iv. io PINEAL EYES OF LAMPREYS 103 



of the extensive sucking apparatus, innervated from the trigeminal 

 nerve. 



The forebrain consists of a pair of large cerebral hemispheres and 

 these open by the foramina of Munro into a median third ventricle, 

 whose walls constitute the diencephalon or between-brain (Fig. 65). 

 This diencephalon, besides connecting the forebrain with the mid- 

 brain, includes the thalamus and serves important functions of its own. 

 Its ventral part, the hypothalamus, is well developed in all vertebrates 

 as a central organ controlling visceral activities and the internal life 

 of the organism. Nerve-fibres from the supraoptic nucleus of the 

 hypothalamus proceed to the pars nervosa of the pituitary and, as in 

 other vertebrates, are filled with granules of neurosecretory material, 

 which presumably controls pituitary action. A simple portal system 

 of blood-vessels connects the hypothalamus with the pituitary. 



10. The pineal eyes 



The diencephalon is also the region of the brain from which the 

 eyes are formed. In lampreys, besides the usual pair of eyes, there is 

 also, attached to the roof of the between-brain, the so-called third, 

 epiphysial, or median eye, better developed in these animals than in 

 any other living vertebrate except perhaps certain reptiles. 



This organ is actually not median but consists of an unequally 

 developed pair of sacs, that on the right, the pineal, being larger and 

 placed dorsal to the morphologically left parapineal (Fig. 66). The 

 sacs form by evagination from the brain and remain connected with 

 the dorsal epithalamic or habenular region of the between-brain by 

 two stalks. The two organs are similar in structure, consisting of 

 irregular flattened sacs with a narrow lumen. Both upper and lower 

 walls of each organ contain receptor cells, with processes that project 

 into the lumen and nerve-fibres directed outwards. These fibres 

 apparently mostly end within the organ, in contact with ganglion 

 cells whose axons run to unequal right and left habenular ganglia. 

 In addition there are supporting and pigment cells in the retinas. 

 Knowles has shown that the retinal cells of the pineal make movements, 

 being arranged differently under conditions of illumination and dark- 

 ness. The significance of these photomechanical changes is unknown 

 but they demonstrate that the pineal cells are sensitive to light. 



The structure of these pineal organs shows that they consist of 

 portions of the diencephalic wall where the ciliated cells of the epen- 

 dyma are specialized as photoreceptors. They show the same general 

 plan as the paired eyes, but with no differentiated dioptric apparatus. 



