CHANGES OF FORM IN CELLS. 17 



formation, the importance of which cannot at present be 

 estimated. 



Hering* has endeavoured to explain the passage both of v 

 coloured and of colourless blood corpuscles through the walls 

 of the vascular system, by comparing it with the filtration of 

 colloidal substances. But in whatever way the process may 

 be explained, the fact remains that the white corpuscles leave 

 the interior of the vascular system, and are thus enabled to 

 traverse various regions of the body. 



In stating that protoplasm is capable of active or vital movements, 

 we have by no means admitted the existence of an immaterial force. 

 Ed. Weber f has expressed himself very decidedly upon this point, 

 and at the present day the position he took up is still tenable. " Ac- 

 cording to my view," said Weber, " the movements of any living body 

 are not dependent upon two kinds of force namely, first upon forces 

 which are exerted on this body by other bodies, and secondly upon 

 forces which are exerted on this body by life ; but there is only one 

 kind of force on which the movements of all bodies depend namely, 

 the force which is exerted on it by other bodies." We name the 

 movements of certain bodies "vital," in the sense that the forces 

 which we then call into play are subject to certain other varying 

 influences, and we denominate the apparatus and the processes of 

 which these influences are the result, "organization" and "life." 



It is customary also to call the vital movements of protoplasm 

 Spontaneous. But this only shows that we are ignorant of the forces 

 by which the movements are originated and sustained. We no longer 

 term the movement of striated muscle spontaneous, because we know 

 the external influences or stimuli through which it can be excited. 

 And so also there can be no doubt that as soon as we have acquired 

 a knowledge of all the external influences by which movements in 

 protoplasm can be induced, we shall cease to term them spontaneous. 

 Thus, in an analogous case, we say the production of heat by coal is 

 immediately dependent on our placing it on the fire, i.e. on raising 

 its temperature. Here the process of heating is the external influence 

 or stimulus which induces a change in the molecular structure ; and 

 as a consequence of this molecular change, active force is set free, 

 which becomes perceptible to us in the form of heat. The production 



* Wiener Sitzungsberichte, 1868. 

 t Miiller's Archiv, 1858. 



