THE FUNCTIONS 01' OlKi AM/HI) CELLS. 25 



going change, are the most important; there is added the apparatus by 

 means of which the altered food stuffs are absorbed or reach the two 

 systems of blood-vessels already mentioned, and a muscular apparatus 

 contained in the walls of the intestinal tube by means of which that 

 part of the food which is not fit for absorption is removed from the 

 body. In addition to this excretory apparatus we have another, the 

 kidneys, which are concerned with the removal of certain substances 

 from the blood which have served their purpose in the economy. 



Then we have the muscular system, which by its special power of 

 contraction is capable of bringing about all the movements of the body 

 those of the frame, the head, arms, legs, etc., as well as those of the 

 heart, the vessels, the alimentary canal, and the like. The nervous sys- 

 tem, by the aid of which the processes of the living body may be regu- 

 lated and controlled. Lastly, we have a special system that of the 

 generative system, by means of which the reproduction of the species 

 may take place. 



To these subjects, the merest outline of which has been here 

 sketched, our attention has to be given in the succeeding chapters, but 

 it may be well to mention as a preliminary that the information about 

 them which we have at our disposal has been derived from many sources, 

 the chief of which are as follows: 



(1.) From actual observation of the various phenomena occurring in 

 the human body from day to day, and from hour to hour, as, for exam- 

 ple, the estimation of the amount and composition of the ingesta and 

 egesta, the respiration, the beat of the heart, and the like; 



(2.) From observations upon other animals, the bodies of which we 

 are taught by comparative anatomy approximate to the human body 

 in structure; 



(3.) From observations of the changes produced by experiment upon 

 the various processes in such animals; 



(4.) From observations of the changes in the working of the human 

 body produced by disease; 



(5.) From observations upon the gradual changes which take place 

 in the functions of organs when watched in the embryo from their 

 earliest beginnings to their completed development. 



In accordance with the plan sketched out above, the next chapter 

 will be devoted to a consideration of the minute structure of the ele- 

 mentary tissues, and the one after that to a preliminary account of the 

 chemical composition of the body. These two chapters will serve as an 

 introduction to the study of the problems of physiology proper, which 

 will be commenced in Chapter V. 



