98 ANATOMY OF THE CENTRAL NEETOUS SYSTEM. 



It is probable that this nucleus is identical with one consisting of exactly- 

 similar cells, which one finds in amphibians and reptiles quite near the 

 median line of the Tectum opticum, or roof -nucleus, of the midbrain, which 

 is especially large in turtles. 



All trigeminus nuclei receive important afEerent bundles from the fillet. 

 These approach the nuclei after decussation, as is the case with other nerve- 

 nuclei. Besides this they receive from the cerebellum a contribution regard- 

 ing which it is uncertain whether it ends in the nucleus or leaves the brain 

 direct with the nerve. This accretion from the cerebellum to the lobus 

 trigemini is very large. ■ 



Up to the present we have, for the sake of clearness, let it be understood 

 that the medulla contains nothing but the nuclei of the cranial nerves, with 

 their associated tracts and connections. But there are still other fibers which 

 enter the medulla on their way from the spinal cord to the cerebellum, as 

 well as some which pass from the cerebellum into the nuclei of the medulla 

 itself. Moreover, the medulla contains, besides the cells and fibers of the 

 association-system, a number of special ganglia. Only one of these— the 

 Oliva superior — has been mentioned in connection with the description of 

 the Acusticus. But in all vertebrates there are still other groups of nuclei. 

 In mammals the largest of these is the group designated Oliva inferior, 

 which stands in close relation to the cerebellum, by which fact it is char- 

 acterized. Up to the present time (if one stands by this characteristic) an 

 oliva inferior has not been discovered in any vertebrate except in the mam- 

 mal. We may, indeed, find, in reptiles, birds, and even in amphibians, nuclei 

 which are similarly located in the medulla and which, in reptiles, are of simi- 

 lar structure (see Pig. 46); but a positive interpretation of this group is, as 

 yet, lacking. The same is true of nuclei which are everywhere demonstrable 

 at the ventral margin of the medulla, and especially toward the posterior 

 end. At this point one finds, in mammals, the nuclei arciformes. Of all the 

 details given, or possible still to give, remember especially that the numer- 

 ous longitudinal bundles whose cross-sections one sees in the ventral and 

 lateral regions of all cross-sections of the medulla contain, besides the fasciculi 

 of the association-system, connecting-fibers to the mesencephalon and thala- 

 mencephalon. We have already met the tracts from the thalamencephalon 

 to the spinal cord when considering the lateral columns. Even in the 

 medulla this tract, especially well-marked in fishes, lies in the lateral region. 

 It is much larger than in the spinal cord and gradually decreases posteriorly. 



There are two fiber-systems of the medulla which we must now con- 

 sider somewhat in detail, because they are of especial importance physio- 

 logically. 



The first one of these is the fascicle from the nuclei of the posterior 

 columns to the fillet, the most anterior fasciculi of the Tractus tecto-spinalis. 



