ees SSS 
808 CIRCULATING SYSTEM. 
either white or transparent ; and is conveyed into 
the venous system, by means of particular vessels 
called lacteals, where it mixes with the blood. 
Other vessels similar to the lacteals, and compos- 
ing with them one arrangement, called the lym- 
phatic system, convey into the venous system 
those nutritive particles which have either escap- 
ed the lacteals, or have been absorbed through 
the cuticle or outer skin. Before the blood is 
fitted to renovate the substance of the several 
parts of the body, it must receive, from the sur- 
rounding element, through the medium of res- 
piration, that modification which we have al- 
ready noticed. One part of the vessels belonging 
to those animals, which possess a circulating sys- 
tem, is destined to convey the blood to certain 
organs, where it is distributed over a large extent 
of surface, in order that the action of the surround- 
ing element may be the more energetic. When 
the animal is adapted for breathing the air this 
organ is hollow, and called lungs; but when the 
animal only breathes the air dissolved in water, 
the organ projects, and is called branchie, or gills. 
Certain organs of motion are always arranged so 
as to draw the surrounding element either within 
or upon the organ of respiration. 
In animals which do not possess a circulating 
system, the air penetrates into every part of the 
body, through elastic vessels called trachee,; or 
else water acts upon them, either by penetrating, 
in a similar manner, through vessels, or simply 
by being absorbed through the surface of the 
skin. In man, respiration is performed by means 
of the pressure and elastic force of the air, which 
rushes into the lungs, where a vacuum would 
otherwise have been formed by the elevation of 
the ribs, and the depression of the diaphragm. 
Muscular force then expels the air, after the ne- 
cessary purification of the blood existing in the 
lungs has been performed ; and the same actions 
are again repeated. The blood, which was of a 
dark purple colour, while slowly travelling from 
all parts of the hody to the heart, has no sooner 
been purified by yielding its excess of carbonic 
acid to the surrounding air, and by absorbing 
oxygen, than its colour changes into a bright ver- 
milion. In birds, it was necessary to combine 
lungs of small bulk with an extensive aeration of 
the blood; and, accordingly, the blood not only 
passes into the lungs, but through them into 
capacious air cells; from which, by the action of 
the chest, it is again expelled. The lungs thus 
act twice upon the same portion of air. The 
change of the tadpole into the frog is accompa- 
nied by extraordinary alterations in its respira- 
tory organs. In the first, or tadpole state, the 
organs are branchial, in the frog they are pulmo- 
nary. The arrangements are striking and sin- 
gular. 
All respiration must be either aquatic or atmo- 
spheric. In the former case, the respiration is 
said to be cutaneous or branchial, according as it is 
performed through the skin or through gills. On 
CIRSIUM. 
the other hand, atmospheric respiration may be 
either tracheal or pulmonary, according as it is 
performed through the air-tubes called trachez, 
or by means of lungs. 
After the blood has been purified by respira- 
tion, it is fitted to restore the composition of all 
parts of the body, and to execute the function of 
nutrition properly so called. The wonderful pro- 
perty, possessed by the blood, of decomposing it- 
self so as to leave precisely, at each point, those 
particular kinds of particles which are there most 
wanted, constitutes the mysterious essence of 
vegetative life. We lose all traces of the secret 
process by which the restoration of the solids is 
performed, after having arrived at the ramified 
extremities of the arterial canals. But in the 
preparation of fluids we are able to trace appro- 
priate organs, at once varied and complicated. 
Sometimes the minute extremities of the vessels 
are simply distributed over extended surfaces, 
from which the liquid exudes; and sometimes | 
the liquid runs from the bottom of minute cavi- 
ties. But the more general arrangement is, that 
the extremities of the arteries, before changing 
into veins, form particular vessels called capillary, 
which produce the requisite fluid at the exact 
point of union between these two kinds of ves- 
sels. The blood-vessels, by interlacing with the 
capillary vessels which we have just described, 
form certain bodies called conglomerate or secre- 
tory glands. 
With all animals destitute of a circulation, and 
especially with insects, the nutritive fluid bathes 
the solid parts of the body; and each of them 
imbibes those particles necessary for its suste- 
nance. If it become requisite that any particular 
fluid should be secreted, capillary vessels, adapted 
for this purpose, and floating in the nutritive 
fluid, imbibe, through their pores, the elements, 
necessary for the composition of the fluid to be 
secreted. It is thus that the blood continually 
renovates all the component parts of the body, 
and repairs the incessant loss of its particles, re- 
sulting necessarily from the continued exercise 
of the vital functions. The general idea which 
we are able to form of this process is sufficiently 
distinct, although the details of the operations 
performed at each particular point are involved 
in obscurity, from our ignorance of the precise 
chemical composition of each part, and our con- 
sequent inability to determine the exact condi- 
tions necessary for their reproduction. 
CIRSIUM. A large genus of hardy herbaceous 
plants, of the thistle division of the composite 
order, It comprises the greater portion of the 
old genus cnicus or plume-thistle, about a dozen 
species which were formerly included in the ge- 
nus carduus or true thistle, and a large number 
of species peculiarly its own. The total number 
of its species is nearly one hundred; and the 
number of its indigenous species is eight. Some 
of the latter are excessive pests on poor soils and 
badly cultivated lands, and are well and sorrow- 
