754 



THE NERVOUS SYSTEM 



The mesial diverticulura soon after its formation divides into two secondary vesicles, each of 

 which represents a cerebral hemisphere and each of which gives off ventrally from its anterior 

 part a narrow, tube-like diverticulum which is transformed into the olfactory bulb and olfac- 

 tory tract of the adult encephalon. As development proceeds the cavities of the olfactory diver- 

 ticula become occluded in man. However, in many of those animals in which the olfactory ap- 

 paratus attains greater relative development than in man the cavities persist as the olfactory 

 ventricles. The optic vesicles never persist as ventricles in the adult. 



FIG. 559. DIAGRAMS OP SURFACE VIEWS AND SECTIONS OF OEKMIXAL ABEAS SHOWING THE DE- 

 VELOPMENT OF THE NEURAL GROOVE. 



A. Earlier stage. B. Later stage. A'. Section through area A along the line a. B'. Section 



through area B along line 6. 



FORE-BRAIS 



KMnitroxic 



I /;/. I 



PRIMITIVE 

 STREAK 



S.VBRY- 



oxw 



A HE A .1 



A 



SMBRYOlflC 

 AKKA 



brat hcmitjikercs) 



OPTIC VESICLE 



FIRST PRIMARY 



VESICLE 



f!t:ri,\n PRI- 

 MARY VESICLE 

 THIRn PRIMARY 

 VESICLE 



PRIMITIVE 

 STREAK 



KEUIiAL CREST 

 NEURAL TUBE 



The adult human brain is characterised by the preponderant development of the cerebral 

 hemispheres. These expand till, held within the cranial cavity, they extend posteriorly com- 

 pletely over the thalamencephalon and mesencephalon and overlap the cerebellum to its posterior 

 border. Their cavities, which persist from the divided mesial diverticulum of the first or ante- 

 rior embryonic vesicle, are correspondingly large, and comprise the two lateral ventricles (two 

 of the four ventricles) of the adult brain. The third ventricle lies between the thalami and 



FIG. 560. DIAGRAMMATIC SAGITTAL SECTION OF A VERTEBRATE BRAIN. (After Huxley.) 

 4, fourth ventricle; s, aqueduct of Sylvius; 3, third ventricle. 



CORPORA qUADRIGEMINA 



CERERELL VM 



MEDULLA 



OBLUMiA TA 



(hind-brain) 



PONS VAROLII 



MID-BRAIN 



EP1PHYSIS 



LA TERA I. 

 VENTRICLE 



ci-:i;i:nRAL 



lll-'.MISPUERE 

 CORPUS 



,S77,'/.I TDI 

 OLFACTORY 

 DIVERT1CULVM 



CEREBRAL PEDUNCLE TIIALAMUS HYPOPHYSIS 

 INTER-BRAIN 



FORAMEN OF UONRO 



represents the middle of the three embryonic vesicles, while the fourth is between the cerebellum 

 and medulla oblongata and represents the posterior of the primary vesicles. The four ventri- 

 cles remain continuous with each other, as well as continuous with* the central canal of the spi- 

 nal cord. Both they and the central canal represent the persisting portion of the cavity of the 

 original neural tube. The inter-ventricular foramina connecting the third ventricle with the 

 two lateral ventricles are known as the foramina of Monro; that connecting the third ventricle 



