PROTOPLASMIC MOVEMENTS. 85 



movements. If, on the other hand, cold be applied to moving 

 protoplasm, the motions become less and less active, and com- 

 monly cease at a temperature about or a little over C. 

 (2) Mechanical irritation also produces a marked effect on the 

 movements of protoplasm. This may well be seen in the behavior 

 of a protoplasmic cell of frog's blood under the microscope. It 

 is spherical when first mounted, owing to the rough treatment it 

 goes through while being placed on the glass slide and covered ; 

 shortly its movements become obvious by its change in form, 

 which may again be checked by a sudden motion of the cover 

 glass. (3) Electric shocks given by means of a rapidly-broken 

 induced current cause spasm of the protoplasm, the cell becoming 

 spherical. (4) Chemical stimuli also have a marked effect ; car- 

 bonic acid causing the movements to cease, and a supply of oxy- 

 gen making it active. The movements and other activities of 

 protoplasm are, during life, frequently modified and controlled 

 by nerve influence, as will appear in the following pages : this 

 may readily be seen in the stellate pigment cells of the frog's 

 skin, which can be made to contract into spheres by the stimula- 

 tion of the nerves leading to the part. 



The motions of protoplasm are thus seen to be greatly affected 

 by external influences, but the most careful observer cannot find 

 physical explanations of the various movements which have been 

 described. It is necessary, therefore, to ascribe this power of 

 motion to some property inherent in the protoplasm, and hence 

 the movements are called automatic. We are unable to follow 

 the chemical processes upon which the activities of the proto- 

 plasm depend, and therefore we call them vital actions; but we 

 must assume that these so-called vital properties depend on cer- 

 tain decompositions in the chemical constitution of the proto- 

 plasm. We know that some chemical changes do take place, as 

 we can estimate the products which indicate a kind of combus- 

 tion ; but we know little or nothing of the details of the chemical 

 process. 



From the foregoing description of the manner in which proto- 

 plasm responds to external stimuli, it may be gathered that it is 

 capable of appreciating impressions from without ; indeed, we may 



