Johnston, The Brain of Petromyzon. 37 



branches with small offshoots. These main branches extend 

 far laterad or dorsad in the walls of the inferior lobes and may 

 even reach the thalamus. Their position is determined mainly 

 by the disposition of the fiber tracts among which they lie. 

 The neurites are directed laterodorsad, and incline caudad in 

 the caudal part of the lobe and cephalad in the cephalic part. 

 In the cephalic part of the lobe the neurites cross behind the 

 chiasma, forming the greater part of the postoptic decussation. 

 These, together with those from the middle and caudal parts of 

 the lobes, which do not cross, pass upward and backward 

 through the lateral walls of the thalamus and mid-brain, form- 

 ing the large tractus lobo-bulbaris et cerebellaris. A small part 

 of the tract enters the cerebellum and the remainder passes back 

 into the medulla. The tract corresponds in every way to the 

 tract of the same name in Acipenser, except that a relatively 

 smaller part goes to the cerebellum. 



The cells of the mammillare are smaller than those of the 

 inferior lobes and both the central processes and dendrites are 

 more slender. These smaller and more slender elements are 

 found also along the ventral border of the inferior lobes, as in 

 Acipenser, and in the postoptic recess. There seems to be no 

 essential difference in the relations of the cells of the mammil- 

 lare and the inferior lobes. The neurites from the mammillare 

 proper form distinct paired bundles which pass latero-dorsad 

 over the walls of the corpus and bend caudad in the base of the 

 mid brain as the tractus mammillo-bulbares (Fig. 30). The 

 fibers of these tracts are more slender than those of the tractus 

 lobo-bulbaris, corresponding to the smaller size of the cells of 

 origin, but the presumption is that the two tracts have essen- 

 tially the same relations in the medulla. 



In the connections between the hypothalamus and fore 

 brain there seems to be one important difference between Pe- 

 tromyzon and Acipenser, namely, that only a small part of the 

 ascending fibers run by way of the anterior coiuniissure, the 

 most crossing in the postoptic decussation. I have been unable 

 to demonstrate fibers to the anterior commissure and the very 

 small size of the commissure is reason for thinking that they 



