80 



THE MITOCHONDRIAL CONSTlTlTENTS OF PROTOPLASM. 



volumes, especially in experiments. This difficulty is almost insurmountable in 

 muscle-cells, but it is comparatively negligible in nerve-cells. 



Accordingly, Thurlow (1917, p. 37), working in this laboratory, selected nerve- 

 cells for study. She managed to enumerate the mitochondria with surprising accu- 

 racy by inserting in the ocular a glass disk on which a square of known dimensions 

 had been ruled. Using a 1.5 mm. apochromatic objective and sections of known 

 thickness, the number of mitochondria per cubic millimeter of cytoplasm was 

 easily calculated. Observations were confined to the cells of the nuclei of the 

 cranial nerves, because they may be most readily referred to the different func- 

 tional types. She carefully controlled her counts and found that the experimental 

 error was never more than 1.3 per cent. She found that there is a constant num- 

 ber of mitochondria per unit volume of cytoplasm in normal nerve-cells of the 

 corresponding cranial nerves of different white mice; further, that the constant 

 differs for cranial nerves of different types, so that certain groups of nerve-cells can 

 be distinguished by the number of mitochondria within them. The amount does 

 not depend upon whether the cells are sensory or motor in character. A casual 

 inspection of the sections showed at once that the mitochondria are most abundant 

 in the cells of the mesencephalic nucleus of the fifth nerve and least numerous in 

 the nucleus of the tenth nerve. The counts disclosed that there are 284,378,159 

 per cubic millimeter in the former and 178,210,313 per cubic millimeter in the latter. 

 She examined all the cranial nerves, this being only a case in point. 



It is highly desirable that these studies should be 

 extended to other varieties of nerve-cells in the central 

 and peripheral nervous systems, and the study of late 

 embryos and young animals might tell us when these 

 remarkable differences in number become first manifest, 

 for we know that in undifferentiated nerve-cells the 

 mitchondria are fairly uniform in number. We want, 

 of course, to discover whether or not these differences 

 in amount arise at the time of functional maturity ; but 

 whichever way it turns out we shall have obtained a 

 much-needed clue to their significance. Furthermore, 

 this work supplies us with a new criterion of nerve- 

 cell changes which has the rare merit of being quantita- 

 tive, and, as such, may well be compared with the nucleus 

 cytoplasmic ratio which we owe primarily to Hertwig. 

 The quantity of mitochondria does not depend at 

 all upon their form. They niay be heaped up in the 

 condition of granules, rods, or filaments. Similarly, 

 when the mitochondrial content is reduced to a mini- 

 mum either form may predominate. 



It may be argued that these counts of mitochon- 

 dria do not give us any real information about the cells 

 in question, because tissues, kidneys especially, which 



Fig. 6 



-Spinal ganglion cells of the 

 pigeon, showing (a) great increase in 

 the amount of mitochondria as con- 

 trasted with the average amount (b). 



