238 H- F. Osborn 



(Menobranchiis. Menopoma), where the nucleus is wanting, while 

 the tract is strongly developed and its course forwards is readily 

 observed (fig. 14). The further course of the tract in Ran a is pa- 

 rallel with that of the cerebral peduncles and, in carmine prepara- 

 tions, it can be faintly distinguished by its deeper red color (fig. 10 

 p.ol.), and by the fineness of its fibres. As the sections pass for- 

 wards, we observe fine fibres given off towards these tracts from 

 the cells of the central gray. It is not determined how far this 

 bundle forms a commissure of the olfactory lobes. 



The lower tract of the lower bundle can be readily observed 

 in sagittal and horizontal sections. After following the course of the 

 upper tract for some distance as a small compact bundle, it turns 

 backwards, above the cerebral peduncles, and is distributed in the 

 postero- lateral region of the stem of the hemispheres (fig. 10^^^). 

 When followed from side to side in sagittal sections I find that it is 

 distinct from the upper tract throughout and may be regarded as an 

 independent bundle. Along the ventral region of the inner mantle, 

 upon the horizontal level of the anterior commissure is observed a 

 fine tract of fibres which pass backwards and cross each other in 

 front of the anterior commissure (fig. 1 1 ol.i) . After crossing, the 

 fibres scatter below the anterior commissure. This is the inner 

 olfactory tract. Anteriorly the fibres pass into the olfactory 

 lobes, and cannot be followed further. There is a strong resem- 

 blance, between the relations of this tract with the main part of the 

 lower bundle p.ol and the inner olfactory tract ol.i. with the ol- 

 factory commissure o.cm in the carp's brain. 



The upper bundle ascends in the medial hemisphaeral walls, 

 forming the posterior boundary of the foramen of Monro, and then 

 turns forwards roofing the foramen. In vertical sections there is a 

 well marked division of the commissural portion into two parts of 

 equal size. This is also seen in horizontal sections and gives the 

 impression that this is not a true commissure but in large part a 

 chiasma, or crossing of tracts which pass from the hemisphere of 

 one side into the optic thalamus of the opposite side. This bundle 

 has in fact been described and figured as a chiasma by Bellonci. 

 If successive horizontal sections, from the level of the commissure, 

 are carefully followed upwards, it appears that the two divisions 

 reunite above into one bundle, the whole of which turns forwards 

 into the hemispheres (fig. 12). The same fact is shown still more 

 clearly in vertical sections, where the two divisions are finally ob- 



