425 



1) The vagus has no communication (as in the gill-region of Pe- 

 fromyzon) with the spinal nerves ; and correspondingly 



2) the large fibres a. are entirely absent, and 



3) the ganglion cells in the trunk of the nerve lack the segmen- 

 tal arrangement. 



Thus the Vagus is inherently compound in its nature. 



Firstly, in Petromyzon it contains large motor anterior-root fibres, 

 comparable to a spinal accessory (in part). The difference in this 

 respect between the vagus oî Petromyzon and the Myxinoids is referable 

 to the different function of the branchial skeleton. In Petromyzon the 

 water both enters and leaves by the external branchial orifice moved 

 by a pump-action of the branchial basket: in Myxine it is inspired 

 through the nose, by the agency of the cilia therein. 



Secondly, apart from this, in Petromyzon the vagus in the bran- 

 chial region includes small commissural (?) fibres derived from poste- 

 rior roots, and clumps of ganglion cells derived (?) from posterior-root 

 ganglia (cf. On odi), that is to say the typical elements of a ,sympa- 

 thetic'. 



No segregated parts of spinal ganglia, and no such commissural 

 fibres of posterior-root origin occur in the post-branchial region of 

 Petromyzon. The , sympathetic' is in this animal not only associated 

 closely with the vagus, but, it is furthermore peculiar in not extending 

 as a commissural system, backwards beyond the branchial region. 



In Bdellostoma^ though a nerve from the vagus reaches back to 

 the anus, even the branchial region is devoid of communications be- 

 tween successive spinal posterior roots, that is to say of a commissural 

 sympathetic system. 



But the cranial origin of the vagus in Petromyzon is from the 

 fusion of four gangliated dorsal roots, which according to A h lb or n, 

 are likewise connected with the four similar roots of the glossopha- 

 ryngeal, and in turn with the origins of the seventh and (though less 

 certainly) of the fifth also. 



Now the fibres of these vagus-roots are as has been abeady said 

 exactly similar to those of spinal posterior roots; and the vagus 

 ganglion (g. jugulare) is in its bilobed form precisely like a large spinal 

 ganglion. So all the visceral fibres in the vagus, whether cranial or 

 spinal in origin, arise from precisely similar posterior roots, — a fact 

 with important physiological bearings. 



We suppose the connections between the (5*^) 7*^', 9*'' and 10*^'' 

 nerves and between the successive roots of the two latter to be in part 

 commissures of visceral fibres. 



In the embryo dog-fish the second or ventral commissure de- 



