| OCR Text |
Show 12 INTRODUCTION. which their nutritive fluid penetrates all other parts through pores or vessels, which are a ki~d of inte:nal roots. The organization of this cavity and Its appurte.nances required varying, according to the nature of the aln~en~, ~nd the operation it had to undergo, before it could furmsh JUICes :fit for absorption; whilst the air and earth present. to vegetables nought but elaborated juices ready for absorptwn .. T he animal whose functions are more numerous and varied ' . than those of the plant, consequently necessitated an orgam~a-tion much more complete ; besides this, its parts not bemg capable of preserving one fixed relative position, there w~re no means by which external causes could produce the motw.n of their fluids which required an exemption from atmospheric influence • fr:m this originates the second character of animals, their cir:ulating system, one less essential than that of diges~ tion, since in the more simple animals it is unnecessary. The animal functions required organic systems, not needed by vegetables- that of the muscles for volun~ary motion, an~ nerves for sensibility; and these two systems, hke the rest, actin? only through the motions and transformations of the fluids, It was necessary that these should be most numerous in animals, and that the chemical composition of the animal body be more complex than that of the plant; and so it is, for one su?sta~ce more (azote) enters into it as an essential element, whilst In plants. it is a mere accidental junction with the three other general elements of organization, oxygen, hydrogen, and carbon. This then is the third character of animals. From the sun and atmosphere, vegetables receive for their nutrition water, which is composed of oxygen and hydrogen; air, which contains oxygen and azote; and carbonic acid, which is a combination of oxygen and carbon. To extract their own composition from these aliments, it was necessary they should retain the hydrogen and carbon, exhale the superfluous oxygen and absorb little or no azote. Such, in fact, is vegetable life, whose essential function is the exhalation of oxygen, which is effected through the agency of light. Animals also derive nourishment, directly or indirectly, ft·om the vegetable itself, in which hydrogen and carbon form INTRODUCTION. 13 the principal parts. To assimilate them to their own composition, they must get rid of the superabundant hydrogen and carbon in particular, and accumulate more azote, which is performed through the medium of respiration, by which the oxygen of the atmosphere combines with the hydrogen and carbon of their blood, and is exhaled with them in the form of water and carbonic acid. The azote, whatever part of the 'body it may penetrate, seems always to remain there. The relations of vegetables and animals to the surrounding atmosphere are therefore in an inverse ratio-the former reject water and carbonic acid, while the latter produce them. The essential function of the animal body is respiration, it is that which in a manner animalizes it, and we shall see that the animal functions are the more completely exercised, in proportion to the greatness of the powers of respiration possessed by the animal. This difference of relations constitutes the fourth character of animals. Of the forms peculiar to tlie Organic Elements of the .!lnimal Body, and of the principal combinations of its Chemical Elements. An areolar tissue and three chemical elements are essential to every living body; there is a fourth element peculiarly requisite to that of an animal; but this tissue is composed of variously formed meshes, and these elements are variously combined. There are three kinds of organic materials or forms of texture, the cellular membrane, the muscular fibre, and the medullary matter, and to each form belongs a peculiar combination of chemical elements, as well as a particular function. The cellular substance is composed of an infinity of small fibres and laminre, fortuitously disposed, so as form little cells tha: communicate with each other. It is a kind of sponge, Which has the same form as the body, all other parts of which traverse or fill it, and contracting indefinitely, on the removal of the causes of its tension. It is this power that retains the body in a given form and within certain liQJ.its. |
