180 A MANUAL OF ANATOMY. 



Each hemisphere is further subdivided into lobes, for 

 convenience of reference, by the fissures. 



The fissures of the brain may be divided into three 

 classes : 



(i) The fissure between the halves of the cerebrum, and 

 that between the cerebrum as a whole and the rest of the 

 brain. To this class belong the great longitudinal and 

 transverse fissures. (2) Fissures entirely within the cere- 

 brum itself, which are formed by the involution of the entire 

 thickness of the cerebral wall. These are the primary 

 fissures and comprise the Sylvian, dentate or hippocampal, 

 parieto-occipital, calcarine, and collateral fissures. Cor- 

 responding to the external depressions of the primary fis- 

 sures are internal elevations within the lateral ventricles of 

 the cerebrum. (3) Fissures within the cerebrum, which are 

 formed by the upgrowth of the outer layer (cortex) of the 

 cerebrum along certain lines, depressions being left between 

 these ridges. The ridges are called convolutions, the depres- 

 sions fissures and sulci. (The former term being restricted 

 to the deeper and earlier appearing depressions of this class). 

 These constitute the secondary fissures (and sulci) and are 

 the Rolandic and callosomarginal fissures and all the re- 

 maining sulci of the cerebrum. 



The Fissure of Sylvius. Figs. I, 7, 23, 24, 25, 28. 



The Sylvian fissure, though strictly a primary fissure, 

 belongs to the class of fissures formed by the outgrowth of 

 the cortical wall rather than by an involution of the entire 

 thickness of the cerebral wall. 



It appears during the third month of development, oppo- 

 site the floor of the interbrain, where the mass of cells 

 which is to form the corpus striatum is found. 



Its shape is first triangular, and at the bottom of the tri- 



