HISTOLOGY OF THIi NERVOUS SYSTEM. 297 



this nervous mass, corresponding to the sides of the head, are usually 

 thicker than the band stretching between them, though we are not 

 warranted by the histology in regarding them as two ganglia united 

 by a commissure, as Blumberg^ and others have done. A strong 

 nerve cord runs backwards from each of these lateral swellings, and 

 since they are almost as broad as the latter, they look like direct 

 prolongations. Each passes outwards in a curved course, and, enter- 

 ing the body, becomes one of the above-mentioned lateral nerves 

 which run down the middle layer close by the border,^ to which they 

 sometimes send an external branch. Such is the case in Twnia 

 perfoliata, where also one can observe, opposite the origin of the 

 lateral nerves, another branch, which runs forward between the 

 suckers, evidently designed to supply the head. 



It is difficult to analyse the histological structure of these nerves, 

 and the accounts of their nature vary widely. In Twnia -perfoliata I can 

 recognise, like Kahane, a distinctly fibrous texture in the finely granular 

 clear mass of the cephalic ganglion, and also lying between the fibres, 

 numerous small (0-015-0-025 mm.) ganglion cells with a nucleus, and 

 membraneless, granular protoplasm. These are to be found both in the 

 median commissure and in the lateral swellings, and have usually an 

 oval or triangular form, which is probably to be regarded as related to 

 the presence of fibrous processes, especially since the long diameter of 

 the cell is always in a line with the direction of the fibres. Although 

 these cells are by no means sparsely distributed through the ganglion 

 mass, I cannot at all agree with Blumberg in regarding the latter as 

 merely a conglomerate of ganglion cells. In Steudener's preparations 

 the cells had fallen into pieces, and their presence was only indicated 

 by the " somewhat large round nuclei with nucleoli " which remained. 



The histological analysis is rendered more difficult by the fact 

 that the nerve substance is penetrated throughout by a fine meshwork 

 of supporting tissue, which is not striking in Tmnia perfoliata, but is 

 in some other cases strongly developed (in Tcenia crassicoUis, accord- 

 ing to Steudener). This meshwork is also found in the lateral nerves, 

 where it sometimes predominates so much over the fibres that the 

 early observers spoke of it as " spongy cords." Here and there 

 Kahane and I have both been able to detect distinct cellular struc- 

 tures; but, on the other hand, I have never been able to discover the 



' Blumberg shows (loc. cit. Tab. i., Fig. 1), besides the two main masses, also a third 

 in the sagittal space between the suckers, and speaks in the text of several aggregations of 

 ganglion cells connected by nerve filaments. 



' We have already noted how Sommer erroneously places the lateral nervous cords of 

 Tamia aaginata inside the longitudinal vessels ; Zeitsckr. f. wiss. ZooZ., Bd. xxiv., Tab. xliii. 

 and xliv., 1874. 



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