REACTIONS OF ANIMALS. 



was usually increased activity (period of heightened sensibility) 

 followed sometimes by erratic movements (period of over-stimula- 

 tion), which was followed by depression, apparent fatigue (the 

 depression period). The first and last periods were always 

 evident. The second in some cases only. 



(a) Heightened Sensibility. 



The irritability of the animals is evidently increased by a small 

 loss of water, as indicated by the periods of heightened sensibility 

 and over-stimulation referred to on page 86. The same phe- 

 nomenon is shown by the increasing avoidance of the air of high 

 evaporating power after several entrances into this air. The 

 increased sensibility is probably due to the concentration of the 

 blood and tissue fluids. Dr. A. P. Mathews and Dr. Carlson have 

 both informed me that when muscle preparations dry during the 

 usual study, contractions and twitchings result. Clearly the 

 drying of the surface of muscles may not only increase the con- 

 centration of the ions but may also interfere with neutrality. 

 In the gradients and in the killing experiments the skin of the 

 amphibians clearly dried slightly in the dry air. The same is 

 probably true of the sense organs and tracheal surroundings of 

 the arthropods. The CO 2 output may be interfered with by the 

 drying (Krog, '03, '04; Winterstein, '12). An increased concen- 

 tration of CO 2 may be a cause of the increased irritability, as 

 Waller ('96) has found that a very small increase of this substance 

 increases activity of nerves. 



Carlson ('06) and Meek ('06) secured decreased vigor of con- 

 traction in heart and other muscle preparations by the with- 

 drawal of water by means of sugar and glycerine solutions 

 and increased it by diluting the isotonic solutions surrounding 

 the preparations. The cause of the decreased irritability in the 

 case of water withdrawal by hypertonic solutions, is not so 

 apparent and an explanation is not so easily reached, especially 

 when we consider the fact that Paramcecium gives the avoiding 

 reaction when water is withdrawn by sugar solution (Jennings, 

 '06). Still the water withdrawal may have been so rapid that 

 an increase in irritability was overlooked because of its transi- 

 tory character and only the depression period which follows 

 noted (see below). 



