PART I. 



MICROSCOPIC ANATOMY. 



I. CYTOLOGY. 

 THE CELL. 



Since 1838 it has been known that all plants and animals are composed 

 of small structural elements called cells (Latin, cellula; Greek, *vros). 

 The lowest forms of animals and of plants are alike in being single cells 

 throughout life. The more complex organisms are groups of cells, which 

 have been derived by process of repeated division from a single cell, the 

 fertilized ovum. Thus the human body, which begins as one cell, becomes 

 in the adult an aggregation of cells variously modified and adapted to 

 perform special functions. Since the liver is a mass of essentially similar 

 cells, the problems of its functional activity are the problems of the func- 

 tions of a single one of its cells. The diseases of the liver are the result of 

 changes occurring in these cells, which must be restored to a normal con- 

 dition to effect a cure. As this is equally true of other organs, it is evident 

 that cytology, the science of cells, is a basis for both physiology and 

 pathology. 



A cell may be defined as a structural element of limited dimensions, 

 which under certain conditions can react to external stimuli and perform 

 the functions of assimilation, growth, and reproduction. Because of 

 these possibilites a cell may be considered an elementary organism. It is 

 described as a mass of protoplasm containing a nucleus. A third element, 

 the centrosome, is found in the cells of animals, but it is doubtful whether 

 it exists in the cells of the higher plants. It becomes prominent when a 

 cell is about to divide. Some authorities regard the centrosome as a tempo- 

 rary structure, which forms shortly before division begins and disappears 

 after it is completed. Others consider it as a permanent and essential 

 part of a cell, which accordingly consists of protoplasm, nucleus, and 

 centrosome. 



