839 
ZOOLOGY. 
up of two distinct sets of fibres laid upon one another, in 
the form of strata; those which compose the outer set have 
their origin around the root of the aorta, and in a spiral 
manner surround the ventricle to its apex or point, where 
they terminate, after having made a close half turn. The 
fibres of the inner set, or stratum, are similar to those of the 
outer in their origin, in the mode of surrounding the cavity, 
and in their termination ; but their direction is quite the re¬ 
verse; they decussate the outer set in their whole course; 
and where the two sets terminate, they are both blended into 
one mass. There is an advantage gained by this disposition 
of fibres over every other in the body, which adapts the ven¬ 
tricle so perfectly for its office, that it would almost be im¬ 
possible to construct it in any other way, so as to answer the 
purpose for which it is intended, in this muscle, the fibres, 
by their spiral direction, are nearly one-fourth part longer 
than the distance between their origin and termination ; and 
the actions of the two sets being in different directions, ren¬ 
ders only one-half the quantity of contraction in each fibre 
necessary, that would have been otherwise required; while 
the turn both sets make in opposite directions at the apex of 
the ventricle, fixes it, and prevents lateral motion. In the 
action of the ventricle, two different effects are produced; 
the first brings the apex nearer to the basis, by which means 
the vis inertin', of the blood will be overcome where the re¬ 
sistance is least, and a direction given to its motion in the 
course of the aorta; the second brings the sides nearer toge¬ 
ther, which will accelerate the motion of the blood already 
begun ; and tire spiral direction of the fibres renders the 
power which is applied more uniform through the whole of 
that action, than it could have been made by any other 
known form of muscle. By this beautiful mechanism, the 
muscular fibres of the left ventricle of the heart, perform 
their office with a smaller quantity of contraction compared to 
their length (although in themselves proportionally longer,) 
than those of any other muscle in the body, and, conse¬ 
quently, produce a greater effect in a shorter time. The 
right ventricle is situated upon the outside of the left, with 
which it is firmly united; it is not oviform in its shape, but 
triangular ; nor is it uniform in its structure, being made up 
of two portions, whose fibres have a very different distri¬ 
bution. The portion of the ventricle, which makes a part 
of the septum of the heart, consists of only one set of fibres, 
similar in their direction to those of the stratum underneath, 
belonging to the left ventricle, but from being considerably 
shorter, they are more oblique than the spiral ones; and at 
the edge of the cavity they are blended with the fibres of the 
■opposite portion. That portion which is opposite to the 
septum, is composed of three sets of fibres; those of the exter¬ 
nal set are nearly longitudinal, the two others which lie under 
it, decussate each other, and are obliquely transverse in their 
direction, one passing a little upwards, the other downwards, 
and both terminate upon the edge of the septum. In the 
structure of this muscle, we find none of the mechanical ad¬ 
vantages so obvious in the left ventricle: the want of these, 
however, is in tome measure compensated by its situation; 
for the blood contained in its cavity will have the vis inert ice 
overcome, and a direction given to its course by the action of 
the apex of the left ventricle; that motion only requiring to 
be continued and accelerated, for which purpose this muscle 
is very well calculated, and in which it is also assisted by 
the lateral swell of the septum into its cavity during the con¬ 
traction of the left ventricle.” 
Of the respiratory system.—Among the vermes, some 
orders, as those which inhabit corals, the proper zoophytes, 
and perhaps the intestinal worms, appear to be entirely des¬ 
titute of particular respiratory organs ; so that the function is 
carried on in these animals by that invisible imbibition 
which affords them nourishment. 
Those vermes, which are furnished with proper organs of 
respiration, have the same variety in their structure which 
was remarked in insects. Some, as the cuttle-fish, oyster, 
&c. have a species of gills, varying in structure in different 
instances. But the greatest number have air-vessels or tra¬ 
cheae. Several of the testaceous vermes have both kinds of 
respiratory organs. In some of the inhabitants of bivalve 
shells, as the genus Venus, the air-vessels lie between the 
membranes of a simple or double tubular canal, found at 
the anterior part of the animal, and capable of voluntary ex¬ 
tension and retraction. It serves also for other purposes, as 
laying the eggs. The margins of its mouth are beset with 
the openings of the tracheae. 
In the terrestrial gasteropodous mollusca, of which we 
may instance the snail and slug, there is a cavity in the 
neck receiving air by a small aperture, which can be opened 
or shut at the will of the animal. The pulmonary vessels 
ramify on the sides of the cavity. 
Many aquatic insects, as the genus cancer, have a species 
of gills near the attachment of their legs. The others, and 
particularly the land-insects, which constitute, as is well 
known, by far the greatest number of this class of animals, 
are furnished with air-vessels or tracheae, which ramify over 
most of their body. These tracheae are much, larger and 
more numerous in the larva state of such insects as undergo 
a metamorphosis (in which state also the process of nutrition 
is carried on to the greatest extent) than after the last, or, as 
it is called, the perfect change has taken place. 
In this class of animals the scorpions, being also provided 
with fins, present an extraordinary instance of an animal, 
which, though living nearly in the air, breathes like fishes. 
A large air-tube (trachea) lies under the skin on each 3ide 
of the body of larvae, and opens externally by nine aper¬ 
tures (stigmata): it produces on the inside the same number 
of trunks of air-vessels, (branchiae) which are distributed 
over the body in innumerable ramifications. 
Both the tracheae and branchiae are of a shining silvery 
colour; and their principal membrane consists of spiral 
fibres. The most numerous and minute ramifications are 
distributed on the alimentary canal; particularly on the 
corpus adiposum. 
There is a great variety in the number and situation of 
the external openings by which insects receive their air. 
In most instances the stigmata are placed on both sides of 
the body. The atmospheric air enters by an opening at the 
end of the abdomen in several aquatic larvae, and even per¬ 
fect insects. A very remarkable change in this respect takes 
place in several animals of this class during their metamor¬ 
phosis. Thus in the larva of the common gnat (culex pi- 
piens), the air enters by an opening on the abdomen ; while 
in the nympha of the same animal, it gains admission by 
two apertures on the head. 
Fishes breathe with gills or branchiae; which are placed 
behind the head, on both sides, and have a moveable gill- 
cover (operculum kranchiale), which is wanting in the order 
of pisces chondropterygii only. By means of these organs, 
which are connected with the throat, the animal receives its 
oxygen from the air contained in the water; as those ani¬ 
mals which breathe, derive it immediately from the atmos¬ 
phere. They afterwards discharge the water through the 
branchial openings (aperturae branchiales). 
The gills receive the venous blood by means of the bran¬ 
chial artery, and this blood is sent into the aorta after its 
conversion into the arterial state. The distribution of these 
vessels on the folds and divisions of the gills constitutes one 
of the most delicate and minute pieces of structure in the 
animal economy. 
Each of the gills consists, in most fishes, of four divi¬ 
sions, resting on the same number of arched portions of bone 
or cartilage, connected to the os hyoides. Generally there 
is only a single opening for the discharge of the water; but 
in many cases, particularly among the cartilaginous fishes, 
there are several openings. 
The lungs of amphibia are distinguished from those of 
warm-blooded animals, both by a great superiority in point 
of size, as well as by a greater looseness of texture; which 
circumstances are serviceable in swimming in many of these 
animals. 
It is well known that the lungs of turtles and frogs do not 
collapse on opening the animals, like those of mammalia, but 
often remain expanded, at least partially, for some time. 
In 
