30 -^n Introduction to Zoology. 



recovered at the dawn of day, when they at once proceed to decompose 

 the stock of carbonic acid that has accumulated during the night, in addi- 

 tion to the fresh supplies continually afforded to them from the source above 

 specified, and thus send forth pure streams of oxygen, fitted for the purpose 

 of animal respiration, thence to return to them fraught with the carbon 

 they require in this wonderfully adjusted circulation of nature. 



The sap then, thus modified by the additional quantity of the great 

 vcetable nutriment, carbon, which it has acquired, and now being alto- 

 gether analogous in its functions to the blood of animals, descends from 

 the leaves, through the intercellular spaces of the innermost layer of bark 

 and outermost of the wood, and deposits in each part that it traverses, the 

 materials required for its development or support ; while by vital powers, 

 analogous to those which reside in animal glands, it is made to yield at 

 particular points the specific secretions and products required, whether 

 gummy, saccharine, amylaceous, or ligneous. 



Such is the simplest form of vital nutrition, that whicli prevails in the 

 vegetable kingdom j having traced which, we shall proceed to its more 

 complicated relations in the animal kingdom. 



Animals do not, like vegetables, immediately imbibe the nutritive fluid 

 from the surrounding elements, in a state of nearly perfect preparation 

 from the first, but they have to derive it from the animal or vegetable 

 substances on which they feed, by a long train of previous preparation and 

 elaboration, which is called the digestive process. Digestion is carried on 

 in the alimentary caual, which for this purpose is expanded into cells 

 called stomachs, and thence protracted into a long intestinal tube, con- 

 torted in order to pack the greatest length into the smallest space. The 

 food passes from the mouth through the gullet, or sesophagus, into the 

 stomach, being first comminuted either by tiie teeth of the mouth, or in 

 some instances by muscular gizzards, or small bony masses, accommodated 

 for the purpose, and placed in an introductory stomach. In the true 

 stomach the food thus presented is dissolved and digested, by the chemical 

 action of a specific fluid, the gastric juice, secreted in the glandular struc- 

 tures of the coats of the stomach, at the commencement of its passage 

 through the intestine. 



In all except the vi.ry lowest animals, the nourishment becomes mingled 

 with other peculiar fluids, the bile secreted by the liver, and generally with 

 a second fluid, the pancreatic juice from the pancreas. The nutritive por- 

 tion of the food thus becomes converted into a peculiar milky fluid, called 

 chyle, nearly agreeing in its chemical composition with blood, while the 

 non- nutritive portion is prepared for excretion. In the lowest animals, 

 however, as the zoophytes, mere digestion in the stomach seems to be all 

 that is required, the nutritive fluid becoming thus properly prepared, and 

 ready, as it appears, to be absorbed and diffused through the pores of their 

 mass, for they are supposed to be destitute of any regular apparatus of 



