GENERAL CONCLUSIONS 477 



ditions appear to have been absent in the majority of cases of pineal 

 tumour. 



An accurate knowledge of the immediate anatomical relations of the 

 pineal body, is essential in order to clearly distinguish the symptoms due 

 to implication of neighbouring parts and those due to a supposed special 

 function of the pineal gland. Some of the structures in close relation 

 with the pineal body are : the aqueduct of Sylvius, the quadrigeminal 

 plate, the geniculate bodies, the nuclei and nerve tracts of the ventral 

 part of the midbrain, the thalamencephalon, the hypothalamus and 

 " portal system " of vessels supplying the pituitary gland, the cerebellum, 

 and the related intracranial nerves and blood-vessels. The mere enumera- 

 tion of these parts which are liable to be involved in a growth of the pineal 

 body will indicate that when the pressure symptoms are eliminated from 

 the total " symptom complex " accompanying the growth of such tumours 

 there is little left in support of the contention that the human pineal gland 

 has a regulating influence on the normal development of the body and the 

 genital organs, and more especially in the direction of inhibiting or retard- 

 ing their growth. 



In Fig. 323 we have tried to show in a diagrammatic manner the 

 general distribution of the different types of median and lateral eyes in 

 the animal kingdom. We have not attempted to include in this scheme 

 any of the aberrant forms of eye such as those met with on the back of 

 the Chitons, or " coat-of-mail shells," or invertebrate eyes with inverted 

 retina; such as those on the back of Oncidium or at the edge of the mantle 

 in Pecten, since these are not specially concerned in the phylogeny of 

 either the paired median or paired lateral eyes of vertebrates, and although 

 of great interest in showing how special organs are sometimes evolved in 

 anomalous situations in adaption to special needs, they do not assist in 

 tracing the general evolution of the eyes of vertebrates. We hope that 

 the diagram will be of some assistance in showing graphically how very 

 far removed the more highly organized classes of living vertebrates are 

 from the highly organized living invertebrates ; and, although the form 

 and dimensions of the " tree " are not intended to accurately represent 

 the periods of time which have elapsed since the divergence of the various 

 classes took place in the course of evolution, that it will give some indica- 

 tion of the way in which certain of the simple types have persisted to the 

 present day without, it may be presumed, having undergone marked 

 modifications in general form and structure, while others have diverged 

 from the primary simple type, but have nevertheless retained some of 

 their older traits, which appear either in a simple form in the early larval 

 condition, or may be present in the adult, in a modified and highly differ- 

 entiated form. We have limited the term " parietal eye " to the parietal 



