— n 



Chap. I.] THE CELL. 3 



the upper part of the ventral cavity are two small cavities, the 

 buccal cavity, or mouth, containing the tongue, teeth, salivary 

 glands, etc., and the nasal cavity, containing the organ of smell. 



Structural elements of the body. — When any part of the body 

 is separated by the aid of the microscope into its simplest parts, 

 such parts are called its structural elements. The structural 

 element of every part of the body is the cell. 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 

 definite conception of these tiny elementary bodies. 



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

 (protoplasm) which is sometimes enclosed in a membrane (cell 

 membrane). It consists of a semi-fluid, 

 often granular, part (cytoplasm) sur- 

 rounding a more solid part (the nu- 

 cleus). The nucleus differs somewhat 

 from the cytoplasm in function and in 

 chemical composition. Yryf A { Y F F^ c 



The study of physics shows us that 



all matter, of whatever kind it may be, -n. ^ t^ 



' J ■> Fig. 2. — Diagram of a 



is made up of little particles, or mole- Cell, n, nucleus; c, cyto- 

 cules, so small that they are perfectly ^ ^^™' 

 invisible to the human eye, even when aided by the most power- 

 ful microscope ; and it is only when a great number of these 

 molecules are collected together that they become perceptible. 

 Again, a study of the chemical properties of matter teaches us 

 that these molecules are in turn composed of still smaller parti- 

 cles called atoms. There are only about seventy different kinds 

 of atoms, whereas the different sorts of molecules which are 

 formed by combination of atoms are innumerable. The prop- 

 erty of atoms of uniting together to form molecules is known 

 as their chemical affinity, while that which binds the molecules 

 together is called cohesion.^ The strength of chemical affinity 



1 As examples of atoms and molecules we may mention the following : 

 hydrogen (H) and oxygen (0) unite by chemical affinity to form the hydro- 

 gen monoxide (H2O), or water molecule. Such molecules, when gathered 

 together in great numbers and united by their property of cohesion, form 

 the water which we can perceive by our senses. So also sodium (Na) and 

 chlorine (CI) unite to form sodium chloride (NaCl), or common table salt. 



