1913] Gee: Behavior of Leeches 267 



the plum cureulio (Gee and Latlirop, 1912), by a very slight 

 contact stimulus. The changed responsiveness of the leeches to 

 the light stimuli of jars and shadows seems to be due, then, to 

 the "fatigue sensorielle" of Bohn, involving perhaps both a 

 dulling of the sensibility of the receptors and slight changes in 

 the nerve centers involved. 



IV. Normal Fatigue from Repeated Contact Stimuli 



The study of muscular fatigue and mental fatigue has re- 

 ceived much consideration in comparatively recent years. The 

 work of Mosso (1905) is a classic in this field. Lee (1907) dis- 

 cusses the physiological effect of paralactic acid, mono-potassium 

 phosphate, and carbon dioxide, the substances at present known 

 to be formed in the muscular tissues of the higher animals during 

 fatigue. Yoakum (1909) in an extensive monograph has con- 

 sidered the matter of mental fatigue and gives a rather full 

 review of the literature bearing on this subject. With regard 

 to the phenomenon of fatigue of skeletal muscle, Ranke (1865) 

 says: "Every one knows that the first twitch of the muscle is 

 not its greatest. With the stimuli uniform in strength the later 

 contractions are stronger than the earlier ones." Marey (1866) 

 discusses the fact that in continued activity the height of the 

 muscle curve at first increases and then decreases. Lee (1907) 

 in a paper discussing the cause of the "Treppe" says in regard 

 to this phenomenon: "In its literal sense this term signifies 

 the fact that the repeated responses of a tissue to repeated and 

 equal stimuli increase for a time in intensity. In a more general 

 sense, the phenomenon may be characterized as augmentation of 

 activity resulting from previous activity." He finds that the 

 fatigue products, if present in small quantities or in moderate 

 quantities for a brief time, cause an augmentation of activity, 

 characterized by an increase in irritability and working power. 

 If present in moderate quantities, or smaller quantities for a 

 longer time, each of the fatigue substances is depressing or 

 fatiguing. 



Bohn (1909) has pointed out in Cerianthus among other 

 forms that there is an increased sensibility of the tentacles to 

 a certain stage, reactions of a certain type becoming more and 

 more easy. When that stage has been reached there is a eorre- 



