176 A MANUAL OF ANATOMY. 



the development of a great mass of connecting fibres, which 

 unite the cord below with the rest of the cerebrospinal 

 system above. 



The cavity within the hind and the afterbrains dilates to 

 form the fourth ventricle. 



The Formation of Fissures. As the forebrain increases 

 in size and enlarges upward and backward there is left be- 

 tween the two halves a deep fissure, which is the great 

 longitudinal fissure of the adult. As the forebrain enlarges 

 it extends backward, so as to overhang the rest of the brain, 

 and the interval thus left between the forebrain and the 

 other parts of the brain becomes the transverse fissure (of 

 Bichat) in the adult. 



The above constitutes one method of formation of the 

 fissures of the brain. 



Another method is by the involution of the entire thickness 

 of the cerebral surface itself. To this second classification 

 belong the fissures of Sylvius, parieto-occipital, calcarine, 

 and hippocampal. 



A third class of fissures is formed by the unequal 

 growth of the outer layers of the cerebral surface. The 

 most important one of these is called the fissure of Rolando. 

 To these depressions the name of sulci is applied in dis- 

 tinction to those grooves formed between parts of the brain, 

 or by the involution of the entire thickness of the brain 

 wall, which are called fissures. 



The formation of the fissure of Sylvius is taken as an 

 example of the method by which the second class of 

 fissures is formed. 



At the second month the foetal brain (forebrain) pre- 

 sents a smooth surface, but beginning with this period 

 there appears upon the middle of its outer lower surface a 

 depression. 



