INTRODUCTION. 19 



tain its own nutrition, incorporating the food-materials that are 

 accessible to it and using them in such a way as to keep its struct- 

 ure in a normal condition. But, aside from this duty which is com- 

 mon to all, each part has a duty to perform for the good of the 

 whole organism ; and, as we shall see, this duty often appears to be 

 paramount, the activities which it necessitates being carried on 

 even if they involve a sacrifice in the nutrition or structure of the 

 individual part. 



Each part of the body has some particular kinds of work assigned 

 to it, which constitute its functions, and which it performs for the 

 benefit of the whole body. The development and life-history of 

 each part has direct reference to those functions, through which it 

 co-operates with all the other parts in maintaining the integrity and 

 normal activities of the whole body, all the parts being interde- 

 pendent upon each other and subservient to the general needs. 



The foregoing considerations prepare us for the fact that the 

 structure of the various parts of the body differs in its details. 

 The study of those finer details can only be pursued with the aid 

 of the microscope, for the microscopical constituents of the tissues 

 are the elements which confer upon them their particular properties 

 and powers. This study is called histology. 



Investigation has shown that there is one form of tissue-element 

 which is always present in all parts of the body. This is the cell. 

 It does not always possess the same form or internal structure, but 

 in all its variations the same general plan of construction is adhered 

 to. These cells are the essentially active constituents of the tissues. 

 It is within them that the transformations of matter and energy are 

 chiefly carried on, and it is due to their activities that the tissues 

 forming the body are elaborated and enabled to perform their sev- 

 eral functions. These marvellous powers possessed by the cell 

 have created our conception of life, and, in spite of eager study, 

 remain inscrutable. We do not know why a living cell differs from 

 a dead cell, but we do know that the mysterious vital powers are 

 only derived from pre-existent living cells and are not antagonistic 

 to the chemical and physical laws governing unorganized matter. 



All the cells of the body are descendants of a single cell, the egg, 

 from which they arise by successive divisions, and throughout the 

 existence of the body they retain some of the characters of the 

 original cell. But as the body develops the cells of the different 

 parts display divergent tendencies, which finally result in the for- 



