SENSE OF SIGHT. 



189 



one body on each side — the optic lobes or corpora bigemina. As 

 the human foetus grows older, each lateral lobe becomes divided 

 into an upper and lower part by the formation of a transverse 

 groove, the upper and lower pairs of the corpora quadrigemina being 

 thus formed. The upper pair are connected with sight. In tbe 

 mole they are vestigial, but in compensation the inferior corpora 

 are well developed as they are connected with the sense of hear- 

 ing, which is very acute in that animal. 



cerebrum 



t corp. bigem {optic lobe) 



cerebellum 



olfact. 



lamina termin. 



optic nerue fttuit. 



Fia. 154.— Mesial Section of the brain of a Lizard showing the resemblance to the 

 human foetal brain (Fig. 153) especially in the development of the Corpora 

 Bigemina. 



The internal geniculate body also belongs to the mid-brain 

 (mesencephalon) ; the pulvinar and external geniculate body, in 

 which the upper division of the optic tract ends, are developed on 

 the wall of the 3rd ventricle (thalamencephalon). 



(3) Tbe optic radiations connect the basal optic centres just 

 named with the mesial surface of the occipital lobes. The fibres 

 join the posterior part of the internal capsule and pass under and 

 round the posterior horn of the lateral ventricle to end in the 

 cortex of the calcarine fissure and neighbourhood. 



(4) The occipital lobe and calcarine fissure. — A mesial view 

 of the 5th month foetal brain is shown in Fig. 155. The 

 occipital lobe is already well formed ; its inner aspect shows 

 the calcarine and parieto-occipital fissures. A section across the 

 occipital lobe is shown in Fig. 156 ; the posterior horn is large; 

 the calcarine fissure indents its inner wall, giving rise to the 

 calcar avis or hippocampus minor. 



The calcarine is one of the first fissures to appear on the brain ; 



