NORMAL HISTOLOGY. 



CHAPTER I. 



THE CELL AND THE TISSUES. 



HISTOLOGY, literally, the science of tissues, represents that part 

 of general morphology which treats of the structural elements of 

 organisms, by the various arrangement of which the textures and 

 organs of the body are formed. The term is, evidently, equally 

 applicable to the structural components of plants as well as to those 

 of animals; "histology," however, is usually accepted as relating 

 especially to animal tissues, "vegetal histology" expressing the 

 extension of the study to the tissues of plants. 



At first sight apparently complex and numerous, the structures 

 composing the animal economy are really made up of but few 

 elementary tissues ; these latter may be divided into four funda- 

 mental groups: 



Epithelial tissues; 



Connective tissues; 



Muscular tissues; 



Nervous tissues. 



Each of these tissues may be further resolved into the compo- 

 nent morphological constituents, the cells and the intercellular 

 substances. All animal cells are the descendants of the embryonal 

 elements derived from the division of the primary parent cell the 

 ovum; the intercellular substances, on the other hand, are formed 

 through the more or less direct agency of the cells. The animal 

 cell may exist in either the embryonal, matured, or metamorphosed 

 condition. 



The embryonal cell, as represented by the early generations of 

 the direct offspring of the ovum, or by the lymphoid or colorless 

 blood-cells of the adult, is a FlG 



small irregularly round or 

 oval mass of finely granular 

 gelatinous substance the 

 protoplasm or cell-contents 



in some part of which a smaller and often indistinct spherical 

 body the nucleus lies embedded. In the embryonal condition, 



