after a course in the neck vary-
ing with the length of this part, at its entrance
into the lungs, divides into two primary
bronchi, one for each lung. At the place of
this bifurcation there exists, in most birds,
a complex mechanism of bones and cartilages,
moved by appropriate muscles, and consti-
tuting the true organ of voice. This part is
known as the inferior larynx.* The trachea
is composed of rings of cartilage which are
not deficient at the posterior third of the
circle, as in quadrupeds. The successive
rings are linked together into a cylindrical
form by means of a highly extensile and elastic
membrane. The whole cylinder is embraced
in a second concentric cylinder of muscular
fibre which belongs to the voluntary or striped
variety. In this particular it differs from the
trachea of mammals. In the latter, only the
deficient portion of the rings is composed of

* See arts. LARYNX, VOICE, and art. AVE&



RESPIRATION.



277



muscular fibre, and that too of the involuntary
or unstriped kind. This muscular layer in birds
extends from the superior larynx to the com-
mencement of the bronchi : these latter are,
however, unsupplied by muscular fibres. They
are exclusively membranous.* The bronchi
in the case of birds, on entering the substance
of the lungs, divide and subdivide without
decreasing in diameter (a, a, a, jftg. 225.)
Patches of cartilage appear in the parietes only
of the largest order of these tubes. They are
distinguishable into two principal classes :
those, first, which course superficially along the
inferior or sternal surface, and which terminate
by wide openings in the thoracic and abdo-
minal air- receptacles. This class of tubes is
perforated by the inter-cellular passages only
on one side, the other being strengthened by
cartilaginous semi-rings. The deep bronchi,
resembling cylindrical tubes, traverse the
lungs in many directions, and freely commu-
nicate with each other, not, however, to
form a network, for they run in nearly pa-
rallel directions. These tubes are always
patulous on dissection, and seem incapable
of contraction and dilatation. They are
lined internally by a well-marked ciliated epi-
thelium. The submucous tissue in the true
bronchi is strong and dense, composed chiefly
of elastic fibres, none of a muscular character.
It constitutes a distinct fibrous layer, like that
which lines the trachea of quadrupeds. Those
bronchi which do not end in open orifices
on the surface of the lung terminate ccecally.
These ccecal extremities are perfectly defined
by a prominent lining of fibrous and mucous
membrane. It was first proved by Mr. Rainey
that in the lungs of birds the mucous mem-
brane does not extend inwards in the direc-
tion of the interior of the lungs beyond the
limits of the bronchi. By the words mucous
membrane Mr. Rainey desires to indicate that
flocculent covering which is so well seen in
his injected preparations. By this observer
it is maintained that all parts of the lungs of
birds beyond the extremes of the bronchi are
literally devoid of all epithelial covering
whatever, the extreme capillary vessels being
included in nothing but their own proper
tunics. It has been already shown that
Mr. Rainey has mistaken the cessation of
the ciliated epithelium at the ends of the
bronchi for the termination of all the other
elements of this covering. The apparently
naked vessels of the air-ctlls are really in-
vested by a hyaline epithelium, coinciding with
that which, in the instance of reptiles, will
afterwards be described. The abrupt termina-
tion of the bronchial tubes marks the abrupt
commencement of the intercellular passages.
These passages contrast remarkably in struc-
ture with the bronchi. The membranous
walls of these parts are reduced to the ut-
most state of thinness; those of the former
are furnished with cylindrical epithelium and
a dense fibrous coat. But, what is extra-
ordinary, the dense mass of vessels which
* See art. AVES, by Prof. Owen.



bound these passages are not joined together
into a continuous partition. Each vessel is
separate from and unconnected with those
adjacent. " A wall " thus constructed is at
every point between the vessels permeable to
air. These " intercellular passages " (b t 6, I,
fig. 225.) arise, with singular uniformity, from
the sides of the bronchi, at right angles to the
axes of the latter. This is so constant as to
become a characteristic point of structure in
the bird's lung.

The "spaces" between the vessels forming
the walls of the intercellular passages lead to
no definitely bounded cells or chambers.
They lead only to the interval which divides
the contiguous bronchi from each other
(c,c). This interval is filled densely with the
ultimate pulmonary vessels. (n,fig. 225.) It
was first determined by Mr. Rainey that
these vessels, in the bird's lung, are arranged
in a peculiar manner. They do not form
plane reticular definitively bounded air-
chambers. Each ultimate capillary crosses
an air-space of its own. It is thus surrounded
by air. The ultimate vessels interlace and
interloop in every direction, forming a cubic
mass of capillaries permeated everywhere by
the air. The apparently naked loops of the
ultimate vessels may be seen projecting into
the areas of the intercellular passages. No-
thing can be conceived more mechanically
perfect than this arrangement of the vessels
for the exposure of the blood to the opera-
tion of the air. The latter is in immediate
contact with each individual vessel (B, Jig.
226.) It surrounds the blood-current borne

Fig. 226.




