i6 



NERVOUS SYSTEM OF VERTEBRATES. 



as well as from without, certain ridges and grooves will be seen 

 which indicate a division of the myelencephalon into longitudinal 

 zones (Fig. 3). The ventral portion of the myelencephalon 

 appears to be a continuation of the ventral part of the spinal cord 

 with slight modification. In the floor of the fourth ventricle is 



Cerebellum 



Fasc. long. med. 

 Lobus visceralis 



Fig. 3. — The medulla oblongata and cerebellum of the lake sturgeon (Acipenser 

 ruhicimdus), to show the longitudinal zones. A, dorsal view with the choroid 

 plexus removed. B, C and D, sketches of sections at the levels indicated by the 

 reference lines. The dark area with light circles is the continuation of the ventral 

 horn of the cord. The dark area with rectangular spaces is the continuation of 

 4he lateral horn. The area with oblique lines is the visceral sensory column (lobus 

 visceralis). The area with vertical lines is the somatic sensory column. 



to be seen a deep median groove bounded by two narrow but 

 usually high ridges. The fiber bundles which make up these 

 ridges bear important relations to the motor nerve roots. The 



