422 



intervals in the extreme lateral region of the cord, isolated from the 

 rest of the grey matter, a. and b. are connected with fibres of the 

 posterior roots, c. and apparently also d. send processes to the an- 

 terior. 



On entering the cord, the anterior roots decussate considerably; 

 the posterior roots do not. 



Spinal nerves: Petromyzon. The posterior and anterior roots 

 leave the spinal cord not in the same plane but alternately. There is 

 considerable want of regularity in their arrangement, and often the 

 two posterior roots are themselves not in the same plane. 



The anterior and posterior roots remain separate throughout the 

 body : the former are placed opposite the myomeres, the latter oppo- 

 site the intermuscular septa. The fibres of both roots are non-me- 

 duUated, but possess a nucleated sheath ; the anterior-root fibres are 

 much the larger. 



No ganglion cells exist in the anterior roots. 



On each posterior root, outside the neural arch, is a ganglion, 

 composed of large bipolar ganglion- cells each of which possesses a 

 nucleated capsule continuous with the sheath of the nerve. The peri- 

 pheral fibre of each ganglion-cell is somewhat larger than that running 

 to the cord. In the ganglion the nerve-root bifurcates into a smaller 

 dorsal and a larger ventral ramus, and the ganglion presents a corre- 

 sponding bilobed form, since its cells continue some little way along 

 each ramus. 



Each anterior root similarly divides into a dorsal and a ventral 

 ramus. 



The dorsal rami of the anterior roots pass up over the lateralis 

 nerve to the muscles of the dorsal region. 



The dorsal rami of the posterior roots likewise pass up to the skin 

 of the back, but appear also to send fibres into the lateralis. 

 (For this statement we at present rely only on sections ; but me hope 

 shortly to test it by dissections of the large P. ynurèms.) 



The X*^ cranial nerve (Vagus) arises by four posterior roots, (iden- 

 tical in appearance with the other posterior roots), which unite to form 

 a large bilobed ganglion from which proceed a ventral and a dorsal 

 ramus exactly as in the case of the spinal ganglia. 



The VII *'' nerve (Facialis) presents also a ganglion from which a 

 strong branch winds round the ear-capsule to join the ganglion and 

 dorsal ramus of the Vagus. The ganglion on the VII^'^ is also said 

 (Ahlborn) to be joined by a branch to the ganglion of the V' nerve. 



Lateralis. This nerve consisting of small fibres like those of 

 posterior roots, and containing (except at one point) no ganglion cells. 