Slightly oblique section through a bronchial tube.
{After Rainey.}

a, cavity of the tube ; b, its lining membrane,
containing blood-vessels with large areolae ; c, c,
perforations in this membrane, where it ceases at
the orifices of the lobular passages (rf, rf); e, e, spaces
between contiguous lobules, containing the terminal
pulmonary arteries and veins supplying the ca-
pillary plexus (/,/) to the meshes of which the air
gains access by the lobular passages.

by the latter. Every part of the circum-
ference of this current, less than ^^ of an
inch, is under the direct agency of the aerat-

T 3



278



RESPIRATION.



ing clement. In the bird's lung there exist,
therefore, no air-cells.

It is argued by Mr. Rainey that the ultimate
vessels in the bird's lung, as in the mammal's,
are literally naked; that is, that they have no
other covering whatever than their own
proper coats, of which at irregular intervals
the cell-nuclei may be distinguished. In
other words, that the epithelium, so percep-
tible on the bronchi, is not under any shape
continued beyond the termination of these
tubes. To this view it has already been
objected that it is at variance with all ana-
logy ; the branchial and pulmonary vessels
of fishes and amphibia are provided, as will
be subsequently shown, with pavement epithe-
lium, the scales of which may be seen to be
continuous with those of the ciliated divi-
sion of the membrane ; that a law of ana-
tomical structure applying to the respiratory
organs of the lower vertebrata must also
govern that of the higher. It is impos-
sible to demonstrate on the injected vessels
of the bird's lung the presence of a separate
investment of epithelium. The vessels do
appear to be literally naked. But in the
recent structure, in their sections through
the bronchi and intercellular passages, it is
perfectly easy to the practised eye to trace
the epithelium of the bronchi over the larger
vessels amid the intercellular passages just
before the former break into the mass of the
ultimate capillaries. The continuity of the
pavement epithelium of the larger vessels with
the cylindrical of the bronchi may be un*
doubted ly traced by the eye. Now, what is
true of the larger vessels is very probably true
also of the smaller. Although, therefore, it
cannot be directly proved at present that in
the bird's lung the ultimate capillaries, as in
the branchiae of fishes and the saccular
lungs of amphibia, are invested by a separate
epithelium, the conclusion first stated appears
at present to be most reasonable and most in
accordance with analogy. In these examina-
tions it is important not to mistake the out-
line of the red corpuscles in the vessels for
that of the epithelial scales on their pafietes.
According to the measurements of Mr. Rainey
the arcol<B between the capillary vessels,
which in the bird's lung are the real air-
spaces, equivalent to air-cells, are gene-
rally smaller in diameter than the capillaries
themselves, and average in diameter about
W*re of an inch. An epithelial cell taken
from the lining membrane of the bronchi in
a pigeon measures in length -gi^and in breadth
Woo- It is therefore certain that, as Mr.
Rainey contends, epithelium of such magni-
tude could not, by physical possibility, line
spaces the diameter of which did not exceed
eVo- f an mcn - The error here committed
consists in overlooking the difference between
the dimensions of the cylinder epithelium
which lines the bronchi, and those of the won-
drously attenuated hyaline epithelium which
belongs to the true respiratory, capillary, areas
of the lungs of birds, reptiles, and mammalia.
A similar distinction between the epithelium



which lines the caecal ext) .mities of glandular
ducts, and that covering the merely convective
or excretory stages of the same ducts, obtains
in nearly all the simple and compound glands
of the animal body. How singular if a prin-
ciple so widespread should be violated in the
instance of the lungs !

Respiratory Organs of Reptiles.

Temporary branchiae of Amphibia. In the
life of all batrachian reptiles, the period which
immediately follows the emergence of the
young from the ovum is remarkable for the
existence of organs capacitating the animal to
live in water. In different genera these or-
gans vary in duration of existence. The larvas
of the frog retain the external branchiae only
for a few days, after which these organs be-
come internal. Those of the toad remain in
the egg state for a longer, and in that of the
fish condition for a shorter, time than those of
the frog. The tadpoles of the terrestrial sala-
manders of this country retain the external
gills only for a brief interval, early assuming
an exclusively atmospheric life. Those of
the aquatic species, exemplified in the familiar
tritons of our pools, carry the external bran-