20 INTRODUCTION. 



hence chemistry and physics cover the atomic and molecular 

 structure ; histology, the minute molar structure of organisms. 



The ultimate and smallest perceptible histologic elements 

 of distinct individual character may be divided into two 

 classes, cells and non-cellular or extracellular elements. By 

 the union of these elements tissues are formed, as fibrous tis- 

 sue, muscular tissue ; by the union of tissues organs are 

 formed ; a combination of organs all working together for 

 the accomplishment of some general function constitutes a 

 system or tract, as the reproductive system, the alimentary 

 tract; while all the systems together make up the complete 

 organism. 



The ascending grades of aggregation in organic structure 

 are, then, after atoms and molecules, cells and extracellular 

 elements, tissues, organs, systems, the organism. 



The subject will here be elaborated in the order correspond- 

 ing to this analysis, beginning with the fundamental elements, 

 cells, etc., then taking up their aggregations in tissues, and 

 finally studying the structure of the organs making up the 

 various systems of organic activity. 



Histologic technique : The histologic examination of tissues 

 is carried out entirely by microscopic methods. To gain an 

 adequate idea of histology, actual examination of the tissues 

 under the microscope is necessary. The use of the microscope 

 and the preparation of specimens of tissues for microscopic 

 examination involve numerous delicate procedures, the mastery 

 of which requires much special study and experience. Objects 

 cannot be examined in bulk under the microscope, but require 

 special treatment before their structure can be made visible. 

 For one thing they have to be very thin, so thin as to be 

 transparent; hence, solid tissues have to be cut into thin 

 slices, or sections, about yj-^th millimetre thick. In their 

 natural state most tissues are colorless, so that the details of 

 the structure cannot well be made out. To make the details 

 more visible it is customary to stain the sections with certain 

 dyes, of which a considerable number are available for vari- 

 ous purposes. Some stains have an affinity for some parts of 

 the tissues, other stains for other parts ; thus by the employ- 

 ment of various materials and methods different structures can 

 be brought plainly into view and differentiated. The nuclear 



