THE NERVOUS SYSTEM. 467 



The chief difference consists in this, that whilst in the Mam- 

 malia, from all we know, the roots of the nerves have no direct 

 connection with the nerve-cells, and merely pass through the 

 ganglion, in Fishes, all the radical fibres are connected with 

 the cells, so that each fibre is interrupted by a bipolar cell, 

 and independent ganglion-fibres are wholly wanting. It. Wagner 

 has thought, that what obtains in the Fish might be applied, 

 unconditionally, to all the Vertebrata, and asserts, that the occur- 

 rence of bipolar cells in the course of the posterior radical fibres 

 is in accordance with Bell's doctrine, and a necessary contingent 

 in the mechanism of the sensitive fibres; and moreover, that in 

 this case the highly important and long-sought distinction 

 between sensitive and motor primitive fibres, has been dis- 

 covered. In opposition to this I have expressed the opinion, 

 that it is not a necessary postulate, that what is found in the 

 Fish should be applied to Man, and that the interruption of a 

 sensitive fibre by a nerve-cell does not distinguish it from a 

 motor fibre. Although Wagner has very recently characterised 

 this opinion of mine as unphysiological, he will not, at the same 

 time, convince any one that the spinal ganglia of the Mammalia 

 are constructed as he thinks, and I shall therefore wait to see 

 whether further observations will confirm my observations or 

 not. In order to complete them, I will moreover mention, that 

 direct measurement of the sensitive roots above and below the 

 ganglia, shows a not inconsiderable difference in favour of the 

 latter situation (vid. ' Mikroskop. Anat./ II, p. 509), which as 

 differences in the thickness of the entering and emergent 

 nerve-fibres, and divisions of them within the ganglion do not 

 exist, can only be referred to the fibres which originate in the 

 ganglion and proceed towards the periphery, a view which is 

 also confirmed by direct observation (fig. 154). With respect 

 to the interesting observations on the structure of the spinal 

 ganglia of the lower animals, and particularly of Fishes, I 

 would refer especially to the works of R. Wagner, Bidder, 

 Robin, and Stannius, cited below.] 



§ 121. 



Further course and termination of the Spinal Nerves. — Below 

 the spinal ganglion, the sensitive and motor roots unite to form 

 a common trunk, their fibres being intermixed in diverse ways, 



