CHAPTER VI. 

 The Oblongata and the Nuclei of the Ceanial Nekves. 



Let us now turn our attention to that part of the central neryous 

 system which supplies the region of the head with nerves. Rememher that 

 the trunk portion of the nervous system — the spinal cord — ^represents a 

 more or less independent center, joined to the cephalic segment through 

 several tracts varying among different classes of animals; that an animal 

 can live and can make approximately normal movements after the spinal 

 cord has been completely severed from the cephalic part of the central 

 nervous system. Indeed, if the cephalic portion, which contains im- 

 portant nerves for respiration and circulation, remains intact, and if 

 the animal be protected from certain damaging conditions which mani- 

 fest themselves under su.ch circumstances, its existence will not be inter- 

 -rupted through the complete loss of the trunk portion. This is true even 

 for mammals (Goltz); for lower vertebrates it is probable that even the 

 cephalic portion also may remain functionless for a certain period without 

 causing death. 



We come now to the consideration of the complex of nerve-centers 

 which are associated with those already described and which are not physio- 

 logically dependent upon them, though they may be influenced by them or 

 may, in turn, influence them. 



At the cephalic end of the spinal cord one may notice both macroscop- 

 ically and microscopically marked changes of structure; the spinal cord 

 merges into the Medulla Oblongata. These changes are with slight varia- 

 tions similar in all classes of vertebrates; but, among the lower vertebrates 

 in which the branchial region is to be supplied with especially large nerves, 

 the changes are mtich more apparent and clear than among mammals. 



One always observes that the posterior columns diverge from each other 

 and that the commissura dorsalis, just below them, and the adjacent gray 

 substance about the central canal are visible. The dorsal closure of the 

 central canal of the nervous system is effected by only a thin membrane. 

 Passing anteriorly from the point of divergence of the posterior columns 

 this membranous roof becomes wider and wider with the continued diver- 

 gence of the columns. The widened central canal is the fourth ventricle, 

 and the membranous roof — the Tela chorioidea posterior — merges anteri- 

 orly into the Formatio cerehelli. 



(75) 



