THE PINEAL BODY 201 



The postvelar arch is separated from the caudalmost or epi- 

 physeal arch of the pineal region by a shallow invagination of 

 the diencephalic roof, which usually contains commissural 

 nerve fibers. This is known as the superior commissure or 

 commissura habenularis. In some forms, as in amphibia, it is 

 associated with a small, somewhat thickened area of the roof in 

 which the histological elements are largely neuroglia. This is 

 the pars intercalaris anterior. Although the structure, or its 

 homologue, occurs in such a limited number of animals, its 

 recognition as a distinct part seems advisable in the description 

 of this area of the brain. In cyclostomes, prosaurians, and 

 saurians, the superior or habenular commissure seems to be 

 connected with the parapineal or parietal nerve and, perhaps, 

 through this relation is brought into connection with the end- 

 vesicle of the parapineal organ. If such is the case, it may well 

 be that this commissure in cyclostomes, in prosaurians, and in 

 saurians is related to an organ of special sense. In this light 

 the superior commissure must be accounted as engaged in the 

 organization of a specialized neural mechanism, and thus be- 

 comes the first of the structures encountered in the pineal region 

 to show this tendency in differentiation. The significance of the 

 pars intercalaris anterior is not altogether clear, although it is 

 possible that it may represent a residue of an unutilized susten- 

 tacular area developed in the interest of the commissural forma- 

 tion. The presence in it of a few nerve fibers would seem to 

 substantiate this view. 



The caudalmost or epiphyseal arch is by far the most complex 

 of the three arches in the pineal region. In order that its de- 

 scription may be comprehensive enough to include all verte- 

 brates, a number of different elements are to be recognized, 

 either as appearing in the embryo or giving rise to definite adult 

 structures whose composit may, for convenience, be termed the 

 epiphyseal complex. This complex, then, consists of two princi- 

 pal organs, namely, the pineal organ and the parapineal organ. 

 Each of these organs is in turn susceptible of subdivision into 

 certain portions as follows: 



