THE PINEAL GLAND 291 



a bundle of nerve-fibres, known as Meynert's bundle, or tractus 

 habenulo-peduncularis, or fasciculus retrqflexus, runs backwards 

 and downwards to the base of the brain through the substance 

 of the corresponding optic thalamus, and, as we shall see later 

 on, the condition of these fibre-tracts in the lampreys affords 

 most important evidence with regard to the paired nature of 

 the pineal organs. 



In front of the superior commissure the thin epithelial roof 

 of the thalamencephalon, in the embryo, is usually more or less 

 strongly bulged outwards to form the " dorsal sac " (D.S.), at 

 the anterior limit of which a transverse fold of the ependyma, 

 known as the velum transversum (Vel.), commonly projects into 

 the brain-cavity and marks the junction of the thalamencephalon 

 with the prosencephalon. Immediately in front of the velum 

 transversum, again, a median outgrowth of the prosencephalic 

 roof grows upwards and backwards to form the " paraphysis " 

 (Par.), discovered by Selenka in 1890. It was, of course, 

 inevitable that the paraphysis should at first have been inter- 

 preted as the vestige of another " unpaired " sense-organ, but 

 this view cannot be maintained. It never exhibits any trace 

 of sensory structure, and is simply part of the same system of 

 folds and outgrowths of the ependymal epithelium which gives 

 rise to the velum transversum and to the choroid plexuses of 

 this region. It may (e.g. in Sphenodon) become largely developed 

 and assume the character of a follicular or tubular gland, which 

 is probably concerned in the secretion of the cerebro-spinal 

 fluid. 



The two pineal outgrowths, the dorsal sac and the paraphysis, 

 developed to varying extents in different types, have been the 

 subjects of almost endless confusion in nomenclature, and in 

 some cases (Lacertilia) they all seem to have been included 

 together under the term "epiphysis" by the older writers. 

 Their true relationships, as indicated by recent investigations, 

 are shown in a diagrammatic manner in fig. 1. 



Having thus briefly surveyed the general topographical 

 relations of the pineal organs, we may pass on to consider 

 the condition in which these organs are actually present at 

 the present day and the manner in which they are developed 

 in the different vertebrate groups. We shall find that the cases 

 in which they still exhibit the structure of light-perceiving 

 organs are comparatively few and distributed in an extra- 



