22 INTRODUCTION. 
fices. there are many in which the nutritive juices being ab- 
sorbed by the parietes of the intestine, are immediately dif- 
fused throughout the whole spongy substance of the body: 
such, it would appear, is the case with all Insects. But from 
the Arachnoides and Worms upwards, the nutritive fluid circu- 
lates in a system of closed vessels, whose ultimate ramifications 
alone dispense its molecules to the parts that are nourished by 
it; the vessels that convey it are called arteries, those that 
bring it back to the centre of the circulation, ves. The 
circulating vortex is here simple, and there double and even 
triple (including that of the vena porte); the rapidity of its 
motion is often assisted by the contractions of a certain fleshy 
apparatus called a heart, which is 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 from the intestines, white or transparent, and is then 
termed chyle; it is poured into the veins where it mingles 
with the blood, by two peculiar vessels called /acteals. Ves- 
sels similar to these lacteals, and forming with them an arrange- 
ment called the lymphatic system, also convey to the venous 
blood the residue of the nutrition of the parts and the pro- 
ducts of cutaneous absorption. 
Before the blood is fit to nourish the parts, it must expe- 
rience from the circumambient element the modification of 
which we have previously spoken. In animals possessing a 
circulating system, one portion of the vessels is destined to 
carry the blood into organs in which they spread it over a 
great surface to obtain an increase of this elemental influence. 
When that element is air, the surface is hollow, and is called 
lungs; when it is water, it is salient, and is termed branchiz. 
There is always an arrangement of the organs of motion for 
the purpose of propelling the element into, or upon, the organ 
of respiration. , 
In animals destitute of a circulating system, air is diffused 
through every part of the body by elastic vessels called tra- 
chez ; or water acts upon them, either by penetrating through 
vessels, or by simply bathing the surface of the skin. The 
respired, or purified blood is properly qualified for restoring 
a = 
