CHAPTER II. 



STRUCTURAL BASIS OF THE HUMAN BODY. 



BY dissection, the human body can be proved to consist of various dis- 

 similar parts, bones, muscles, brain, heart, lungs, intestines, etc., while, 

 on more minute examination, these are found to be composed of different 

 tissues, such as the connective, epithelial, nervous, muscular, and the 

 like. 



Cells. Embryology teaches us that all this complex organization has 

 been developed from a microscopic body about y-J-0- in. in diameter 

 (ovum), which consists of a spherical mass of jelly-like matter enclosing 

 a smaller spherical body (germinal vesicle). Further, each individual 

 tissue can be shown largely to consist of bodies essentially similar to an 

 ovum, though often differing from it very widely in external form. They 

 are termed cells : and it must be at once evident that a correct knowledge 

 of the nature and activities of the cell forms the very foundation of 

 physiology. 



Cells are, in fact, physiological no less than histological units. 



The prime importance of the cell as an element of structure was first 

 established by the researches of Schleiden, and his conclusions, drawn 

 from the study of vegetable histology, were at once extended by Schwann 

 to the animal kingdom. The earlier observers defined a cell as a more or 

 less spherical body limited by a membrane, and containing a smaller body 

 termed a nucleus, which in its turn encloses one or more nucleoli. Such 

 a definition applied admirably to most vegetable cells, but the more 

 extended investigation of animal tissues soon showed that in many cases 

 no limiting membrane or cell-wall could be demonstrated. 



The presence or absence of a cell- wall, therefore, was now regarded as 

 quite a secondary matter, while at the same time the cell-substance came 

 gradually to be recognized as of primary importance. Many of the lower 

 forms of animal life, e.g., the Rhizopoda,were found to consist almost entire- 

 ly of matter very similar in appearance and chemical composition to the 

 cell-substance of higher forms: and this from its chemical resemblance to 

 flesh was termed Sarcode by Dujardin. When recognized in vegetable 

 cells it was called Protoplasm by Mulder, while Remak applied the same 

 name to the substance of animal cells. As the presumed formative mat- 

 ter in animal tissues it was termed Blastema, and in the belief that, 

 wherever found, it alone of all substances has to do with generation and 



