THE BRAIN. 519 



disappear wholly or in chief part : and (ii) the secondary sulci, 

 which are mere grooves on the surface of the hemisphere, and 

 consequently do not give rise to corresponding internal projec- 

 tions ; these appear late, but persist throughout life. 



(i) The primary sulci appear towards the end of the second 

 month, and occur on both the mesial and the outer walls 

 of the hemispheres ; they attain their maximum development 

 between the third and fourth months, and by the end of the 

 fourth month have disappeared almost completely. It has been 

 suggested that their formation is due to the brain increasing in 

 size more rapidly than the skull, and consequently becoming 

 thrown into folds ; while at a later stage, when the skull 

 enlarges, most of the folds become flattened out and obliterated. 



On the mesial wall of each hemisphere a long curved sulcus, 

 the fissura arcuata, appears towards the close of the second 

 month. It runs parallel to the upper border of the hemisphere, 

 and a little distance from this ; and extends from the anterior 

 end of the frontal lobe round to the temporo-sphenoidal lobe 

 (cf. Fig. 224). From the fissura arcuata a series of furrows, 

 usually six to eight in number, radiate outwards towards the 

 margin of the hemisphere. 



On the outer wall of the hemisphere the primary sulci are 

 less regularly arranged. In a general way, they start from the 

 margin of the hemisphere and converge towards the Sylvian 

 fissure, but do not meet this. 



The obliteration of the primary sulci is mainly a process of 

 unfolding, progressing from, the ends of the sulcus towards its 

 middle ; the sulcus becoming shorter and shorter, and ultimately 

 disappearing. 



It is not quite certain whether any of the primary sulci 

 normally persist as permanent sulci ; but it appears that three 

 or four of the most strongly marked ones do persist as a rule, or 

 else are replaced by permanent sulci which are formed along the 

 same lines. The hippocampal, and portions of the calcarine and 

 parieto-occipital sulci belong to this category. The Sylvian 

 fissure is also a permanent one, but it differs in some respects 

 from the primary sulci, and can only doubtfully be referred to 

 the same group as these. 



(ii) The secondary snlci. During the fifth month, and the 

 early part of the sixth month, the surface of each hemisphere is 



