18 INTRODUCTION. 



The canal in which this first act of nutrition is performed is a con- 

 tinuation of the skin, and is composed of similar layers ; even the fibres 

 that encircle it are analogous to those which adhere to the internal sur- 

 face of the skin, called the fleshy pannicle. Throughout the whole in- 

 terior of this canal there is a transudation 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 re- 

 sembling 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 Arachnides and Worms upwards, the nutritive 

 fluid circulates in a system of closed vessels, whose ultimate ramifica- 

 tions alone dispense its particles 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 

 portas) ; the rapidity of its motion is often assisted by the contractions 

 of a certain fleshy apparatus called the 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 pre- 

 viously spoken. In animals possessing a circulating system, one portion 

 of the vessels are destined to carry the blood into organs, in which they 

 spread it over a great surface to obtain an increase of this elemental in- 

 fluence. When that element is air, the surface is hollow, and is called 

 lungs ; when it is water, it is salient, and is termed branchcei. 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 



