488 THE NERVOUS SYSTEM 



3. The posterior median septum rapidly becomes a deep sulcus 

 which soon broadens out to form the fourth ventricle, thus push- 

 ing the diminished remains of the posterior columns, with the 

 adjacent dorsal horns, farther and farther lateral and ventralward 

 until the dorsal horns finally come to occupy a position in which 

 their long axis is directed, from the grey commissure, lateralward 

 and but slightly dorsal ward. 



The fibre paths of the posterior columns, consisting now of 

 secondary neurones, having crossed, almost in a body, to the fillet 

 of the opposite side in the ventral portion of the medulla, uncovers 

 the dorso-mesial surface of the dorsal grey horns, leaving them 

 exposed in the floor of the fourth ventricle, which is formed by 

 the much expanded central canal and posterior median fissure. 

 It will thus be seen that the nuclei of the centripetal cranial nerve 

 fibres (sensory nuclei), whose homologues in the spinal cord were 

 found in the dorsal horns, in the medulla oblongata are to be 

 found in the floor of the fourth ventricle, the grey matter which 

 forms this region being both homologous, and nearly continuous, 

 with the dorsal horns of the spinal cord. 



The nuclei of the centripetal paths (motor nuclei), which in 

 the spinal cord included the various cell groups of the ventral 

 horns, are found in the medulla oblongata in two regions : first, a 

 median group near the anterior median raphe, which, in the lower 

 levels of the medulla oblongata, lies just ventro-lateral to the 

 central canal and grey commissure, but higher in the medulla is 

 found on either side of the median line beneath the floor of the 

 fourth ventricle, from which it is separated by a layer of grey 

 matter continued upward from the grey commissure of the spinal 

 cord ; and second, a lateral group representing the detached tips 

 of the ventral horns, which is now found well toward the lateral 

 surface of the medulla oblongata, and which in the lower part 

 of the medulla forms the lateral nucleus, but higher up becomes 

 the nucleus ambiguus of the ninth and tenth cranial nerves. 



The laterally displaced dorsal horns are now separated from 

 the detached portions of the ventral by an insignificant column of 

 white fibres, the upward continuation of the lateral columns of 

 the spinal cord. 



It will later be shown that the motor fibres of the twelfth, 

 sixth, fourth, and third cranial nerves take their origin from the 

 medial group of nuclei, while the motor portions of the tenth, 

 ninth, seventh, and fifth nerves arise from the lateral group. The 



