THE 1TITITAKY L!t>I>V. 155 



doubt ; (4) the large commissure of transverse fibres found in the 

 roof of the third ventricle ; whether the fibres decussate or not is 

 uncertain (Koppen). 



The fibres of the large commissure of the roof are, in part, con- 

 tinued into the strand of the Tuber-cinereum above mentioned, and 

 thus conducted to. the posterior parts of the hemispheres (Koppen). 

 A group of fibres (Meynert's band, Koppen) is found in each lateral 

 wall of the third ventricle; they pass from the region of the nucleus 

 parvits downwards in a curved course almost parallel with the ex- 

 ternal border of the thalameiicephalon. 



A distinct group of cells may be noted in this section of the brain, 

 an arciform or circular group (Xt/deit* jtan-u.x, Reissner ; ganglion 

 of the hahenula, Koppen) of large spindle cells (average diameter 

 o - oi6 mm.), placed under the upper border close to the third ven- 

 tricle; the group extends alongside the whole length of the 

 ventricle. A second group lying in the middle and posterior parts 

 beneath the ventricle, Koppen names the ' ventral nucleus ' of the 

 thalamencephalon. 



The pigment in the posterior part of the thalamencephalon is 

 arranged in a manner similar to that in the optic lobes and crura 

 cerebri -, anteriorly where the third ventricle is prolonged forwards 

 and downwards the arrangement is different; the pigment lies in 

 a curved line above the process of the ventricle, with its concavitv 

 directed downwards, each end bifurcating, in order that one branch 

 may pass upwards, the other down wards. 



The pineal body T is a small vesicle placed underneath the skin 

 above the fronto-parietal bones ; in the embryo it is connected with 

 the third ventricle by means of the pedicle 2 already mentioned ; the 

 skin covering the body is always paler than the surrounding skin, and 

 the usual cutaneous glands are absent in this part ; the paler spot on 

 the head may always be found, but is more distinct in Sana teinpo- 

 raria than in Rana excidenta. The structure on the roof of the third 

 ventricle, which is usually known as the pineal body, is nothing more 

 than a thickened portion of the choroid plexus, and consists of a 

 group of convoluted vessels surrounded by pia wafer, which is de- 

 scribed by "VVyman as being covered with ciliated epithelium (R. 

 The true pineal body is a small body with an outer con- 



1 Compare Elilers, Ueber die Zirbel der Haifische, Zeit. f. wiss. Zool. 1878, To . 

 XXX ; and Balfour, Development of the Elasmobranch Fishes, chap. ix. 



2 Wiedersheim states that the pedicle is hollow, and regards the part formerly 

 known as the pineal body as a thickened portion of this pedicle. 



