Johnston, The Brain of Petroviyzon. g 



rior horns. The posterior part (hemispheres of Ahlbokn) cor- 

 responds to the olfactory area of other fishes. The anterior 

 part is the olfactory lobe. The periphery of the olfactory lobe 

 contains glomeruli which appear larger and better defined than 

 in most fishes. 



The floor of the fore brain ventricle contains the small 

 anterior commissure just in front of the preoptic recess and is 

 thin from that point forward, constituting the lamina terminalis. 

 At the dorso-cephalic border of this lamina is the olfactory 

 commissure. 



III. Minute Anatomy. 



It is not my purpose in what follows to give a full and de- 

 tailed account of all parts of the brain for purposes of descrip- 

 tion merely. Since this detailed account has been given for the 

 brain of Acipenser the mere description is of little value except 

 where it is necessary in order to distinguish certain nuclei or 

 centers or to set forth accurately the functional connections be- 

 tween various parts of the brain. I shall therefore omit all 

 detailed description of structures which closely resemble the 

 corresponding structures in Acipenser, or which present no 

 peculiarities of general significance. 



A. Hind Brain. 



a. Motor nuclei and motor nerve roots (Figs. 6-13). 



The existence of two distinct motor columns in the med- 

 ulla has been mentioned above. The ventral column lies at 

 either side of the ventral groove of the ventricle and is a direct 

 continuation of the ventral horn of the cord. The lateral col- 

 umn makes its appearance slightly caudal to the calamus and 

 extends forward just ventral to the lateral groove or angle of 

 the ventricle. In the caudal part of the medulla, i. e., nearly 

 up to the root of VIII, neither column makes any appreciable 

 projection or ridge in the cavity. There is no mingling or inter- 

 locking of the cells of the two columns, but there is everywhere 

 a considerable space between them. In front of IX both col- 

 umns.grow smaller and practically disappear for some distance 



