66 Journal of Comparative Neurology. 



HousER (oi) describes in Mustelus a distinct fasciculus com- 

 munis formed of root fibers of VII, IX, and X, whi :h runs 

 back into the cord. He does not mention a commissura infima 

 Halleri or a median nucleus. In the lobus vagi he finds only 

 cells of the II type, which he thinks come into relation with 

 the cells of the viscero-motor nucleus. It seems probable that 

 HousER has worked with imperfectly impregnated preparations, 

 since cells of the I type giving rise to a longitudinal tract are 

 present in teleosts, ganoids, and Petromyzon. The fact that 

 cells of the II type are few or absent in Petromyzon shows that 

 the I type cells and secondary vagus tract constitute the more 

 fundamental part of the central apparatus for the fasciculus 

 communis system. Moreover, the direct connection of II type 

 cells of the sensory nucleus with the motor cells is not probable 

 on general grounds. It is not possible that all the visceral re- 

 flexes should be carried on by so simple an apparatus as sug- 

 gested by HousER (p. 90, 94). 



The general result of the study of the sensory systems of 

 Petromyzon is to add support to every one of the theoretical 

 considerations brought forward in discussing the brain of Aci- 

 penser. 



B. The Mid, ' Tzvccn, and Fore Brain. 



The only paper on these regions of the Petromyzon brain 

 based on work by modern methods is that by F. Mayer ('97). 

 A discussion of this papfer will lead to the chief questions which 

 I wish to consider. It is to be regretted that the final paper of 

 this author has not as yet appeared as the preliminary paper is 

 so brief that, in the absence of illustrative figures, it is impos. 

 sible to know the author's meaning in all cases. P"or the same 

 reason it is impossible to criticize the author's work in detail. 

 There are certain statements of Mayer's which, in the opinion 

 of the present writer, receive no confirmation from any work 

 which has hitherto been done on the brain of lower vertebrates. 

 These points can not be discussed at least until the complete 

 paper appears, but they are enumerated here. They are : the 

 existence of a tractus thalamo-olfactorius arising from cells of 



