CENTRAL NERVOUS SYSTEM 



537 



The diencephalon, where the central canal persists as the third ventricle, 

 has peculiar visual and secretory functions in addition to the important 

 integrative activities of the optic thalamus. From its ventro -lateral aspects 

 in the embryo the primary optic vesicles which form the lateral eyes 

 emerge as out-pouchings, and it is interesting that in many sjjecies there 

 is evidence that the cells of this region, particularly those of the ependyma 

 lining the ventricle, appear to retain some photosensory functions. 



Thalamo-striatal tract ~"~ 



Xerve II - — _ 

 Brachiuin tecti 



Lat. geniculatL' body 

 Nucleus dorsalis (liffusus 



TECTUM 



Nucleus dcirsalis ant. 

 Isthmo-tectal tract 

 Torus seniicircularis 



l.iantrlion isthnii 



Lat. lemniscus 



Nerve VIII 



Nerve V 



Tost, root ganglia 



Fig. 71: 



-The Visual Pathways ix a Typical A.mphibiax. 



This direct photosensitivity of the central nervous system, especially the 

 diencephalon, has been established in experiments on the action of light upon 

 gonadotropic activities ^ and changes in the chromatophores of the integument in 

 a niunber of .species by several workers, notable among whom are von Frisch (1911) 

 (fish), Scharrer (1928) (minnow), Nowikoff (1934), Young (1935) (lamprey), and Benoit 

 (1937) (ducks). We have already seen that Benoit and his collaborators (1952-53) 

 showed that the direct stimulation of this region by light enhances the gonadotropic 

 activity of ducks from which both eyes had been removed ; and Parker and his 

 colleagues (1952) have pointed out that the central nervous systein of most Birds and 

 Mammals contains a coproporphyrin pigment wath absorptive properties which could 

 account for this direct photosensory response. 



The secretory activities of the diencephalon are equally important.- 

 From its thin roof which consists merely of a single layer of ependymal 



'■ p. 16. ^ p. .558. 



