Cuvierian System of Zoology, 101 



the nourishing juices, absorbed by the coat of the intestines, 

 are immediately spread through the whole spongy mass of the 

 body {la spongiosum du corps) ; for this appears to be the case 

 in the whole class of insects. 



In some of the orders of -^rachnides and of worms, the 

 nourishing juices circulate in a system of closed vessels, the 

 extreme ramifications of which distribute the molecules to the 

 parts that are to be supported by them. The vessels which 

 carry this nourishing fluid to the parts are called arteries ; 

 those which bring it back to the centre of circulation are 

 called veins. The circulation is sometimes simple, and some- 

 times double, or even triple, reckoning that of the abdo- 

 minal, or biliary, system. The rapidity of the circulation is 

 often aided by the contraction of certain fleshy organs (ap- 

 pareils), which are called hearts, and are placed at one or 

 other of the centres of circulation, and sometimes at both. 



In verteb rated animals, with red blood, the nourishing fluid 

 issuing from the intestines is white, and is called chyle ; it is 

 carried by particular vessels, called lacteals, into the venous 

 system, where it is mixed with the blood. Vessels similar to 

 the lacteals, and forming with them a system called the lym- 

 phatic, bring back into the venous blood the residue of the 

 nutrition, and the products of cutaneous absorption. 



In order that the blood may be rendered suitable for the 

 nourishment of the parts, it is necessary that it should receive 

 by respiration a certain modification from the ambient element. 

 In animals which have a system of circulation, a part of the 

 vessels carry the blood into certain organs, where they subdi- 

 vide it, and spread it over a great surface, that the action of 

 the ambient element may be more powerful. When the ele- 

 ment is air, the surface of the organs is cellular {cretise), and 

 they are called the lungs ; when the element is water, the sur- 

 face of the organs is saliant, and they are called gills. There 

 are always organs of motion, disposed in a manner to convey 

 the ambient element either into, or upon, the respiratory 

 organs. 



In animals which have no circulation the air is spread upon 

 all points of the body, by elastic vessels, called tracheae ; or the 

 water either penetrates the body through vessels, or only 

 bathes the surface of the skin. 



The blood which has undergone the process of respiration, 

 is fit to repair the composition of all the parts, and to effect 

 what is properly called nutrition. The facility with which the 

 blood is decomposed in every point of the body, so as to leave 

 there precisely the species of molecules which are requisite for 



H 3 



