196 FREDERICK TILNEY AND LUTHER F. WARREN 



7. DISCUSSION 



1. Significance of the pineal region 



It is now possible, with the facts presented as evidence, to 

 discuss the problem of the pineal body and, perhaps, to formu- 

 late some conclusions concerning it. 



The question uppermost about the epiphysis to-day is whether 

 the structure is a mere vestige or whether it has, in mammals 

 and more especially in man, some definite function. Besides 

 this highly important consideration there is still another which, 

 in its way, has an even more far-reaching significance, namely, 

 the value of the pineal structures as one of the indices which may 

 point out the lines of evolution running through the vertebrate 

 phylum and those leading back to the invertebrate, ancestral 

 stock. 



If the pineal body is a vestige, it is essential to ascertain to 

 what previously active structures it is related and for what 

 reasons it has become vestigial. In this sense the survey of its 

 phylogenetic relations cannot be too broad and should include 

 the entire environment of the organ. If, as is held by many, the 

 pineal organs have significance as a connecting link between 

 the vertebrates and invertebrates, then, on the basis of embry- 

 ology and comparative morphology, the effort must be made to 

 homologize not one, but all of the parts associated with or 

 adjacent to the pineal body. In such a light every derivative 

 of the roof-plate of the primitive forebrain becomes funda- 

 mentally important, and no discussion of the pineal body could 

 be complete which did not recognize the character of the pineal 

 region as a whole. 



The portion of the brain known as the pineal region was first 

 so designated by Minot 277 in 1901. It has also been termed the 

 parietal region. It extends from the dorsal extremity of the 

 lamina terminalis to the caudal limit of the posterior commissure 

 and comprises a,ll of the structures which develop from the roof- 

 plate of the primitive forebrain. It presents, according to 

 Minot, 277 a series of three arches or vaults, arranged one in 

 front of the other. The most cephalic of the three arches is 



