CELLS AND THEIR ASSOCIATION 41 



and linings of the organs. An epithelium may consist 

 of a single layer of cells or of more than one. The thin- 

 nest developments of this character are exemplified in 

 the partitions between the air and the blood in the 

 lungs. In calloused portions of the skin the number of 

 layers of cells is large and those on the outer surface are 

 lifeless, flattened, and dry. Epithelial cells, as a rule, 

 keep growing and subdividing throughout life to make 

 good the loss by degeneration and detachment that is 

 constantly going on. 



Connective tissues have the general function which the 

 name indicates. Bone is an example and what the 

 skeleton does for the body as a whole is done for each 

 organ by a supporting web of material belonging to this 

 class. Under this head we may mention the tendons 

 which join muscles with bones and the ligaments which 

 unite the bones at the movable joints. As already 

 suggested the cells of connective tissue are not much in 

 evidence. It is the intercellular substances, in the form 

 of fibers, concrete, or mineralized deposits that fit such 

 tissues to serve their purpose. 



Contractile tissues are those which produce movement 

 by an energetic change of form. They are able to do 

 this because they can derive power from certain chemical 

 compounds which they decompose. The problems 

 of muscle physiology confront us here. It is well to 

 point out that the tissues which display the property 

 of contractility are heat-producing tissues at the same 

 time. 



The nervous tissues are in many respects the most 

 remarkable and baffling of all. An attempt to deal 

 with them will be made later on; it may be said at this 

 point that their chief service is to maintain the relation- 

 ship between the sense-organs and the contractile tissues. 

 In other words their function is that of coordination. 

 This seems a very inadequate statement when we con- 

 sider the mysterious correlation between the brain and 

 our consciousness, yet we shall find that most of the 



