599 



and the fibres of the ascendiDg bundle just mentioned, all help to 

 complicate and confuse the structure in this region to the last degree. 

 A similar complication of structure in other forms may account for 

 the failure to trace the fibres to their final endings, or it is possible 

 that in the embryological material on which Van Gehuchten ('94) 

 and Cajal ('96) worked, the fibres of Meynert's bundles had not yet 

 reached their full growth. Among or adjacent to the decussating 

 fibres of Meynert's bundles I have found in several series a few small 

 cells, with few and short dendrites, which belong to the Corpus inter- 

 pedunculare. The number and size of these cells show them to be 

 wholly inadequate to serve as the terminal nucleus of the bundles in 

 question. However, they may serve in fishes as a nucleus of secondary 

 importance which becomes of greater importance in higher vertebrates. 

 The neurites of one or two cells in the C. interpedunculare were found 

 directed dorsad. 



Most or all of the fine fibres of these bundles have a different 

 destination from that described for the coarse fibres. Many of them 

 decussate at an acute angle ventral to the decussation of the coarse 

 fibres, but some at least diverge and run to their destination without 

 decussating. The further course of these fine fibres I have been 

 unable to determine with full certainty, but I believe that they are 

 identical with certain fine fibres which course around the lateral 

 surface of the anterior end of the medulla and end in a nucleus of 

 small cells on the lateral surface of the Lobus lineae lateralis. This 

 collection of small cells is in fact an extension of the granular layer 

 of the cerebellum (Fig. 8). 



The cells of the terminal nucleus in the base ot the mid-brain 

 are closely similar in their form and relations to some of the 

 commissural cells of the medulla. I am inclined to regard them as 

 a specially modified group of such cells. Their neurites pass into the 

 ventro- lateral tracts of the opposite side and probably come into 

 relation finally with the motor cells of the medulla and cord. This 

 brings the Ganglia habenulae into close relation with the motor 

 centers, and the very profuse end-branching of the fibres of Meynert's 

 bundles and the large size of the terminal nucleus show that this path 

 is of considerable importance. It gains this importance from the 

 fact that the G. habenulae are important secondary nuclei of the 

 olfactory paths. 



The chief results stated in this paper may be summarized as 

 follows : 



