ANATOMY OF A CYCLOSTOME BRAIN 647 



laterale (figs. 10 and 11; cf. Johnston '02, p. 29). Moreover, 

 Tretjakoff describes ('09, p. 729) neurones of this region whose 

 neurites pass into the postoptic decussation to end in the oppo- 

 site thalamus, these fibers being comparable with the mammalian 

 commissure of Gudden from the corpus geniculatum mediale. 



The hahenula has been briefly commented upon in connection 

 with the description of the lateral surface. 



Sulcus subhabenularis. On the left side the habenula is bounded 

 below by a sharp deep crescentic subhabenular sulcus on the ven- 

 *tricular surface; but on the right side this condition prevails 

 only under the anterior and posterior ends of the habenula. The 

 middle part of the sulcus is partially obliterated by irregular 

 swellings occupied by fibers of the large fasciculus retroflexus 

 (figs. 4, 10) and by clusters of small cells which resemble those 

 of the habenula. 



Below the habenula there is a distinct eminence which has 

 already been mentioned as visible on the lateral surface, the 

 lohus subhabenularis thalami, which is present on both sides, 

 though smaller on the left. On the left side it is smooth, but 

 on the right it bears on the ventricular surface two vertical 

 ridges. This lobe is traversed by the fasciculus retroflexus, which 

 is small on the left side and exerts no influence on the ventricular 

 surface. On the right side, though the fasciculus is very large 

 and lies immediately adjacent to the ventricular ependyma which 

 covers the subhabenular lobe (fig. 9), it does not produce either 

 of the vertical ridges just mentioned. In fact, the thickest part 

 of the fasciculus lies in the floor of the groove between the two 

 ridges (sulcus thalamicus 3, see below). The development of the 

 subhabenular lobe is undoubtedly due primarily to a proHferation 

 of cells in this region (fig. 10), and the appearances strongly sug- 

 gest that on the right side the enlargement of the fasciculus re- 

 troflexus results in a displacement forward and backward of some 

 of these cells, thus producing the two vertical ridges. In front 

 of the first of these ridges and separated from it by a light sul- 

 cus (sulcus thalamicus 2, cf. fig. 3) is another vertical ridge, the 

 eminentia thalami, which is present in identical relations on both 

 sides. 



