FRONTAL SECTIONS OF EMBRYO OF 12 MM. 



297 



morphologically dorsal limit of the ventral zone is marked by the exit of the lateral 

 roots (Fig. 203, L.R). The ventral limit of the dorsal zone is marked by the en- 

 trance of the sensory or ganglionic fibers (Fig. 203, G.tri; Fig. 204, Fac}. Toward 

 the dorsal side the dorsal zone gradually thins out and passes over into the 

 ependyma, epen. The great development of the lateral roots is perhaps the most 

 important single characteristic of the medulla oblongata. They furnish the principal 

 motor or efferent nerve-tracts of the brain and form 

 an important constituent part of four nerves: first, 

 the trigeminal or fifth; second, the facial or seventh; 

 third, the glosso-pharyngeal or ninth; and fourth, 

 the vagus or tenth. There are no lateral roots 

 known to occur anterior to the medulla oblongata, 

 unless possibly the fourth nerve, the relations of 

 which in many respects are peculiar, should turn 

 out to be a lateral root. In the spinal cord we 

 find lateral roots in the upper cervical region, and 

 it is not improbable that they may yet be found 

 associated with the dorsal roots of spinal nerves 



lower f'down. But even in the cervical cord the 







lateral roots attain but a slight development. The Card 

 contrast with other portions of the central nervous 

 system makes the great development of the lateral 

 roots in the medulla oblongata all the more strik- 

 ing. The dorsal zone of the hind-brain lags con- 

 siderably behind the ventral zone in its develop- FIG. 203 

 ment, and at all stages the ventral zone forms a 



PIG, 12.0 MM. FRONTAL 

 SERIES 6, SECTION 284. 



larger proportion of the medulla th n does the Card > Anterior cardinal vein - D - Z ' U P~ 



per portion of the dorsal zone of His. 

 EC, Ectoderm, epen, Ependymal 



roof of the fourth ventricle. G.tri, 

 Ganglion trigemini. L.R, Lateral 

 root of the trigeminal nerve, mes, 

 Mesenchyma. T.S, Tractus soji- 

 tarius of W. His. X 22 diams. 



Section through the Trigeminal Roots (Fig. 203). 

 The section passes through the widest part of the 

 hind-brain, the cavity of which is enormously dis- 

 tended. It is bounded on the dorsal side only 

 by the very thin ependymal roof, epen, which 



does not form any part of the true nervous structure, although it passes into and 

 is directly continuous with the dorsal zone, D.Z, which is thus seen to be only a 

 thickened portion of the wall of the neural tube, just as the ependyma is the 

 attenuated deck-plate. The trigeminal ganglion, G.tri, is very large and sends 

 its sensory fibers upward into the dorsal zone to form there a distinct bundle of 

 nerve-fibers which persists throughout life and is known in the adult as the tri- 

 geminal tract, T.S. The entering sensory fibers fork; their ascending branches form 

 the relatively short ascending tract, their descending branches the much longer 

 descending tract, which gradually grows through the length of the medulla oblon- 



