THE PINEAL GLAND 



293 



"eye" with the brain. Some, at any rate, of these fibres pass 

 to the right habenular ganglion (G.H.R.), others may be con- 

 nected with the posterior commissure, and possibly with 

 Reissner's fibre. The histological structure of the pineal eye 

 is highly remarkable. It consists of a hollow, somewhat flat- 

 tened vesicle, whose upper wall, the " pellucida " (Pell.), is 

 composed of long columnar cells free from pigment, while the 

 lower wall forms the so-called "retina" (Ret), backed by a 



R.P£. 



LP.E 



Ef.q. 



FlG. 2. — Diagrammatic sagittal section through the pineal organs and adjacent parts 



of the brain in a lamprey {Geotrid). 



At., "Atrium" of right pineal organ; C.H.S., superior (habenular) commissure; C.P., 

 posterior commissure ; D.S., dorsal sac; Ep.G., epithelium of ependymal groove; G.H.L.A., 

 anterior division of left habenular ganglion ; G.H.L.P., posterior division of same ; G.H.R., 

 right habenular ganglion; L.P.E., left (anterior) pineal organ; L.T., lamina terminalis ; Pell., 

 pellucida of pineal eye; PL Ch., choroid plexus of mid-brain ; P.N., pineal nerve, lying in 

 remains of pineal stalk; Ret., retina of pineal eye; R.M.B., direction of right Meynerfs 

 bundle ; R.P.E., right (posterior) pineal organ (pineal eye) ; T.H., middle division of left 

 habenular ganglion (tractus habenularis) ; V. 3, third ventricle. 



thick layer of ganglion-cells and nerve-fibres. The retina con- 

 tains sense-cells and pigment-cells. The former are long 

 slender rods, with knob-like extremities projecting into the 

 lumen of the vesicle and connected by delicate threads with 

 the inner surface of the pellucida. The latter are columnar 

 cells filled with minute granules of a glistening white material, 



