ELEMENTARY VITAL PHENOMENA 219 



1. The Production of Mechanical Energy 



All living substance moves, i.e., the single points of its material 

 system change their positions in space. There results, according 

 to the special conditions, a shifting of the single particles, the ex- 

 ternal form remaining the same, a change in the external form, 

 a change of place of the whole (locomotion), or several of 

 these changes at the same time. But although motion in itself 

 is a general phenomenon of life, all forms of living substance do 

 not show the same kind of motion. The variety of modes of motion 

 that may be observed in different organisms is very great. Never- 

 theless, all may be classified in accordance with the manner of 

 their occurrence into a few large groups, of which only certain ones, 

 on account of their wide distribution, possess any considerable im- 

 portance. Since the motion of living substance is the most 

 evident vital phenomenon, and special interest is therefore lent to 

 it, we are justified in considering it somewhat in detail. 



It is useful first to classify the various modes of motion into : 



(a) Passive movements. 



(b) Movements by swelling of the cell-walls. 



(c) Movements by change of the cell-turgor. 



(d) Movements by change of the specific gravity of the cell. 

 (*) Movements by secretion on the part of the cell. 



(f) Movements by growth of the cell. 



(g) Movements by contraction and expansion of the cell- 



body : 



Amoeboid movement. 

 Muscular movement. 

 Ciliary movement. 



a. Passive Movements 



In passive movements the cause lies outside the part that is 

 moved. Passive movements in living substance are, therefore, not 

 a vital phenomenon of the elements that are moved, but the ex- 

 pression of vital phenomena in the environment. The movement 

 of the red blood-corpuscles, the streaming of the blood-plasma in 

 the blood-vessels of the human body, are passive movements ; for 

 the blood-corpuscles and the plasma possess no intrinsic power of 

 movement; they are only passively driven by the activity of the heart, 

 which works like a suction- and force-pump in the system of branch- 

 ing tubes filled with blood. This streaming of the blood in the 

 fine capillary vessels can be observed very beautifully under the 

 microscope, if a frog, paralysed by the South American arrow 

 poison, curare, be placed upon a cork plate and the web between 

 the toes of the hind leg be stretched out by needles over an opening 



