INTRODUCTION. 25 



It may be well to point out here a distinction that divides the 

 tissues of active cellular function into two groups. The first group, 

 including the various modifications of epithelium, displays its ac- 

 tivity in the elaboration of material products, taking the form of 

 either new cells which are continually being produced, or of certain 

 chemical substances which appear as a secretion. The second 

 group, comprising the muscular and nervous tissues, exercises its 

 functional activities in the storage of latent energy in such sub- 

 stances of unstable chemical nature and in such a manner that it 

 can be liberated when required and directed toward the accomplish- 

 ment of some definite purpose. The functions of both groups 

 require an active intracellular metabolism, resulting in the forma- 

 tion of particular chemical substances. In this they are alike. 

 But in the first group the production of those substances is, in 

 itself, the functional purpose of the process, while in the second 

 group those substances are merely a means for holding energy in 

 the latent condition. If we may so express ourselves, the first 

 group utilizes energy for the elaboration of material, the second 

 group elaborates material for the utilization of energy. 



In the adult, under normal conditions, each kind of cell, if it 

 reproduce at all, gives rise to cells only of its own kind. But when 

 the conditions are morbid, this is not always the case, the progeny 

 of a given cell under abnormal conditions may appear less highly 

 specialized than the parent stock or may differentiate in a some- 

 what different way. These departures from the normal appear 

 never to transcend the degree of specialization that is marked by 

 the formation of the three embryonic layers in the early history 

 of development ; for example, epithelium which springs from 

 either the entoderm or ectoderm never gives rise to bone or any 

 other variety of connective tissue normally developed from the 

 mesoderm. 



The cells of the body differ greatly in their individual perma- 

 nency. Some are apparently destined to endure throughout the life 

 of the individual. Among these are the nerve-cells and those form- 

 ing muscular tissue. Other cells have a comparatively short life- 

 history. This is true of many of the epithelial cells, which undergo 

 transformations, ultimately leading to their destruction — e. g., the 

 superficial cells of the epidermis and those cells which yield the 

 secretion of sebaceous glands ; or they are destined to proliferate, 



