106 ELEMENTARY PARTS OF ANIMAL STRUCTURES. 



forming the several changes, by which the fabric itself is built up and 

 maintained in a condition fit for the performance of its peculiar opera- 

 tions. These operations are the phenomena of sensation, of spontaneous 

 motion, and of mental action. They are the great objects of Animal 

 existence ; just as the combination of elements into organic substances, 

 that are to furnish the materials of the Animal fabric, seems to be the 

 great purpose of Vegetative Life. The vital phenomena which are 

 peculiar to Animals, are manifestations of the properties of certain forms 

 of organized matter, the Nervous and Muscular tissues, which are 

 restricted to themselves ; just as those which are common to Animals 

 and Plants, are effected by organized structures which are found alike 

 in both kingdoms. Here, then, we have one essential distinction between 

 these kingdoms ; namely, the presence in Animals of a peculiar appa- 

 ratus, and the consequent possession by them of peculiar endowments, 

 which are totally wanting in Plants. There are, it is true, many species, 

 indeed whole tribes, in which it is impossible to say with certainty, how 

 far sensibility and spontaneity of action may be justly inferred from the 

 movements they exhibit ; so that, their structure being so simple as to 

 afford no distinctive characters, our assignment of them to the Animal 

 or to the Vegetable kingdom must be determined entirely by the mode 

 in which they obtain the materials of their nutrition ( 62, 63). 



165. All the operations, then, which are common to Animals and 

 Plants, are concerned in the building up of the organized fabric, in the 

 maintenance of its integrity, and in the preparation of the germs of new 

 structures, to compensate for the loss of the parent by death. These 

 operations, as formerly explained (41), involve a series of very distinct 

 processes ; which, though all performed by the simple cell of the humblest 

 plant, are distributed in more complex structures through a number of 

 parts or organs, whose several actions are almost as separate as those 

 of the dissimilar machines of the cotton-mill, although, like them, sus- 

 tained by the same powers, and so far mutually dependent, that neither 

 of them can be suspended without in a short time putting a stop to the 

 rest. Now just as in each of the machines of the cotton-mill we may 

 have similar elements, such as wheels, levers, pulleys, bands, &c., put 

 together in different methods, and consequently adapted for different pur- 

 poses, as carding, spinning, weaving, &c., so shall we find in the animal 

 body, that these different organs are composed of very similar elements, 

 and that the individual actions of these elementary parts are the same; 

 but that the difference of result is the consequence of the variety in their 

 arrangement. Thus we shall find that the growth of cells, their absorp- 

 tion of certain matters from the surrounding fluids, and their subsequent 

 liberation of these by the bursting or liquefying of the cell-wall when 

 their term of life is come to an end, are means employed in one part of 

 the body to introduce nutrient materials into the current of the circu- 

 lation, whilst in another the same processes are used as means to with- 

 draw, from that very same current, certain substances of which it is 

 necessary to get rid. Now certain combinations of elementary struc- 

 ture, adapted to the performance of a set of actions tending to one pur- 

 pose, and thus resembling one of the machines of a cotton-mill, is termed 

 an organ ; and the sum-total of its actions is termed its function. Thus 



