4 ANATOMY FOR NUKSES. [CHAP. I. 



Thus, though we may say the greater bulk of the body is made 

 up of a combination of four distinct tissues, the epithelial, 

 connective, muscular, and nervous, there are parts in which 

 these tissues are so intimately mixed that we cannot distinguish 

 any distinct variety, and we are therefore obliged to class them 

 by themselves. 



Structural elements of the tissues. When any tissue is sep- 

 arated by the aid of the microscope into its simplest parts, such 

 parts are termed the structural elements of the tissue. The 

 simplest structural element of every tissue is a cell or fibre, 1 

 and however diversified the tissues of the body may appear to 

 be, they all originate as collections of cells. All the varied 

 activities of the body are the result of the activity of the cells 

 which compose it, and it is very desirable, owing also to their 

 being the foundation of all structure (the bricks, as it were, out 

 of which the tissues are built), that we early acquire some defi- 

 nite conception of these tiny elementary bodies. 



The cell. A cell is a minute portion of living substance 

 called protoplasm, which is sometimes 

 enclosed by a cell membrane, and al- 

 ways contains a vesicle which is known 

 ^ as the nucleus. 



Up to 1865 it was universally believed 

 2?* that protoplasm had no definite parts, 

 or, in other words, was structureless ; 

 A but > whei \ examined under the highest 

 CELL, n, nucleus ; p, proto- microscopical power, it appears as an 



exceedingly fine network of delicate 



fibres. The width of the meshes varies to some extent ; some- 

 times they are narrow and close, and sometimes wider and more 

 open. The interspaces are filled with a clear soft semi-fluid 

 substance and minute particles or granules of variable size. The 

 microscope can tell us little more than this, though there are good 

 grounds for supposing that there is structure that cannot be 

 directly observed. We have to turn to the chemical nature 

 of protoplasm for light as to the cause of its remarkable prop- 

 erties. 



All matter of whatever kind is made up of little particles or 

 atoms, so small that they are perfectly invisible to the human 

 1 A fibre is merely a modified cell. 



