THE BRAIN. 517 



wHcli give rise to the Sylvian fissures ; but it is more correct 

 to describe the corpora striata and Sylvian fissures as both 

 alike due to relatively rapid growth of the parts of the hemi- 

 Bpheres in connection with which they arise ; the folds taking the 

 direction of least resistance, and projecting inwardly into the 

 brain cavity rather than outwardly towards the skull. The 

 corpora striata grow rapidly : by the end of the second month 

 they are strongly arched, and have reduced very considerably 

 the size of the foramina of Monro, which they bound ventrally. 



The main lobes of the cerebral hemispheres — frontal, parietal, 

 occipital, and temporo-sphenoidal — are established during the 

 fifth and sixth months ; they are formed by subdivision of the 

 original hemispheres, and not as separate outgrowths from these. 

 The olfactory lobes, on the other hand, arise as hollow out- 

 growths from the under surfaces of the hemispheres, which first 

 appear about the end of the fourth week or beginning of the 

 fifth. Each olfactory lobe early becomes divided by a constric- 

 tion into two portions, of which the anterior forms the bulbus 

 and tractus olfactorius, and the trigonum olfactorium of the 

 adult ; while the posterior portion gives rise to the anterior 

 perforated space, and adjacent parts of the brain. 



The commissures of the cerebral hemispheres require special 

 notice. 



Towards the end of the second month, as the cerebral 

 hemispheres extend bacl^wards over the thalamencephalon, 

 closely embracing this latter, extensive fusion occurs between 

 the superficial white matter of the corpora striata, and of the 

 optic thalami which these overlap. 



This tendency to fusion of originally distinct parts of the 

 brain occurs in other regions as well. During the third month, 

 the inner or mesial surfaces of the right and left hemispheres 

 come in contact, and fuse, in front of the lamina terminalis, 

 or anterior wall of the thalamencephalon ; and from this 

 fused portion the great commissures of the hemispheres are 

 developed. The fusion takes place round the margins of a 

 triangular patch (Fig. 224, SP), immediately in front of the 

 lamina terminalis. The triangular area itself remains free, as a 

 narrow vertical chink between the two hemispheres, which 

 becomes the fifth ventricle of the adult. Of the margins of the 



