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, veins. The 

 circulating vortex is here simple, and there double and even 

 triple (including that of the vena portse) ; 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 lacteals. 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 hranchise. 

 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- 

 chese; 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 



