INTRODUCTION. 27 



the whole interior of this canal there is a translation which has 

 some connexion with the cutaneous perspiration, and which becomes 

 more abundant when the latter is suppressed ; the absorption of the 

 skin is even very analogous to that of the intestines. It is in the 

 lowest order of animals that the useless residuum is rejected by the 

 mouth, their intestines resembling a sac, with but the one opening. 



Even among those where the intestinal canal has two orifices, 

 there are many in which the nutritive juices being absorbed by the 

 parietes of the intestine, are immediately diffused throughout the 

 whole spongy substance of the body : such, it would appear, is the 

 case with all Insects. But from the Arachnoides and Worms up- 

 wards, 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 portae) ; the rapidity of its 

 motion is often assisted by the contractions of a certain fleshy appa- 

 ratus 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. 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 branchiae. 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 trachea ; or water 

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



