32 PROTOPLASM AND THE CELL. 



The movement is then seen to be more rapid and vigorous in one 

 direction than in the other, all the cilia working together like 

 the oars of a row-boat acting in concerted motion. By this 

 action a definite current is produced in the surrounding medium 

 (in this case the mucus of the trachea) flowing in the direction 

 of the more vigorous movement. In the trachea this movement 

 is upwards towards the mouth, and mucus, dust, etc. , are thus 

 removed from the lungs and windpipe. In many lower animals 

 and plants, especially in the embryonic state, cilia are used as 

 organs of locomotion, serving as oars to drive the organism 

 through the water. The male reproductive germs of plants and 

 animals are also propelled in a similar fashion. 



In all these forms of vital action the protoplasm is visibly at 

 work. In most cases, however, no movements of the protoplasm 

 in cells can be detected. But it is certain from indirect evidence 

 that protoplasm is no less active in those modes of physiological 

 action that give no visible outward sign, as for example in an 

 active nerve-cell or a secreting cell. This activity being molec- 

 ular arid chemical is beyond the reach of the microscope, but it 

 is none the less real ; and the play of these invisible molecular 

 actions is doubtless far more tumultuous and complicated than the 

 visible movements of the protoplasmic mass displayed in Nitella, 

 or in a nettle-hair. It is of the utmost importance that the stu- 

 dent should attain to a full and vivid sense of the reality and 

 energy of this invisible activity even in protoplasm which (as is 

 ordinarily the case) under the closest scrutiny appears to be abso- 

 lutely quiescent. 



The Sources of Protoplasmic Energy. Whence comes the 

 power required for protoplasmic action, and how is it expended? 

 The answer to this question can be given at this point only in 

 very general terms. It is certain that protoplasm works by 

 means of chemical actions taking place in its own substance; 

 and it is further certain that these actions are, broadly speaking, 

 processes of oxidation or combustion; for in the long run all 

 forms of protoplasmic action involve the taking up of oxygen 

 and the liberation of carbon dioxide. Energy is therefore set 

 free in living, active protoplasm somewhat as it is in the com- 

 bustion of fuel under the boiler of a steam-engine, and in this 

 process the protoplasm, like the coal, is gradually used up, disin- 



