Chapter VIII 

 THE VITAL UNITS CALLED CELLS 



E. V. COWDRY 



THE human body is made up of a mass of living units, 

 which are known as cells. The term, cell, is a misnomer 

 and a relic of the past, yet it is sanctioned by usage. 

 Vital units are not empty spaces, as the word suggests, but 

 filled with a fluid substance called protoplasm, which is the 

 basis of all vital activity. Recognition of their existence 

 dates back almost one hundred years. The cell theory intro- 

 duced by Schleiden and Schwann (1838), according to which 

 all living things are built up of cells, has played a funda- 

 mental role in biology and medicine comparable in impor- 

 tance only to the conception of the existence of organic 

 evolution. 



SIZE AND SHAPE OF CELLS 



In size, the cells of our bodies are very small. Among the 

 most minute are the white blood cells, or leucocytes, about 

 3^20 of 3,n inch in diameter and altogether too tiny to be 

 seen with the naked eye. Human eggs are the largest (3'^o 

 inch) and are just visible when artificially stained. Since 

 they are so minute the number of these vital units which 

 make up the body of a man weighing about 155 pounds is 

 legion. It has been calculated (Donaldson) that there are 

 about 26,500,000,000,000. 



The shape of cells is highly variable. We can study their 

 form in several ways. One method is to examine them with 

 the microscope in the still living state. A red blood cell 

 which carries the respiratory pigment called hemoglobin, 

 and a leucocyte, viewed in this way, are represented diagram- 

 matically in Figure i (a and b). The former is rather 

 lens shaped but looks circular. Within the leucocyte may 

 be seen an irregular lobated structure, known as the 

 nucleus, which will be referred to again. Some cells are 

 much elongated, like the muscle cell (d), and the nerve 



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