No. I.] THE CRANIAL NERVES OF AMPHIBIA. 189 



" feinkornigem Grundgewebe," the appearance in it of fine 

 fibres which form the chief supply of the IX + X, — all these 

 peculiarities demonstrate the homology here suggested. 



This tract is described by other investigators of the medulla 

 of fishes, and there seems to be essential agreement between 

 all their accounts, with variations respecting the number of 

 enlargements presented by it. Rohon (52) gives a more de- 

 tailed account of it in Selachii, and considers it as representing 

 the summation of a number of nerve nuclei. The nerves 

 originating from these he considers homologous to dorsal 

 spinal roots, — a view which is discussed elsewhere. 



Among the Teleosts, Mayser (41) gives a detailed account 

 of the structure of the lobus vagi, which is here enormously 

 developed. Mayser distinguishes five parts in the lobus vagi : 

 (i) The outermost layer, comprising the bulk of the fine-fibred 

 roots, of which there are two layers — a thicker outer of medul- 

 lated fibres, and a thinner inner layer of non-medullated fibres. 

 (2) The gelatinous substance, consisting of a dense ground- 

 substance with numerous nerve cells interspersed. There are 

 also solitary bundles of fine fibres here which join the first 

 layer. (3) The secondary vagus-tract, consisting of fibres from 

 (2) and from the " spongy substance," which is to be considered 

 the central gray. This tract is joined by the secondary tract 

 from the lobus trigemini, and proceeds cephalad to the higher 

 centres. (4) The origin of the thick-fibred motor vagus-roots. 

 (5) Ependyma. 



Mayser, it may as well be added here, describes the lobus 

 trigemini as having much the same structure as the lobus vagi, 

 and on this ground and from the fusing of their secondary 

 tracts he regards them as practically identical. He finally says 

 that we have thus in Cyprinoids three great cranial nerve roots 

 arising from a common nucleus, i.e.^ the continuous substantia 

 gelatinosa of the medulla and spinal cord, these three roots 

 being the ascending and dorsal geniculate (lobus trigemini) 

 Trigeminus, and the sensory Vagus. The ascending and the 

 dorsal geniculate Trigeminus roots together represent the 

 ascending Trigeminus of the higher vertebrates. 



Mayser apparently includes more in the lobus vagi than, for 



