496 SPECIAL HISTOLOGY. 



a more free condition, the nerve-fibres are more readily excited 

 and able to communicate their conditions ; and as regards the 

 pale nerve-fibres, in this case they would essentially have the 

 same functions as the others, and the absence of the medullary 

 sheath iu them could either be explained on the supposition, 

 that they are less readily excitable, as in the invertebrate 

 animals, and the Cyclostomata, or because they occur in situa- 

 tions where a protective tunic to the nerve-fibres is no longer 

 required, as in the retina, in the nasal mucous membrane, in 

 the grey substance, and in the electric organs, or even where its 

 refractive power upon light would be prejudicial to a certain 

 object, as in the cornea. A similar mechanical function 

 appears to me to be performed by the fine granular substance, 

 which in the higher central organs is found in so many situa- 

 tions supporting the most delicate nerve-fibres, cells, and 

 processes. 



[With respect to the methods to be employed in investiga- 

 tions of the nervous system, the principal have been noticed in 

 the preceding sections. I will, here, once more advert to the 

 importance of preparations made with chromic acid in the in- 

 vestigation of the course of the fibres, and in the examination of 

 the central nerve-cells; and direct attention to the dilute solution 

 of caustic soda for the detecting of nerve-fibres in non-trans- 

 parent tissues, — without which two means very many points 

 would remain in the dark. In this way also the extreme prone- 

 ness to become changed, of the grey and white substances, and 

 particularly the ready disruption of the processes of the nerve- 

 cells, and the varicosity, coagulation, and destruction of the 

 nerve-fibres, are at once removed or avoided. The brain and 

 spinal cord, as well as the elements of the ganglia, are best 

 studied in the human subject, but the course of the fibres in 

 them, and, above all, the terminations of the nerves, are best 

 investigated in the smaller Mammalia, and only in the second 

 place in Man. In the searching for the minute ganglia in the 

 heart, Ludwig recommends the treatment with phosphoric acid 

 and the solution of iodine in hydriodic acid, the latter so diluted 

 that it has only a tinge of brown. For the development of the 

 nerves, the human and mammalian embryo are quite suitable ; 

 but the batrachian larvae, and if opportunity offer, the electric 



