SECT. 126.] NERVOUS SYSTEM. 263 



between the motor cells of different localities. Even the cells of 

 the cortical substances of the cerebrum, which physiology assigns 

 as the seat of the mental functions, exhibit no peculiarities, per- 

 ceptible by any means at our command. With regard to the 

 there-tubes, anatomy fails to indicate distinctive characters in them 

 in the sensicnt and motor nerves; a fact, however, which affords 

 no adequate reason, in physiology, for assuming identity in their 

 functions. Of the differences in thickness of the nerve -fibres, it 

 may be remarked, that the manifold changes in diameter which all 

 the fibres of the cerebro-spinal nerves undergo in their progress, are 

 well calculated to show, that these conditions have, in general, no- 

 thing to do with the functions of the fibres. Nevertheless, I do not 

 regard their differences of diameter as of no importance; and the 

 attenuation of the tubes when they pass through grey substance, 

 and their fineness as well as, in many cases, the absence of white 

 substance at their origins and terminations, appear to me points of 

 no small moment. On the assumption that the medullary sheath 

 only represents a protecting soft covering for the delicate central 

 fibre, and that the latter alone is the active and conducting me- 

 dium, it can be easily comprehended why the dark-bordered nerve- 

 tubes are more readily stimulated, wherever the medullary sheath 

 is thin or absent, and the central fibre lies more free (as in the 

 nerve-terminations in the eye, ear, olfactory organs, etc.). The 

 finely dotted substance, which is found so often in the higher 

 central masses, and supporting the most delicate nerve-tubes and 

 cells, appears to me to have a similar protecting function. 



Of the methods employed in investigating the nervous system, the chief 

 have been already referred to in the preceding section. I may here call attention 

 once more to the importance of chromic acid in examining the course of the 

 fibres and the central nerve-cells, and to the great value of diluted caustic soda 

 in indicating the nerve-tubes in transparent parts. The extreme proneness of 

 the grey and white substance to undergo changes, particularly the tearing off 

 of the processes of the nerve-cells and the varicosity, coagulation and break- 

 ing down of the nerve-tubes may be once more insisted on. The brain and 

 cord are best studied in man, as also the elements of the ganglia ; but, on the 

 other hand, the course of the fibres in them and the nerve-terminations are 



rt seen in small mammalia. To find out the small ganglia in the heart, 

 Ludroig recommends treatment with phosphoric acid and solution of iodine 

 in hydriodic acid, so diluted as to have merely a brownish tint. For study 

 of the development, human and mammalian embryoes are very well adapted ; 

 but the batrachian larvae, and, when opportunity offers, the electric organs of 

 the embryo of the ray, should not be neglected, since in these the relations 

 are by far most obvious. 



Literature. — J. E. PuBKINJE, in the "Report on the Meeting of the German, 

 Naturalists, in Frag, in 1S37, Frag, 1838, p. 177 ; and in Mull. Archie., 18+5, 



