76 MANUAL OF PHYSIOLOGY. 



made to recover and again commence its movements. If, on the 

 other hand, cold be applied to moving protoplasm, the motions 

 become less and less active, and commonly cease at a temperature 

 about or a little over C. (2.) Mechanical irritation also pro- 

 duces a marked effect on the movements of protoplasm. This 

 may be well 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 ; carbonic acid causing the movements 

 to cease, and a supply of oxygen making it active. The move- 

 ments 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 stimulation 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 protoplasm 

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

 assume that these so-called vital properties depend on certain de- 

 compositions in the chemical constitution of the protoplasm. We 

 know that some chemical changes do take place, as we can esti- 

 mate the products which indicate a kind of combustion ; 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 

 say, it can feel. We can only judge of the sensitiveness of any 



