NORMAL HISTOLOGY. 



CHAPTER I. 

 INTRODUCTION. 



Histology (from the Greek f<jroc, tissue, and ^0/07, discourse) 

 is a branch of anatomy treating of the minute or microscopi- 

 cal structure of living organisms. In its full significance it 

 relates to the entire organic kingdom, all plants and animals 

 alike. Within large classes of animals or plants the structure 

 of the tissues is quite uniform ; but on comparing one large 

 group of organic types with another, differences in the structure 

 appear, although there are a fundamental unity and similarity 

 throughout. 



The histologic structure of man, with which the present 

 treatise deals, is substantially the same as that of all the mam- 

 malia, the points of variation being few and slight; hence 

 human histology and mammalian histology are almost identi- 

 cal. In passing to the classes of birds and reptiles, however, 

 marked differences begin to appear, as in the forms of the red 

 blood-corpuscles, while the general structure remains similar. 



Analysis of histologic structures : In analyzing the organism 

 into its anatomic and histologic elements we find that they 

 are arranged in an ascending series of aggregations. The 

 ultimate elements of all are the atoms and molecules ; as the 

 scope of histologic science does not extend beyond the limits 

 of microscopic vision, the atomic and molecular structure of 

 tissues and cells does not come within its field, but is covered 

 by organic chemistry and physics, with different methods of 

 investigation. 



The smallest elements with which histology deals are masses: 



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