NERVOUS SYSTEM 163 



recognised as the homologue of the so-called parietal organs of 

 the other Vertebrates. There is an exhaustive work on this 

 subject by Dr. Studnicka in Oppel's Text-book of Compara- 

 tive Microscopic Anatomy, Part V. 1 By the term " parietal 

 organs" are apparently understood unsym metrical structures, 

 in saccular or vesicular form, arising by means of protrusion 

 from the cerebral cortex, or in the roof of the mesencephalon, 

 and originally serving to receive and transmit sensations of 

 light ; in the majority of Vertebrates, however, they contain 

 but the merest rudiments of organs for the reception of light, 

 and are finally transformed into organs of an entirely different 

 character, namely, into glandular tissue of unknown function. 

 As a rule the parietal organs tend to lie superficially, and at 

 this point the skull becomes markedly attenuated ; indeed, an 

 opening sometimes occurs in the bone or in the cartilage 

 (foramen parietale). In the region of the parietal organ the 

 tissues are much more transparent and colourless than else- 

 where. Almost invariably a close connection with the brain is 

 found, and the organ is generally enclosed in the interior of 

 the skull. In certain of the Vertebrates (e.g., Anura) the 

 parietal organ lies above the roof of the skull, immediately 

 under the skin. 



The general structure of the parietal organs corresponds to 

 their origin in a protrusion of the cerebral lining and to their 

 original function of (unsymmetrical) visual organs. The walls are 

 found to be constructed of the same elements as those composing 

 the nervous parts of the cerebro-spinal canal and the retina of 

 the permanent eyes. 



The lens-cells of the parietal eyes resemble those of the 

 true eyes, and the content of the lumen of these primitive eyes 

 may be considered as the rudiments of vitreous bodies. 2 



Following, in the light of Studnicka's treatise, the parietal 

 organs from the lowest to the highest classes of the Vertebrates, 

 we are not astonished to find the Cyclostomata (with the ex- 

 ception of the Myxinoides) furnished with well-developed 

 parietal eyes in the form of pediculated vesicles with a retina, 

 or so-called Pellucida, and even a plano-convex lens. 



1 Dr. Studnicka, " Die Parietalorgane der Wirbeltiere," Oppel, vol. v., 1903. 



2 Studnicka, loc. cit., pp. 9, 10. 



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