6 PHYSIOLOGY. 



to those in plant life. Thus we find that every tissue is composed of 

 minute parts known as cells and which in a particular tissue are 

 nearly or quite similar. For instance, in examining a muscular fiber, 

 we find that it is composed of very small, ribbonlike units called 

 muscle-cells. Although differing somewhat in size and development, 

 yet they are otherwise similar ; that is, muscular tissue is composed of 

 muscular units, or cells. Cartilage is composed of oystershell-shaped 

 cells; mucous-membrane cells are gobletlike, and secrete, or give off, 

 mucus. Even though these cells are self-supporting and grow and 

 form other cells, in the higher animals they are grouped and held 

 together by means of a kind of cement, spoken of as "intercellular 

 material." 



Hence a tissue may be defined as a group of similar cells having 

 a similar function. Tissues are different only because they are com- 



IF 



Fig. 1. Vegetable Cell. (DuVAL.) 

 up, Cell-wall of cellulose, n, Nucleus, ch, Chlorophyll bodies. 



posed of different kinds of cells having functions peculiar to ihem- 

 selves. An aggregation of cartilage- and muscle- cells gives us, 

 respectively, cartilage- and muscle- tissues. 



As the result of this knowledge, pl^siology is beginning to 

 develop from a science of the organ and its functions to that of the 

 cell and its functions. But this is only natural as a form of develop- 

 ment, since we first consider the greater and more active functions 

 of the organs and then delve down deeper and deeper until we reach 

 the functions of the cell. 



Cells are characterized by the presence of the elementary func- 

 tions or phenomena of nutrition, growth, reproduction, etc. If 

 physiology has to deal with them, it can do it most successfully by 

 studying them in their seat the cell. 



