38 



QUANTITATIVE STUDIES ON MITOCHONDRIA IN NERVE-CELLS. 



The method of fixation and staining has been described by Cowdry (1916a). 

 The mitochondria, which take the fuchsin, appear as discrete, bright-red granules 

 sharply outlined against a background of light-green Nissl substance, and, with 

 proper optical and lighting facilities, may be readily counted. 



Quantitative estimations were made by carefully counting the mitochondria 

 occurring within a field of constant area. Such a field was obtained by placing 

 within the ocular a glass disk upon which a single square had been ruled accord- 

 ing to the method of Isaacs (1915). The actual area of the optical field covered 

 by the square was determined by the stage micrometer and found to be 19.78 

 square micra. The thickness of the sections was 4 micra; hence the volume of 

 the field counted could easily be determined, and from that the amount of mito- 

 chondria per cubic millimeter was calculated. 



The lenses used were the Zeiss apochromatic 1.5 mm. objective and Zeiss 

 No. 6 compensating eyepiece. Constant conditions of illumination were obtained 

 by the use of the 40-watt Mazda lamp. A mechanical stage was employed. All 

 counts and drawings were made under uniform conditions of magnification and 

 illumination. 



In order to avoid inaccuracies due to possible minor differences in thickness 

 of the sections, the same number of fields was counted in every nucleus occurring 

 in any one section. For example, in the same section may be found cells of the 

 cochlear and vestibular nuclei of the VIII, of the VI, VII, and the mesencephalic 

 nucleus of the V. If four fields were counted in one section in mesencephalic 

 cells, four fields of all the other nuclei were counted in that section. 



OBSERVATIONS. 



Studies were carried out upon five brains which had been cut into partial 

 serial sections. Only such nuclei as were well stained in each series were investi- 

 gated and the results tabulated (table 1). 



TABLE 1. 



The above figures represent the average number of mitochondria present in the field counted, the volume of 

 which was 79.12 cubic micra. It will be noted that there are many gaps in the series. This is due to the fact that, 

 owing to the great difficulty of cutting perfect serial sections 4 micra in thickness, some of the nuclei are missing. 

 Again, perfect definition of outline of the mitochondria is required before they can be counted, and since it is im- 

 possible, even with the most expert technique in staining, to attain this ideal in every section, some of them could 

 not be used. 



