INTRODUCTION. 17 



Even among those where the intestinal canal has two orifices, there are 

 many in whicli the nutritive juices, being absorbed by the parietes of the 

 intestine, are immediately diiFused throughout the whole spongy substance 

 of the body : such, it would appear, is the case with all Insects. But 

 from the Arachnides and Worms upwards, the nutritive fluid circulates 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-portce) ; 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 a set of peculiar vessels 

 called lacteals. Vessels similar to these lacteals, and forming with them 

 an arrangement called 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 fit to nourish the parts, it must experience 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 branchice. There is always an ar- 

 rangement of the organs of motion for the purpose of propelling the ele- 

 ment 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 trachece; or water acts 

 upon them, either by penetrating through vessels, or by simply bathing 

 the surface of the skin. The respired or purifled blood is properly quali- 

 fied for restoring the composition of all the parts, and to effect what is pro- 

 perly called nutrition. This facility, whicli the blood possesses, of decom- 

 posing itself at every point, so as to leave there the precise kind of mole- 

 cule necessary, is indeed wonderful; 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 ra- 

 mifications ; but for the production of fluids the apparatus is more complex 

 and various. Sometimes the extremities of the vessels simply spread 

 themselves over large surfaces, whence the produced fluid exhales; at 



VOL. I. 



