vi HISTOLOGY OF THE FROG 121 



CHAPTER VI 

 HISTOLOGY OF THE FROG 



SINCE Schleiden and Schwann promulgated the cell theory 

 in 1838-1839 \ve have been accustomed to regard organisms 

 as composed of little units or cells. Most cells of the body 

 of the higher organisms are united to form tissues which 

 are aggregations of cells of similar character bound together 

 by means of an intercellular substance. In the bodies of 

 animals the classes of tissues commonly distinguished are 



the following : - 



1. Epithelial. 



2. Connective. 



3. Muscular. 



4. Nervous. 



These broad divisions include nearly all the manifold 

 variety of cells occurring in the body. The blood and 

 lymph are sometimes added as forming a distinct class of 

 tissues, sometimes classed as a form of connective tissue 

 with fluid intercellular substance, and sometimes treated of 

 as if they were not tissues at all. They will be described 

 in a later chapter. 



In the epithelial tissues the cells lie in layers with only a 

 small amount of intercellular substance. We meet with this 

 class of tissue on the surfaces of organs, or lining the cavi- 

 ties of organs, and forming the lining of glands, blood 

 vessels, and ducts of all kinds. The various kinds of epithe- 

 lium are distinguished according to the shapes of the cells. 



