THE NERVOUS SYSTEM 1401 



spoken of as the nerves of Lancisi. External to each mesial stria, 

 and situated under cover of the gyrus fomicatus or caUosal gyrus, 

 there is another band, composed of grey matter, called the stria 

 longitudinalis lateralis. Each lateral stria is sometimes spoken 

 of as the tcsnia tecta (' covered taenia'), from the circumstance that 

 it is covered by the callosal gyrus. 



The striae of each side may be traced posteriorly round the 

 spleniiun into the corresponding gyrus dentatus. Anteriorly each 

 mesial stria, along with the grey matter in which it lies, passes 

 round the genu and backwards on the under surface of the rostrvun, 

 under the name of the geniculate gyrus. This enters the sub- 

 callosal gyrus, and finally passes to the temporal pole. 



The mesial and lateral longitudinal striae of each side, together with the 

 thin layer of gray matter, represent a rudimentary convolation caUed the 

 supracallosal gyras. 



The posterior extremity of the corpus callosum is called the 

 splenium. It is ver^- thick, which is due, as stated, to the great 

 number of fibres which cross in it, on account of there being more 

 of the hemisphere behind it than in front of it. The splenium is 

 rolled upon itself, so that its lower part is directed forwards. It 

 lies over the mesencephalon. 



Anteriorly the corpus callosum is bent upon itself, and passes 

 at first downwards and then backwards. The bent portion is 

 called the genu, and the portion which passes backwards is termed 

 the rostrum. The rostrum terminates in the median line by 

 becoming connected with the lamina terminaUs, and on either side 

 it passes into the peduncles of the corpus callosum, otherwise 

 known as the subcallosal gyri. Each subcallosal gyrus, with 

 the contained stria longitudinalis mesialis, now called, as stated, 

 the geniculate gyrus, passes downwards on the internal surface of the 

 cerebral hemisphere to the locus perforatus anticus, lying in front 

 of the lateral portion of the lamina cinerea. The geniculate gyrus 

 then passes backwards and outwards along the posterior margin 

 of the locus perforatus anticus, and so reaches the temporal pole. 



The inferior surface of the corpus callosiun is divisible into a 

 mesial and two lateral portions. The mesial portion is connected 

 posteriorly with the fornix, and over the remainder of its extent 

 with the septum lucidrun. Each lateral portion enters into the roof of 

 the body and anterior comu of the corresponding lateral ventricle. 



Destination of the Callosal Fibres. — The transverse fibres of the 

 corpus callosmn, on entering the white medullary substance of each 

 cerebral hemisphere, traverse it in a radiating manner as they pass 

 to the cerebral cortex. They constitute the radiatio corporis 

 callosi, and intersect in their course the fibres which pass between 

 the internal capsule and the cerebral cortex, which form the 

 corona radiata. The fibres from the central portion or body and 

 upper part of the splenium of the corpus callosum constitute the 

 tapetum. This forms the roof of the body of the lateral ventricle. 



