1 202 



SCANDINAVIAN FISHES. 



The spinal cord consists of m flattened band, of 

 iiniforiu breadtli and soiiirwliat trianijjular in section, 

 which in a fairly spacious niedullispinal canal, lying on 

 the notochord, follows the latter to its posterior termi- 

 nation, and there extends in a contracted ependymal 

 tilum terminale somewhat hcliind the same (tig. 3.54,/). 

 By means of improved colouring methods the form, dis- 

 tribution, and proces.ses of the ependymal as well as 

 the neurogliar and ganglion cells has been elucidated on 

 M fairly extensive scale in the spinal cord and jjartly 

 too in the brain. 



The nerves (jriginating in jjairs, but not (piite sym- 

 metrically, from the spinal cord (spinal nerves) each 

 have a ventral (raotory) and a dorsal (sensory) root, 

 and tlie latter is furnished with a ganglion. As in 

 Petrowi/zon, all tlie nerve-libres are "pale", being with- 

 out myeline sheatii. The motory nerve-tibres end their 

 terminal ramitications on tlie muscular fibres in free 



I q d 

 Fig. 360. Auditory urgan of the riglit sirle of Mi/xine gluttnosa, 



seen from above and within, X 18. 

 a, internal division witli e, the macula acnstica; b, external division 

 with an ampulla at each end (o, (/) and a crista acustica (/, g) in 

 each ampulla; /, /, branches of the acoustic nerve; //, endolymphatic duct. 



"end-trees" within more or less marked end-hillocks 

 (end-plates). The sensory tibres terminate within the 

 different organs also in free intercellular ramitications. 

 Such is the case too with the nerves in the epithelium 

 of the skin and raucous membranes. No special sensory 

 organs (end-cells, end-buds, sensory corpuscles, etc.) 

 iiave been demonstrated. Even in the tentacles sur- 

 rounding the oral and nasal apertures only free terminal 

 ramitications of the sensory nerves in the epidermis 

 have been traced, liut here there is a slight sign of 

 end-buds, round whose cells the nerve-ends ramify. 



No lateral line witli aijpertaining end-buds ai)pears either 

 on tlie head (jr the bod)'. In the subcutaneous nerve- 

 plexus, on the other hand, especially in the head, nu- 

 merous ganglion cells have been found embedded. Pos- 

 .sibly they belong to the sympathetic nervous system, 

 whereof else no trace has hitherto been discovered. 

 These ganglion cells further remind us by their po- 

 sition in some degree of the cells in the rostral region 

 of the Lancelet, close to the foremost nerve-pair. 



Of the (iryans of sense proper the oJfnctnrii oryan 

 is obviously the most powerfully developed. It forms 

 (tig. 8.59, i) a conical organ situated close before the 

 brain in a cartilaginous basket and con.sisting of nearly 

 vertical and dense folds of raucous membrane, their 

 surface covered with olfactory epithelium, i. e. olfactory 

 cells with supporting cells arranged between them. The 

 filamentous, central processes of the olfactory cells enter 

 the olfactory bulb, and terminate there in numerous 

 ramitications and with free ends in the olfactory glome- 

 ruli, in which other nerve-fibres, from the ganglion 

 cells (mitral cells) of the bulb, running towards the 

 periphery, also terminate in free ramitications. The ol- 

 factory organ of Myxine thus exhibits in the main the 

 same structure as this organ retains throughout the 

 whole of the vertebrates, even including man. Here 

 there is no trace of "olfactory end-buds", as Blaue 

 has endeavoured to demonstrate in the Teleosts. Neither 

 in the latter nor the Cyclostoraes, nor indeed in any of 

 the vertebrates, do such "end-buds" exist as Blaue 

 meant; and his theory of the genetic connexion between 

 the olfactory organ and the sense-organs of the skin, 

 especially the lateral line, is therefore without ground 

 in the structure of the said organs. 



Special oryans of taste have not yet been demon- 

 strated in Myxine, whereas Petromyzon and its larva dis- 

 play witiiin tlie mouth cavity bulbous or genimiform 

 organs tliat may with reason be regarded as represent- 

 ing tiie gustatory organs of the higher vertebrates". 



The oryan of sight in Myxine'' is very rudimentary. 

 It is composed of the two very small, (jblong ej'es dis- 

 covered by JoHANNE.s MuLLEK, which, covered by the 

 skin and a layer of muscles, lie on each side of the 

 fore end of the brain (olfactory brain) and close to the 

 posterior outer angle of the cartilaginous basket invest- 

 ing the olfactory organ (fig. 3.59, k). The two optic 



« Cf. G. Retzius, Biol. Unters., N. Folge, Bd. V, p. 69, taf. XXVII. 

 ' Cf. „ , 1. c, p. 64, taf. XXVI. 



