854 



THE NER VE SYSTEM 



The simple brain tube expands very early in intrauterine life in a sac-like 

 manner, with the formation of three dilatations or pouches the primary brain 

 vesicles demarcated by two constrictions. 1 The vesicles are designated respec- 

 tively the 



Fore-brain (Prose ncephalon). Mid-brain (Mesencephalon). 



Hind-brain (Rhombencephalon or Metencephalon). 



ECTODERM 



FORE-BRAJN 



NEUROPORE 



FIG. 620. Brain tube of embryo salamander, sagittal section, 

 showing neural segmentation (neuromeres): F.I. -III. Fore-brain 

 neuromeres. M. I.-II. Mid-brain neuromeres. H. I. VI. Hind- 

 brain neuromeres. (Adapted from Kupffer.) 



MID-BRAIN 



HIND-BRAIN 



PROTOVERTEBRAl. 



FIG 621. Brain tube of chick (25J4 

 hours), showing partly closed brain 

 tube with eleven folds or neuromeres. 

 (After C. Hill.) 



This classification has been 

 found acceptable from every com- 

 parative standpoint in brain mor- 

 phology, but attempts have been 

 made to establish a further seg- 

 mentation into definite anatomical 

 divisions regarding which opinions 

 and usages differ widely and have 

 proved to be a hindrance rather 

 than an aid to the homologization 

 of brain structures in the vertebrate 

 series. The difficulties in formu- 

 lating a satisfactory schema of the 

 segmental divisions of the brain 

 will be overcome, perhaps, only 

 by distinguishing the neuromeres 

 or neural segments conforming to 

 the general segmental plan of the 

 vertebrate body. The existence 

 of a neuromerism that is akin to the metamerism or serial segmentation of the 

 body, or to the branchiomerism characterizing the arrangement of the brand lial 

 arches, is indicated in several ways, but thus far only the earliest embryonic 



1 The constriction between mid- and hind-brain has been called the isthmus rhombencep/tali by Prof. His. :nu? 

 he regards it as coordinate with the other segments recognized by him; the region, however, no more deserves 

 a definitive segmental value than would the cephalic constriction even if it were dignified by the term isthmus 

 prosencephali. 



Middle peduncle 

 Inferior peduncle 



Medulla oblongata 



FIG. 622. Scheme showing the connection of the several 

 parts of the brain. 



