THE CELL. 5 



for instance is made up of a variety of substances called tissues. 

 There - are bone-tissues, cartilaginous tissues, muscle-tissues, nerve- 

 tissues, etc., all different in structure, yet all bundled up in the mem- 

 ber called the arm and essential to it to perform its various functions. 

 The brain is composed of two distinct tissues the gray and white 

 tissues. So, in like manner, any of the organs of the body may be 

 resolved into various parts known as tissues. 



Thus far anatomy has aided us in our analysis of the various 

 parts of the body, for it has to deal with only the grosser, coarser, 

 and more obvious forms of the body. So, for a long time, physiology 

 was the study of those large and more evident organs. Physiology 

 could not go further until it had more exact and intimate knowledge 

 of the organs. How can we gain correct knowledge of the working 

 of any machine unless we first know and understand the construc- 

 tion of the parts of the machine? 



Chemistry and physics teach us that matter is made up of simple 

 forms, called elements and molecules, respectively. It is assumed 

 that the units, ultimately, of these elements and molecules are definite, 

 though exceedingly small, material particles. These particles are 

 called atoms the word meaning that the particles are unable to 

 be divided without losing their identity. The atom of the chemist 

 and the cell of the physiologist are the final divisions of matter. In 

 the physical world it was found that all phenomena were due to the 

 movements of these small particles the atoms. 



The fact that animals and plants, although very different ex- 

 ternally, are made up of the same anatomical units was not brought 

 to light until the invention of the microscope. These structural units 

 were called cells. The theory that organisms were made up of cells 

 was suggested by the study of plant-structure. At the end of the 

 seventeenth century, scientists, by means of their low-power micro- 

 scopes, discovered in plants small, roomlike spaces, provided with 

 firm walls and filled with a fluid. Because of their similarity to the 

 large cells of the honeycomb these small structures received the name 

 of cells. To the scientists, however, the principal feature seemed to 

 be the firm walls. By study, they found that the cell absorbed nutri- 

 ent material, assimilated it, and produced new material. Although 

 plants were composed of a mass of cells, or even a single cell, it was 

 found that each cell was an isolated whole; that it nourished itself 

 and built itself up. The cell-theory was also applied to animal tis- 

 sues. By its use it was found that many of the tissues were formed 

 also of cells and that these cells appeared to be of similar construction. 



