42 HUMAN PHYSIOLOGY 



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 

 strict physiology of the nervous system can be harmon- 

 ized with itl 



The contractile and the nervous tissues have been 

 called the "master tissues" of the body. It is quite 

 clear that they are more essentially living and active 

 than the others. One can imagine non-living substi- 

 tutes for bones and tendons and, in fact, such are 

 sometimes used but one cannot imagine any lifeless 

 contrivance that could replace a muscle or a section 

 of the nervous system. As to the epithelial tissues, 

 it might be supposed that these would be found purely 

 passive but this turns out not to be the case. It is 

 through the epithelial expanses that transfers of food 



