tf HANDBOOK OF PHYSIOLOGY. 



Corti's demonstration of the rotation of the cell sap in characeae, and in 

 the earlier part of the century by Meyer in Vallisneria, 1827; Robert 

 Brown, 1831, in " Staminal Hairs of Tradescantia." Then came Du jar- 

 din 's description of the granular streaming in the pseudopodia of Rhizo- 

 pods and movement in other cells of animal protoplasm (Planarian eggs, 

 v. Siebold, 1841; colorless blood-corpuscles, Wharton Jones, 1846). 



2. The Power of Response to Stimuli, or Irritability. Although the 

 movements of the amoeba have been described above as spontaneous, yet 

 they may be increased under the action of external agencies which 

 excite them and are therefore called stimuli, and if the movement has 

 ceased for the time, as is the case if the temperature is lowered beyond 

 a certain point, movement may be set up by raising the temperature. 

 Again, contact with foreign bodies, gentle pressure, certain salts, and 

 electricity, produce or increase the movement in the amoeba. The pro- 

 toplasm is, therefore, sensitive or irritable to stimuli, and shows its irri- 

 tability by movement or contraction of its mass. 



The effects of some of these stimuli may be thus further detailed: 



a. Changes of Temperature. Moderate" heat acts as a stimulant; the 

 movement stops below C. (32 F.), and above 40 C. (104 R); be- 

 tween these two points the movements increase in activity; the optimum 

 temperature is about 37 to 38 C. Exposure to a temperature even 

 below C. stops the movement of protoplasm, but does not prevent its 

 reappearance if the temperature is raised; 'on the other hand, prolonged 

 exposure to a temperature of over 40 C. altogether kills the protoplasm 

 and causes it to enter into a condition of coagulation or heat rigor. 



b. Mechanical Stimuli. When gently squeezed between a cover and 

 object-glass under proper conditions, a colorless blood-corpuscle is stim- 

 ulated to active amoeboid movement. 



c. Nerve Influence. By stimulation of the nerves of the frog's cornea, 

 contraction of certain of its branched cells has been produced. 



d. Chemical Stimuli. Water generally stops amoeboid movement, 

 and by imbibition causes great swelling and finally bursting of the cells. 

 In some cases, however (myxomycetes), protoplasm can be almost en- 

 tirely dried up, but remains capable of renewing its movements when 

 again moistened. Dilute salt-solution and many dilute acids and alka- 

 lies stimulate the movements temporarily. Strong acids or alkalies 

 permanently stop the movements; ether, chloroform, veratria, and qui- 

 nine also stop it for a time. 



Movement is suspended in an atmosphere of hydrogen or carbonic 

 acid and resumed on the admission of air or oxygen, but complete with- 

 drawal of oxygen will after a time kill the protoplasm. 



e. Electrical. Weak currents stimulate the movement, while strong 

 currents cause the cells to assume a spherical form and to become 

 motionless. 



