PREFACE 



This book is the outcome of a desire to study the pineal body from the 

 broad standpoint of comparative anatomy, and to correlate as far as possible 

 the structural appearance and connections of the mammalian epiphysis 

 with the conflicting views which are held with regard to its origin and 

 functions. 



It was originally intended to deal with the subject mainly from the 

 practical standpoints of diagnosis and operative technique, based upon 

 the exact anatomical relations and connections of the pineal organ in the 

 human subject. It soon became evident, however, that the study involved 

 a much wider basis than the purely anatomical and clinical, and in order 

 to assess the true significance of the facts which have been observed in 

 connection with the mammalian epiphysis it would be necessary to investi- 

 gate the history of the pineal body and parietal sense-organ from the 

 standpoints of embryology, conparative anatomy, and geology. This, 

 study raises further questions which are of great biological importance 

 such as the influence which heredity appears to have in causing the reten- 

 tion for millions of years of an organ which in the majority of living 

 vertebrate animals has completely lost its original function of a visual 

 sense-organ ; also the problems that are raised by the recent hypothesis 

 that the pineal body of the higher vertebrate classes is evolving as an 

 endocrine organ by transformation of the vestigial parietal sense-organ 

 into a gland of internal secretion, or the alternative supposition, that the 

 mammalian epiphysis is a genetically distinct structure which has arisen 

 independently of the parietal eye of fishes and reptiles. Another problem 

 which has engaged the attention of previous investigators, and which we 

 have discussed in the light of recent palaeontological work, is the explana- 

 tion of the coexistence of a general similarity in structure of the pineal 

 eyes of vertebrates and the median eyes of invertebrates along with certain 

 differences in detail, which also exist, and which were formerly considered 

 to exclude any possibility of the two systems being genetically connected. 

 One of the first fruits of this inquiry was the definite confirmation which 

 we found is afforded by palaeontology and comparative anatomy of the 

 primarily bilateral origin of the parietal sense-organ ; and a second the 

 conviction which was gained of the extreme antiquity of the pineal 

 system — this seems not only to have evolved before the evolution of the 



