386 



PHYSIOLOGY 



CHAP. 



The grey matter of the cord is continuous with that of the 

 medulla, but its shape in the cross-section is considerably altered 

 by the motor and sensory decussations, and by the appearance of 

 the fourth ventricle. This takes place in the upper half of the 

 bulb, where the dorsal columns separate, the grey commissure 

 disappears, and the central canal opens out to form the fourth 

 ventricle or fossa rhornboidalis (Figs. 207, 208). 



FIG. 204. Transverse section of medulla oblongata in the region of the most caudal roots of the 

 hypoglossus. Decussation of pyramids almost complete. (Henle.) Nc, nucleus of funiculus 

 cuneatus ; XII, hypoglossal. Other indications as in preceding ligure. 



When the central canal opens out, the grey matter that 

 surrounded it in the cord comes to lie in the floor of the ventricle, 

 so that the part that was formerly ventro-lateral (representing the 

 base of the ventral horn of the cord) becomes internal or medial, 

 and the homologue for the dorsal horn becomes external and lateral. 

 The nuclei of origin and termination of the cranial nerves lie in 

 this grey matter, which is formed by the breaking up of the 

 motor and sensory columns of the spinal cord. 



There are other grey nuclei in the bulb that are not represented 

 in the cord. After the nuclei of the columns of Goll and Burdach 

 already alluded to, the most important is the nucleus of the olivary 



