NERVOUS SYSTEM 



857 



piphysial structures belong to the twixt-brain and consist of a parietal 

 organ and the pineal gland (Fig. 476). Both of these arise from the roof 

 of the twixt-brain between the habenular ganglion and the posterior com- 

 missure where twixt and mid-brains meet. Sometimes they develop as 

 a. single outgrowth and sometimes as distinct structures. The more 

 anterior of the two is the parietal organ or eye. The posterior is the 

 pineal gland, also known as the epiphysis. These two organs, although 

 varying in the different vertebrates, are usually always present. The 

 parietal organ in at least one group of lizards extends on a slender stalk 

 actually passing out of the skull and forming a sort of median eye on 

 the dorsal surface of the head. In those vertebrates in which the parietal 

 organ does not appear at all, the pineal gland seems to show tracts of 

 structure similar to the parietal organ when it does become an eye. It 

 will be remembered that the brow-spot seen on the frog is really the 

 spot where the pineal gland began growing toward the exterior of the 

 body but was cut off by the developing skull. 



It is interesting to note that, notwithstanding the close relations of 

 the pineal and parietal organs, the former receives its nerve supply from 

 the posterior commissure while the parietal is connected with the supe- 



Optic lobe 

 Epitha 



n.II 



Hypothalam 



Ganglion V 

 Infra-orbital trunk 



Acoustico-lateral area 

 , laterahs VII 



n.E ai 



r. hyomandibularis VII 

 Spiracle 

 . palatinua VII 



.4. 



anglion geniculi VII 



B. 



Fig. 480. 



A, Side view of the brain of the dogfish Mustelus canis. (After Herrick.) 



B, Longitudinal section of brain of Trout, aq, aqueduct ; bo, bulbus olfactorius ; 

 ca,, cK, ci, cp, anterior, horizontal, inferior, and posterior commissures ; cc, central 

 canal ; cl, cerebellum ; cs, corpus striatum ; h, hypophysis ; t, infundibulum ; iv, 

 trochlearis nerve ; cc, optic chiasma ; p. pallium ; pi, pinealis ; sv, saccus vasculosus ; 

 tl, torus longitudinalis ; to, tectum of optic lobes ; v, velum transversum ; *, v*, 

 ventricles; vc, valvula cerebelli. (From Kingsley after Rabl-Riickhard. ) 



