212 



STUDY OF PIG EMBRYOS. 



wholly obliterated. The floor-plate undergoes a great development in later 

 stages and is transformed into the median raphe of the adult medulla. The 

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

 exit of the lateral roots (Fig. 125, L. R). The ventral limit of the dorsal zone is 

 marked by the entrance of the sensory or ganglionic fibers (Fig. 125, G. tri; Fig. 

 126, Fac). Toward the dorsal side the dorsal zone gradually thins out and passes 



over into the ependymal, epen. The great 

 development of the lateral roots is perhaps 

 the most important single 6haracteristic 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-pharyn- 

 geal or ninth; and fourth, the vagus or tenth. 

 There are no lateral roots known to occur an- 

 terior 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 down. But even in the cervical 

 cord the lateral roots attain but a slight devel- 

 opment. The contrast with other portions of 

 the central nervous system makes the great 

 development of the lateral roots in the medulla 

 oblongata all the more striking. The dorsal 

 zone of the hind-brain lags considerably behind 

 the ventral zone in its development, and at all 

 stages the ventral zone forms a larger propor- 

 tion of the medulla than does the dorsal zone. 

 Section through the Trigeminal Roots. — The section passes through the widest 

 part of the hind-brain, the cavity of which is enormously distended. 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 witli 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 



Ec. 



epen . 



D.Z. 



T.S. 



I..R. _ 



cm. 



Card. - 



Fig. 125. — Pig, 12.0 mm. Frontal 

 Series 6, Section 284. 



Card, Anterior cardinal vein. D.Z, Up- 

 per portion of the dorsal zone of His. 

 Ec, Ectoderm. epen, Ependymal 

 roof of the fourth ventricle. G.tri, 

 Ganglion trigemini. I..R, Lateral 

 root of the trigeminal nerve, mes, 

 Mesenchyma. T.S, Tractus soli- 

 tarius of W. His. X 22 diams. 



