424 



Myxitie is quite similar to Bdellostoma. 

 Vagus. Petromyzon. 



1) The spinal nerves of the branchial region in passing over the 

 vagus nerve send down branches into it, their anterior and posterior 

 roots entering the nerve separately. (So long ago as 1827 Born noted 

 the existence of communications between spinal nerves and vagus.) 



2) The nerve-fibres of the vagus trunk are very various in size: 

 We may trace 



a. certain very large fibres, identical with the fibres of the an- 

 terior spinal roots, and forming for the most part a distinct 

 bundle on the side of the vagus in contact with the roots : 

 h. a large bulk of small fibres, precisely like those of an ordinary 



posterior spinal root : 

 c. certain exceedingly small fibres, apparently destitute of a 

 sheath. These are the fibres which pass to the heart, jugular 

 veins, etc.: that is to say,, the endocardial nervous system is 

 not independent of the central nervous system as Owsjanni- 

 kow maintains it to be. 

 Of these three groups of fibres we may say that 

 a. enter by the anterior spinal roots, leave the vagus soon 

 after entering it, and pass to muscles of the body-wall. They 

 are never found in the small visceral branches of the vagus, 

 but are ordinary motor fibres. 

 h. alone constitute the cranial root of the vagus; but enter 



from the posterior spinal roots as well. 

 c. arise by division of h. and probably come in like manner both 



from the cranial and from the posterior spinal roots. 

 The Vagus of Petromyzon ßuviatilis contains in its trunk abun- 

 dant ganglia. 



The nerve-fibres a. are never connected with ganglion-cells. 



h. enter ganglion- cells similar exactly to those 



of the spinal ganglia. 

 c. are connected with smaller gangli on- cells, 

 destitute of a nucleated sheath, and found 

 also in the heart. 

 The ganglion-cells in the trunk of the vagus have a rough seg- 

 mental arrangement, being numerous near the points of contact with 

 posterior spinal roots: facts which suggest the possibility of their origin 

 from the spinal ganglia. 



In Bdellostoma and Myxine the following differences are to be re- 

 marked : 



