THE OBLONGATA AND THE NUCLEI OF THE CEANIAL NERVES. 85 



to Gaskell, the latter fibers enter a mixed spinal nerve, while the former 

 enter the Sympatheticus. The most centrally located segment of the motor 

 columns produce from their lateral divisions the i^f. facialis. The masseteric 

 branch of the Trigeminus receives its fibers from cells which belong to 

 both lateral and ventral divisions of the motor column. 



The posterior horns of the gray matter are also continued through the 

 medulla, where they are encroached upon by the Vagus, the Glosso- 

 pharygeus, the Actisticus, and the Trigeminus after their origin from the 

 nuclei. It was noted in Trigla (Fig. 30, B) that, in the trunk-segment, 

 wherever the sensory nerves are especially strong the terminal nuclei are 

 enormously hypertrophied. The same is quite generally the case with the 

 terminal nuclei of the large medullar nerves. One of these — ^the longi- 

 tudinally extended terminal nucleus of the spinal root of the Trigeminus — 

 has already been described. "We have now to do with a tract which, from 

 its origin in the Gasserian ganglion, passes far back, ending in the cervical 

 portion of the spinal cord. Such tracts are designated descending tracts. 

 All sensory nerves of the medulla have such, but in no other case is the 

 descending tract so large or so well known as in the Trigeminus. Knowing 

 that all posterior nerve-roots after entering the cord give off a descending 

 twig to a part of the gray horn lying farther posterior, one will not be sur- 

 prised to find a similar thing true in the cranial nerve-roots of the medulla. 

 The most posterior cranial nerve — the Hypoglossus — has already been de- 

 scribed. It arises from a cell-group in the anterior horn (see Fig. 43). 



From the same segment of gray matter, but from a somewhat laterally 

 located cell-group, in reptiles, birds, and mammals, arise fibers which, pass- 

 ing laterally, leave the central system as the Aecessorius (see Fig. 43). 



In the lower vertebrates this nerve arises almost in the same manner, 

 but its fibers usually pass higher and leave the medulla with the Vagus, 

 joining the motor part of that nerve. In that case there is no reason why 

 one should not compare the most posterior root of the motor Vagus with 

 the Aecessorius. 



The Vagus itself passes to the dorsal margin of the medulla from the 

 Ganglion jugulare. It penetrates the fiber-system that here lies in its way 

 {i.e., the Fibrse arcuatse interna arising from the nucleus of the posterior 

 columns); also often the ascending spinal root of the Trigeminus, and ends 

 there in a noticeable enlargement of the gray matter belonging to the pos- 

 terior columns farther back. This may be readily seen in the accompanying 

 figures from the medulla of a gold-fish. 



In the figure one notes that the terminal nucleus of the Vagus is rela- 

 tively very large: it has to receive a much larger nerve than in other verte- 

 brates. A veritable tumor — the Lohus nervi vagi — results here. 



In the brain of the sturgeon (Fig. 39) the nucleus of the Vagus is 



