184 THE PINEAL ORGAN 



over the dorsal sac between this and the pineal eye (Fig. 252, Chap. 24, 

 p. 369). The paraphysis is frequently absent, e.g. in the adults of many 

 mammals. 



2. Velum Transversum. — A membranous fold which projects from the 

 roof of the interbrain into the cavity of the ventricle. Its walls are 

 usually smooth and simple, but it may be thrown into folds by the ingrowth 

 of vessels, especially in its lower part. In some cases instead of forming 

 a single transverse fold, it appears as two, paired folds, as in the Dipnoi, 

 or is developed as an unpaired choroid plexus as in tailed amphibia. 



3. The Dorsal Sac. — Fig. 252, Chap. 24, p. 369, and Fig. 130, p. 180, 

 ds. This is an arched membranous fold which lies behind the transverse 

 velum, the posterior layer of which forms the anterior boundary of the 

 lower part of the dorsal sac. It projects towards the roof of the skull, 

 between the pineal stalk and pineal nerve behind and the paraphysis in front. 

 In some cases the dorsal sac grows backward on each side of the stalk of 

 the epiphysis in the form of two blind pouches. The stalk in these cases 

 may be enclosed in a tubular sheath of ependyma, or lie in a groove in 

 the posterior wall of the sac. The wall of the sac is like that of the velum, 

 in some cases smooth and simple, in others thrown into folds which may 

 be vascular and form part of the tela chorioidea superior (Fig. 252). 



4. Commissura Habenularis. — This, which is also named the superior 

 commissure, appears later and is much smaller than the posterior com- 

 missure. It joins the right and left habenular ganglia. In some cases 

 the habenular ganglia are fused in the median plane and the commissure 

 is then enclosed in the centre of the mass, as in Petromyzon. 



5. The Pineal Organs. 



A . The parietal organ — (parietal eye ; pineal eye ; sensory-vesicle) 

 (Fig. 133) : the principal or parietal sense-organ which is socketed in the 

 roof of the skull may be derived from either the right (Cyclostomes) or left 

 (Sphenodon) member of the primarily paired organ. In Cyclostomes it 

 arises further back than the smaller vesicle which lies below it and to the 

 left. The larger vesicle is therefore sometimes described as Epiphysis II 

 whereas the term Epiphysis I is given to the small vesicle — " parapineal 

 organ " of Studnicka — which springs from a point farther forward and is 

 connected with the anterior part of the left habenular ganglion (see Fig. 134, 

 p. 188, and Fig. 22, Chap. 3, p. 28). It must be borne in mind, however, 

 that this attachment in Petromyzon is one which has been secondarily 

 displaced forward along with the anterior part of the left habenular 

 ganglion and that the primary outgrowth of the pineal diverticulum was 

 immediately in front of the posterior commissure. 



