INTRODUCTION. 
27 
arteries, and those which bring it back to the centre of the circulation are termed veins. 
The circulating vortex is sometimes simple, sometimes double, and even triple (includ- 
ing that of the vena porta) ; the rapidity of its movements is often aided by the contrac- 
tions of a certain fleshy apparatus denominated hearts, and which are placed at one or 
the other centres of circulation, and sometimes at both of them. 
In the red-blooded vertebrated animals, the nutritive fluid exudes white or transpa- 
rent from the intestines, and is then termed chyle ; it is poured by particular vessels, 
named lacteals, into the venous system, where it mingles with the blood. Vessels 
resembling these lacteals, and forming with them what is known as the lymphatic 
system, also convey to the venous blood the residue of the nutrition of the parts and 
the products of cutaneous absorption. 
Before the blood is proper to nourish the several parts, it must experience from the 
ambient element, by respiration, the modification of which we have already spoken. In 
animals which have a circulation, a portion of the vessels is destined to carry the blood 
into organs, where they spread over an extensive surface, that the action of the ambient 
element might be increased. When this element [or medium] is the air, the surface is 
hollow, and is called lungs ; when water, it is salient, and termed gillsJ^ There are 
always motive organs disposed for propelling the ambient element into, or upon, the 
respiratory organ. 
In animals which have no circulation, the air is diffused through every part of the 
body by elastic vessels, named trachea ; or water acts upon them, either by pene- 
trating through vessels, or by simply bathing the surface of the skin. 
The blood which is respired is qualified for restoring the composition of all the parts, 
and to effect what is properly called nutrition. It is a great marvel that, with this 
facility which it has of becoming decomposed at each point, it should leave precisely 
the species of molecule which is there necessary ; but it is this wonder which consti- 
tutes the whole vegetative life. For the nourishment of the solids, we see no other 
arrangement than a great subdivision of the extreme arterial ramifications ; but for 
the production of liquids, the apparatus is more complex and various. Sometimes 
the extremities of the vessels simply spread over large surfaces, whence the produced 
fluid exudes ; sometimes it oozes from the bottom of little cavities. Very often, before 
these arterial extremities change into veins, they give rise to particular vessels that 
convey this fluid, which appears to proceed from the exact point of union between the 
two kinds of vessels ; in this case, the blood-vessels and these latter termed especial, 
form, by their interlacement, the bodies called conglomerate or secretory glands. 
In animals that have no circulation, and particularly insects, the nutritive fluid 
bathes all the parts ; each of them draws from it the molecules necessary for its suste- 
nance : if it be necessary that some liquid be produced, the appropriate vessels float in 
the nutritive fluid, and imbibe from it, by means of their pores, the constituent elements 
of that liquid. 
It is thus that the blood incessantly supports all the parts, and repairs the altera- 
tions which are the continual and necessary consequence of their functions. The 
* It may be remarked here, that, in strictness of language, no 
animals respire water, but the air which is suspended in water, and 
which has been ascertained to contain more oxygen than that of the 
free atmosphere. The elements of water, it should he remembered, are 
chemically combined, while those of air are only mechanically mixed. 
To obtain oxygen from the one, therefore, decomposition is required ; 
from the other, no disunion. Tlie only distinction, then, in the 
respiration of animals is, that some breathe the free air, and are sup- 
plied w’ith lungs, and others that diffused in water, and have there- 
fore gills : but even this difference, however, is more apparent than 
real, as in all cases the respiratory surface requires to be moist or wet, 
in order to perform its function. Deprive water of its air by boiling it, 
and it cannot support life. — E d. 
