INTRODUCTION. 27 



arteries, and those which bring it bacK to tne centre ot 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 lacteuls, 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 ])arts 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 (/ills* 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 diiTused through every part of the 

 body by elastic vessels, named trachece ; 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 sijecies 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 

 tlie extremities of the vessels simply spread over large surfaces, \^•hence 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 hquid. 



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, tltat, in striftness of ItLng-uagc, no 

 animals respire water, but the air which is Buspended in water, and 

 wliich has been ascertiiined to contniu more oxygen than that of the 

 free atmL)sphere. The elements of water, it should l>e remembered , are 

 chemically combined, while those of Fiir are only mechanically mixed. 

 To obtrtiii oxyjifcii from the one, therefore, decompositinn is required ; 

 frtim the other, no disunion. The only distinctlnn, then, in tlie 



respiration of animals is, that some breathe the free air, and are mi 

 plied with lungs, and others that diffused in water, and have the; 

 fore gills : but even this difference, however, is more apparent tli 

 real, as in iiU cases the respiratory surface requires to be moist or w 

 in order to perform iti function. Deprive water of ita air by boiling' 

 and it cannot support life. — Ed. 



