PART I. 



MICROSCOPIC ANATOMY 



1. CYTOLOGY. 

 THE CELL. 



Since 1839 it has been known that all plants and animals are composed 

 .of small structural elements called cells (Latin, celhila; Greek, KVTOS). 

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

 the problems of its functional activity are the problems of the functions 

 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 denned as a structural element of limited dimensions 

 which under certain conditions can perform the functions of assimilation, 

 growth, and reproduction. Because of these possibilities a cell may be 

 considered an elementary organism. It is described as a mass of proto- 

 plasm containing a nucleus. A third element, the centrosome, is found 

 in the cells of animals, but not in those of the higher plants. The centro- 

 some becomes prominent when a cell is about to divide. At other times, 

 in many kinds of cells, it has been found as a minute granule which may 

 be in the center of a very small clear spot in the protoplasm. Ordinarily 

 it cannot be seen unless cell division is about to occur. Some authorities 

 regard the centrosome as a temporary structure which forms shortly 

 before division begins and disappears after it is completed. Others 



