CELL DIFFERENTIATION 1 5 



composed, of what tissues and organs it is made up; this can of course be 

 ascertained only by the dissection of the dead body, and thus it comes that 

 Anatomy, the science which treats of the structure of organized bodies, is 

 closely associated with physiology, which treats of the functions of these 

 structures. So close, indeed, is the association that Histology, which is 

 especially concerned with the minute or microscopic structure of the tissues 

 and organs of the body and which is, strictly speaking, a department of 

 anatomy, is often included in works on physiology. There is much to be 

 said in favor of such an arrangement, since it is impossible to consider the 

 changes which take place in any tissue during life, apart from the knowledge 

 of the structure of the tissues themselves. There is indeed an almost insep- 

 arable relation between the structure and the function of the differentiated 

 animal body in which the one is made the means to a knowledge of the other 

 as an end, and vice versa, according to the aims and purposes of the student. 



An equally important essential to the right comprehension of the changes 

 which take place in the living organism is a knowledge of the chemical com- 

 position of the body. Here, however, we can deal directly only with the 

 composition of the dead body, and it is well at once to admit that there may 

 be many chemical differences between living and non-living tissues; but as it 

 is impossible to ascertain the exact chemical composition of the living tissues, 

 the next best thing which can be done is to find out as much as possible about 

 the composition of the same tissues after they are dead. This is the assist- 

 ance which the science of Chemistry can afford to the physiologist. 



Having considered the structure and composition of the body, we' are 

 brought face to face with physiology proper, and have to investigate the vital 

 changes which go on in the tissues, the various actions taking place as long 

 as the organism is at work. The subject includes not only the observation 

 of the manifest processes which are continually taking place in the healthy 

 body, but the conditions under which these are brought about, the laws 

 which govern them and their effects. 



It may be well to mention as a preliminary that the physiological informa- 

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

 chief of which are as follows: i, From actual observation of the various 

 phenomena occurring in the human body from day to day, and from hour to 

 hour, as, for example, 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 com- 

 parative anatomy approximate the human body in structure, and may be 

 supposed to be similar in function; 3, from observations of the changes 

 produced by experiment upon the various processes in such animals, or in the 

 organs and tissues of animals; 4, from observations of the changes in the 

 working of the human body produced by diseases; and 5, from observations 

 upon the gradual changes which take place in the functions of organs when 



