CHAPTER II. 

 CELLS AND EXTRACELLULAR ELEMENTS. 



Elements of tissues : The ultimate elements of which tissues 

 are composed are cells and extracellular (or intercellular) sub- 

 stances. Of these the cells comprise by far the bulk of the 

 tissues and play the chief part in the phenomena of life. The 

 body protoplasm, which is the seat and source of all vital 

 phenomena, is located in the cells. 



The non-cellular elements, produced by the vital activities 

 of the cells, provide a connection, support, and framework for 

 the cells, which, composed as they are of semifluid protoplasm, 

 have little consistency of their own and are scarcely capable 

 of self-support. The cells are alive and the seat of life ; the 

 function of the extracellular elements is merely mechanical, 

 and they do not possess any real vitality of their own. 



The cell : A cell is the smallest perceptible organization of 

 protoplasm having a definite and complete individuality, struct- 

 ure, and vitality of its own. It manifests in miniature all 

 the characteristics of life exhibited by the entire organism. 

 It is a unit of organic structure and organic activity. It is 

 the source and the seat of vital manifestations. 



The name "cell" literally means a small inclosed space. 

 This meaning does not express a correct conception of the 

 nature of cells. The term arose from the fact that vegetable 

 cells, with their prominent walls and their transparent though 

 vital contents, appear like a mass of cavities separated by 

 partitions. The term "corpuscle," often used synonymously 

 with cell, better expresses the idea. 



Classes of cells: All cells have a fundamental similarity in 

 origin, structure, and nature ; but as they actually occur they 

 vary much in form and function in different situations. Two 

 classes, in general, can be distinguished : 



1 . Generalized or undijferentiated, independent, free-living 

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