DISUSE AND ITS EFFECTS. 



43 



to include this parietal foramen, we find it occupied 

 by an organ representing an eye in miniature, con- 

 nected with the pineal body by nerve-fibres. The 

 relation of the parts is indicated in fig. 21, represent- 

 ing a lateral view of the brain and adjoining parts 

 of the skull in the New Zealand lizard, Hatteria 

 (Spkenodon). 



When the eye is examined microscopically, it pre- 

 sents the structural details found in functional eyes, 

 such as cornea, lens, retina, pigment, &c. (fig. 22). 



FIG. 21. Lateral view of the brain of Sphenodon, showing 

 the relation of the pineal eye, P ; the cerebrum, C ; and the 

 medulla, M. (After Baldwin Spencer.) 



Spencer is of opinion that this suppressed eye repre- 

 sents the unpaired eye of larval Tunicata. There is 

 good evidence that it was highly developed in extinct 

 amphibia (Labyrinthodonta), and was probably a sense- 

 organ in animals of pre-tertiary periods. 



It is not unreasonable to suppose that the gradual 

 development and greater utility of the lateral eyes 

 have led to the suppression of the median eye. 



Although the pineal body (regarded by Descartes as 

 the seat of the soul) in man is clearly vestigial, it is 



