SOURCES AND UTILIZATION OF PHYSIOLOGICAL MATERIAL 15 



As the cells of the embryo increase in number in development there is a 

 corresponding differentiation of function among the groups of cells. The 

 various functions which the original cell may be supposed to discharge, and 

 the various properties it may be supposed to possess, become divided among 

 groups of cells in which the work of each group is specialized. As a result 

 of this division of labor the functions and properties are developed and made 

 more perfect, with a view to the more economic and effective accomplishment 

 of the activities of the body as a whole. 



In studying the functions of the human body it is necessary first of all to 

 know of what it is 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 function of 

 the same 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 assistance 

 which the science of Chemistry can afford to the physiologist. 



Having mastered 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 or- 

 ganism is at work. The subject includes not only the observation of the mani- 

 fest 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. 



Sources and Utilization of Physiological Material. It may be well 

 to mention as a preliminary that the physiological information which we have 

 at our disposal has been derived from many sources, the chief of which are 

 as follows: From actual observation of the various phenomena occurring in 



