GENERAL ANIMAL FUNCTIONS. 77 



time come to be made up of millions of cells, each cell 

 of which finally does some part of the general work. Be- 

 cause of this division of work the cells come to look very 

 different from the original cell and from each other more 

 varied indeed than are the single-celled animals in Fig. 7 . 

 Some of these cells, varied to do different kinds of work, 

 may be seen in Fig. 8. The essential contents of these 

 cells are still nothing but protoplasm and the substances 

 which protoplasm "builds up; but it is clear, from the 

 different kinds of work that they do, that there are some 

 real differences in the protoplasm. 



It must be borne in mind that the cells in the complex 

 animal must do their work individually, just as the simple 

 cell did its work. It is their united work that makes the 

 life of the organism. 



Such groups of similar cells are known as tissues. 

 Several kinds of tissue may be associated into organs; 

 and these various complex organs divide up the work 

 necessary to be done among themselves. In this way it 

 is more perfectly done and a higher type of animal body 

 is possible. We shall now examine some of the more 

 important functions, and the organs by which they are 

 performed, in the higher animals. 



82. Metabolism or Nutrition ; The Central Function. 



Under this head we shall include all the processes directly 

 connected with the use of food. It includes the taking 

 of food into the stomach (ingestiori) ; changing it in such 

 a way as to get it from the stomach into the blood (diges- 

 tion); carrying it to all parts of the body (circulation); 

 taking it up from the blood by the protoplasm of the 

 individual cells, and building it up into the cell substance 

 (assimilation); the getting of oxygen from the air or 



