xlviii 



Journal of Comparative Neurology. 



during the act of swallowing. The usual stroke of these cilia is in a 

 direction away from the mouth, as is shown by the motion of particles 

 of carmine or other insoluble substance. But when a piece of meat or 

 a drop of meat juice touches these cilia they reverse their motion after 

 an appreciable latent interval. 



C. J. H. 



The Histology of the Myeloii.^ 



The ^^ J^eissner's" or ^^ Dorsal Cells ^' of the Cord. Many years 

 since, peculiar cells were noticed in the dorsal portion of the spinal cord 

 of Petromyzon. Reissner described them in i860, and, in 1877, 

 Freud succeeded in tracing their axis cylinders into the dorsal root, 

 though it has since been shown by Beard that they do not originally 

 emerge with the sensory fibres and are of a kinesodic nature. Rohon, 

 in 1885, detected similar cells in the trout and they have since been 

 noticed in the embryonic stages of other Gnathostomata. According 

 to Studnicka, however, these cells, though distinguished from adjacent 

 elements by their great size, are not homologous with the "colossal 

 cells " of the spinal cord of Amphioxus. The latter are median in po- 

 sition and are sparsely distributed; their axis cylinder passes laterad and 

 arches ventrad and, after crossing to the opposite side, extends toward 

 the head. .Some of their smaller processes were traced by Koelliker 



®® 



Horizontal loiigitudiiial section of 

 spinal cold of Aiauiocotes. 



ReissD-er's cells from dorsal region 

 of spinal oord of Pristiiirus. 



into the dorsal roots. Respecting these Studnicka suggests that they 

 may probably be regarded as inherited from some invertebrate prede' 

 cessor. He claims that the " medium-sized" cells of Amphioxus cor. 

 respond with the Reissner's cells of higher forms. These are bipolar, 



^Studnicka, F. K. Ein Beitrag zur vergleichenden Histologic und His- 

 togenese des Riickenmarkes. Sitzb. K'dngl. bohm. Gesellsch., 1895. 



