74 Journal of Comparative Neurology. 



It has, however, become apparent that even the coarse ar- 

 chitecture of the nervous system cannot be rightly understood 

 unless clear views of the anatomical and physiological signifi- 

 cance of the finest elements which constitute these organs can 

 be gained. In this connection, the investigations of recent 

 years have brought to light much that is new ; and, although we 

 are still far from definitely proven, generally accepted results, 

 yet there has been a clearing of the conception on very essen- 

 tial points. It therefore seems appropriate to give at this time a 

 brief review of the most recent investigations in this field. 



As histological elements which contribute to the structure of 

 the nervous system, it is customary to distinguish nerve cells 

 (ganglion cells), nerve fibres, epithelium cells, blood-vessels, 

 and, finally, those tissues which I shall comprise under the term 

 connective tissues. Only the two kinds of tissue first men- 

 tioned commonly rank as nervous elements, in distinction from 

 the others, which are not nervous. This distinction, however 

 clear and simple it may seem, can be strictly maintained only on 

 the ground of convenience, not as rigidly correct. The epithe- 

 lium cells of the Iming of the ventricles and the nerve cells arise 

 from the same embryonic structure YUranlage^, the neuro-epithe- 

 lium. His has demonstrated in the greatest detail (Arch. f. 

 Anat. u. Phys., 1889,) how in the early stages of development 

 the epithelium cells of the medullary tube are separated into two 

 forms, of which the one corresponds to the spongioblasts, the 

 permanent ventricular epithelium, the other to the germinal cells 

 of the neuroblasts, or ganglion cells. When we recall that many 

 (Klausner, Freud,) have followed processes from the epithelium 

 cells into the bandies of nerve fibres and that these processes 

 take certain staining reagents (e. g. gold), just as nerve fibres 

 do, we find another justification for believing that the position 

 can be no longer unconditionally maintained that the epithelial 

 cells of the medullary tube are sharply separated from the ner- 

 vous elements.' 



But, on the other hand, more recent works (I refer to the 

 very latest of Retzius, Verh. d. biol. Ver. zu Stockholm, 1891, 



iSee evidence in this number, p. 31, seq., that the so-called nerve 

 fibres of the hypophysis are simply prolongations of epithelial cells. [Tr.] 



