THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 131 



nerve-cells in muscular fatigue. Strongman (1917. p. 169), working in this labora- 

 torj', has undertaken to answer this. White mice were selected for the experi- 

 ments because they are the smallest mammals which can conveniently be used 

 in the laboratory and because more is known of the qualitative (Thurlow, 1917, 

 p. 37) and quantitative (Nicholson, 1916, p. 329) variations in mitochondria in 

 their nervous system than in that of any other mammal. They were fatigued 

 by the very simple method which Professor Tamao Saito uses, of letting them swim 

 in water until they are exhausted. It was soon discovered that they swim more 

 actively when the water is sUghtly agitated and is raised to body temperature: 

 otherwise they soon learn to float and refuse to exercise 



The experiments were controlled in the usual manner by using only mice of 

 known age and by examining, in exactly the same way, an unexercised mouse of 

 the same htter for comparison. Five experiments of this kind were made, the mice 

 swimming 1 to 2 hours continuously before they were completely exhausted. 

 Larger mammals will swim for a day or more before exhaustion. Experience 

 showed that young mice 25 to 30 days old are more suitable than adults, because 

 they are more easily tired. 



In each of the five experiments the fatigued mouse and the control mouse rom 

 the same litter were chloroformed. They were then fixed by the injection of a 

 formaUn and bichromate mixture in accordance with the method advised by Cow- 

 dry (1916a, p. 30). The brains were then removed, mordanted, dehydrated, and 

 cleared together in the same bottle, and they were embedded in the same block 

 of paraffin. Sections, cut 4 microns in thickness, from the fatigued and from the 

 control, were mounted on the same slide so as to avoid variations in the staining. 

 The .sections were then stained with fuchsin and methyl green, the fuchsin color- 

 ing the mitochondria crimson and the methyl green staining the Nissl substance 

 a bluish-green color. Preparations were made of the cortex, the cerebellum, and 

 the spinal cord, in this way, from each of the five experiments. 



The net result of Strongman's five experiments, with their controls, was to show 

 that mitochondria are surprisingly constant in nerve-cells. A fair degree of fatigue, 

 as well as a certain amount of fright, brought about no constant changes in them. 

 It is quite possible that more prolonged exhaustion, if it can be induced, may lead 

 to definite and precise alterations. It is perfectly clear that these results are in 

 complete accord with those which Key and Scott obtained with the pancreas, 

 because they show that in the nerve-cell, also, the mitochondria are not directly 

 concerned with specialized activities. They do not play a part in conduction any 

 more than they do in secretion, which strongly points to the general conclusion 

 that they are concerned in some fundamental process common to all cells. 



METABOLISM. 



Here, as in all other fields of investigation, oliservation has preceded experi- 

 mentation. Mitochondria have been carefully studied in phylogeny as well as in 

 ontogeny. Their wide distribution is amazing. It has already been pointed out 

 (page 72) that they occur from man to the most lowly protozoon and from the 



