80 MANUAL OF PHYSIOLOGY. 



first to its own growth, then to the reproduction of its like, and 

 finally to the formation of a material fitted to act as a mechanical 

 protection to the surface of the skin. Having manufactured a 

 certain amount of this material, the protoplasm dwindles, and 

 finally quite disappears, so that the cell may be said to die. Its 

 horny insoluble and impermeable skeleton, however, has yet to do 

 good service in the outer layer of the skin while it is passing to 

 reach the surface, and in its turn is rubbed off. 



It has already been stated that the material which forms all 

 active cells, protoplasm, is capable of carrying on the many 

 functions required for the independent existence of simple crea- 

 tures. It will be found in the subsequent pages that, not only can 

 protoplasm perform all the activities necessary for the life history 

 of unicellular organisms, but that it can also work out all the 

 functions of the most complex animals. Indeed, the cells which 

 accomplish the most elaborate functions in man, are but proto- 

 plasm more or less modified for the special purpose to be attained. 



The different functions of an independent unicellular organism 

 can be much more completely watched than the changes which 

 take place in any of the cells of the higher animals, both on ac- 

 count of the greater size of the former, and the more obvious 

 character of the changes taking place in them. The student is 

 therefore earnestly advised to spend a few moments in contem- 

 plating the operations which go on in some simple organisms, 

 whose life is not complicated by structural or functional elabo- 

 ration, before attempting to solve the difficult question of the 

 mechanism of man's life. 



The lowest forms of living creatures that we are acquainted with 

 (micrococcus and bacterium) t are placed among the fungi in the 

 vegetable kingdom. On account of their extremely minute size 

 being hardly visible as spherical or elongated specks with a 

 powerful microscope we can say but little about their struct mv. 

 They appear to be translucent and homogeneous. 



Since we use the term protoplasm to mean the material of 

 which the active parts of the simplest forms of living beings are 

 composed, we must assume that bacteria are small particles of 

 that material, but the characters commonly attributed to proto- 



