THE PITUITARY BODY. 155 



doubt ; (4) the larg-e commissure of transvevse fibres found in the 

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

 uncertain (Koppen). 



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

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

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

 A g-roup of fibres (Meynert^s band, Koppen) is found in each lateral 

 wall of the third ventricle ; they pass from the reg-ion of the nucleus 

 2Mrvus downwards in a ciirved course almost parallel with the ex- 

 ternal border of the thalamencephalon. 



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

 an arciform or circular group [Nucleus parvus, Reissner ; gang-lion 

 of the J/aùeunla, Koppen) of large spindle cells (averag-e diameter 

 O'oiô mm.), placed under the upper border close to the third ven- 

 tricle; the group extends alongside the Avhole 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 diiferent ; the pigment lies in 

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

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

 may pass upwards, the other downwards. 



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

 above the f ronto-parietal bones ; in the embryo it is connected \\\t\\ 

 the third ventricle by means of the pedicle - 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, bvit is more distinct in Rana tewpo- 

 raria than in Rana esculenta. 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 surrovmded by pia niafer, which is de- 

 scribed by Wyman as being- covered with ciliated epithelium [R. 

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



' Compare Elilers, Ueber die Zirbel der Haifisehe, Zeit. f. wiss. Zool. 187S, Vo . 

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



^ Wieder.sheim states that the pedicle is hollow, and regards the part formerly 

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



