144 HODGE. [Vol. VII. 



A number of considerations combine to create a strong pre- 

 sumption in favor of the supposition that these changes will be 

 found in normal activity. The processes in a gland have been 

 found to be identical, whether produced by artificially stimu- 

 lating the nerve going to it or by the normal stimulus of food. 

 Electrical stimulation of a nerve causes contraction of muscle 

 exactly similar to that produced by a normal nerve impulse. 

 And here we have the normal impulse producing a stronger 

 contraction than an electrical stimulus. If the same law holds 

 good for centrally as for peripherally passing impulses, for sen- 

 sory as for motor impulses, we should find a greater effect in 

 sensory cells due to the normal stimuli of the animal's life than 

 we are able to cause by stimulating an exposed nerve trunk. 

 But, most of all, the phenomena of daily fatigue, so closely con- 

 nected with the central nervous system, with the absolute neces- 

 sity of not only rest but of long continued sleep for recovery of 

 nervous power, is inexplicable on any ground which does not 

 suppose profound changes within the central nervous system ; 

 and, knowing what we do as to the fatigue of nerve fibres, we 

 may place these changes within the nerve cells themselves. 



If normal daily fatigue is to be studied, first of all it is neces- 

 sary to choose an animal in which a diurnal rhythm of rest and 

 activity is highly developed. The cat we know is not such 

 an animal, although the cat or other laboratory animals might 

 be employed under the compulsion of some sort of exercising 

 machine, and this may be done later. For the present, we wish 

 distinctively to avoid all compulsion and to study only such 

 activity as an animal normally and voluntarily puts forth in the 

 ordinary round of its daily life. 



In no animals is this daily rhythm more constant than in day 

 birds and insects. In both of these classes, too, metabolic 

 changes are known to be vigorous and rapid. The work done 

 in a day by certain kinds of birds or insects is enormous, and 

 could probably not be equalled, per body weight, by animals of 

 any other group. 



Method. 



In a former communication (24, p. 331) the words occur, "It 

 was found that the ganglion cells of two frogs that could not 

 be distinguished externally might differ widely in staining and 



