42 HUMAN PHYSIOLOGY 



strict physiology of the nervous system can be harmon- 

 ized with it. 



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 

 and waste go on between organisms and their environ- 

 ment. The processes involved, secretion and absorp-. 

 tion, cannot be reproduced with non-living membranes. 



Free-living Cells. A cell from the body of one of the 

 higher animals cannot continue to live when separated 

 from its fellows. But nature abounds in organisms 

 which are single cells. These may be reckoned accord- 

 ing to their nutritional requirements in different instances 

 as either plants or animals. So the bacteria are con- 

 sidered to be plants because they can live on classes of 

 supplies which are simpler in their constitution than 

 those which animal life ordinarily demands. The 

 Infusoria of stagnant water, which are seen to devour 

 organic matter and whose lively habits constantly re- 

 mind us of the higher forms, are regarded as animals. 

 But, as has been stat'ed before, the distinctions between 

 plants and animals have only a limited value when 

 applied to the lower orders of life. 



A free-living cell of the animal type has probably all 

 the activities which have been described in the preceding 

 chapter as characteristic of animals in general. That. 

 is to say, it appropriates supplies of complex matter, - 

 using some to promote its own growth but more to furnish, 

 the energy for movement and heat production. This 

 energy it makes available by a respiratory process in 



