ROOTS OF THE VAGUS GROUP. 89 



in fact the more cephalic of the root fibres run over the 

 outer and dorsal surface of the spinal V tract. 



The vagus ganglion seems single macroscopically, like 

 the root, but microscopically it is clearly separable into 

 four ganglia, as shown in Fig. 4, corresponding to the 

 four branchial clefts innervated by this nerve. In each 

 case the ganglion cells are separated from those of the ad- 

 jacent divisions by the entering and emerging fibres, so 

 that in Weigert preparations especially the discreteness of 

 the ganglia is very conspicuous. This is slightly exagger- 

 ated by shrinkage during hardening. Nearly all teleosts, 

 according to Stannius, exhibit such a branchiomeric con- 

 dition of the vagus ganglia, though there is the widest 

 variation in its amount. We have numerous steps to the 

 condition in Raja (Shore, '89), where the widely separate 

 ganglia lie each in its own branchial ramus. And this is 

 especially interesting in view of Cole's more recent dis- 

 covery ('96) that in Chimaera the IX nerve and the three 

 branchials and the r. intestinalis vagi have quite indepen- 

 dent origins and ganglia, like the IX in teleosts. The 

 distinctness of the vagal ganglia in the present case may 

 therefore be looked upon as ve^tigeal, and not merely as 

 an adaptation to the existing branchial apparatus. 



The fourth ganglion in Menidia {g. X. ^+5) is much 

 the largest. It includes, besides the ganglion for the 

 nerves of the fifth gill cleft, which are much smaller than 

 the others of the series, the ganglion for the great visceral 

 and oesophageal rami of the vagus. The ganglia for these 

 various rami are indistinguishably fused. Lying dorsally 

 of this ganglion and only imperfectly separable from it is 

 the jugular ganglion, or ganglion of the rami cutanei dor- 

 sales {g. X. 6). 



Rememberine that these communis fibres of the vagus 



