CHAPTER II 

 THE STRUCTURAL BASIS OF THE BODY 



THE CELL 



ALL the higher animals and plants, when submitted to microscopic 

 examination, are seen to consist of structural units which are spoken of 

 as cells. In each organ we find a mass of these cells closely resembling 

 one another in all respects, and we may therefore regard the function 

 of any organ as the sum of the functions of its constituent cells. Indeed, 

 any given reaction of the whole body is the resultant of the reactions 

 of the unlike cells of which the body is composed. The cell is therefore 

 the physiological as well as the structural unit, and it is necessary to 

 commence our study of the functions of the animal body with some 

 consideration of the functions and reactions which are common to all 

 the structural units. 



This composite structure is peculiar to the higher forms of life. 

 Amongst the lower forms, both animal and vegetable, an immense 

 number of organisms consist only of a single cell. In this cell are 

 represented all the phenomena of life, all the adapted reactions which 

 we associate with the life of the higher organisms. That the uni- 

 cellular condition represents the more primitive stage from which 

 the higher organisms have been evolved in the course of ages is indi- 

 cated by the fact that every one of these higher organisms in the 

 course of its development passes through a unicellular stage, namely, 

 the fertilised ovum. We may assume that the series of changes 

 attending the development of the higher organism from the egg is a 

 repetition in summary of the changes which have determined the 

 evolution of the species from the primitive unicellular type.* 



The general characteristics of the cell present important simi- 

 larities, whether we are considering a cell which forms the whole of an 

 organism or a cell which is but an infinitesimal part of a highly developed 

 animal. 



The name ' cell ' was first applied by botanists to the structural 

 units found by them in plant tissues, and involved therefore the idea 

 of certain qualities which do not enter into our present conception of 

 the term. A section through the stem of a growing plant shows it to 

 be made up of an aggregation of cells in the etymological sense of the 



* This assumption is often spoken of as the 'law of recapitulation.' 



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