42 QUANTITATIVE STUDIES ON MITOCHONDRIA IN NERVE-CELLS. 



Again, it has been shown that mitochondria vary in their solubilities in acetic 

 acid (Regaud, 1910; Nicholson, 1916). In view of this fact objection might 

 be made that the variation in mitochondrial content in different types of nerve - 

 cells was an artefact produced by the solvent action of the reagents used. Such 

 criticism of the results obtained in this investigation is invalid, for no acetic acid 

 was used in the preparation of the specimens, nor was there any other solvent for 

 mitochondria involved in the fixation or staining. The formalin which was used 

 as a fixative for the mitochondria was neutral, and the long mordanting in the 

 bichromate prevented their subsequent solution in alcohol, although their solu- 

 bility in alcohol does not seem to be marked. Hence there is no error in tech- 

 nique which could account for the striking variation in amount. 



DISCUSSION. 



Having established the certainty that there is, in the nerve-cells of the 

 medulla of the species used, a definite amount of mitochondria per unit volume of 

 cytoplasm, there remains to be determined the functional significance of such 

 numerical variation. Other investigators have, in their work on the central 

 nervous system, determined various cell ratios, among which might be mentioned 

 the nucleus-plasma ratio. Dolley (1914) has found that the resting nerve-cells 

 of corresponding type for the same species have a constant nucleus-plasma ratio 

 which is altered temporarily through functional depression or permanently through 

 functional senility. Having once established this constant, he could study path- 

 ological changes following experimental conditions. Donaldson (1911) found varia- 

 tions in the water content of the nerve-cells accompanying functional changes. He 

 makes no further statement than this: the variation in water percentage of the 

 nerve-cell is an index of functional activity. 



The relationship between number of mitochondria and volume of cytoplasm 

 is another such constant. In the animals studied the number of mitochondria 

 per unit volume of cytoplasm was found to be constant for corresponding cells, 

 not only of the same animal, but also of different individuals of the same species 

 (table 1). In nerve-cells of the same type, therefore, we have a cell constant which 

 is definite for animals of the species studied and which can be used for observations 

 of pathological conditions resulting from experiment. That mitochondria do react 

 to conditions which affect the cell has been demonstrated by several authors, as 

 Policard (1910 and 1912), Homans (1915), Scott (1916), and others. 



These observations may be given another application, viz, to the doctrine of 

 neurone specificity. It would be reasonable to suppose that even if all the mito- 

 chondria were identical, such definite and constant variations as are here recorded 

 would be closely associated with a definite and constant functional differen- 

 tiation. Combined, however, with this quantitative difference are qualitative 

 differences (Nicholson, 1916), and this combination serves to strengthen the theory 

 that the activity of the nerve-cells themselves differs in some way. It would be 

 rather extreme to assume that cells differing specifically with respect to so con- 



