RELATIONS OF THE ADULT PINEAL ORGAN 419 



missure also appears to contain fibres which originate or end in nuclei 

 situated in the tectum opticum. 



In cyclostomes, e.g. Petromyzon, in addition to fibres which are 

 associated with the pineal system, fibres of the posterior commissure 

 arise from cells which are widely scattered through the dorso-caudal 

 part of the thalamus and tectum opticum. In Geotria, the Australian 

 lamprey, according to Dendy the larger of the two organs, the right 

 parietal organ (Epiphysis II or posterior pineal organ) is connected 

 by a well-defined tract, the pineal nerve with the right habenular ganglion, 

 and also sends fibres to the posterior commissure and right bundle 

 of Meynert ; whereas the smaller deeply placed left parietal organ 

 (Epiphysis I or parapineal organ of Studnicka) is joined by a few short 

 fibres with the left habenular ganglion which lies immediately beneath 

 it, and also sends fibres to the posterior commissure and the left bundle 

 of Meynert. The morphology of the pineal tract and of the habenular 

 commissure in cyclostomes is discussed on p. 193. Briefly summarized 

 it may be stated that two organs or pairs of organs have been considered 

 by some authors to be comprised in the pineal system, of which the anterior 

 organ or Epiphysis I is related to the habenular ganglia and the habenular 

 commissure, and the posterior organ or Epiphysis II is connected with 

 the posterior commissure. It is this Epyphysis II in a modified form 

 which is said to be represented by the epiphysis of amphibia ; the pineal 

 sac or epiphysis of reptiles ; and the epiphysis of birds and mammals. 



Having considered the immediate relations of the pineal body and 

 the principal connections of the habenular and posterior commissures, 

 it will be advantageous to examine the structures which lie around the 

 pineal zone, and which are liable to be compressed by a tumour originating 

 in these regions, or would have to be borne in mind when approaching 

 the organ with the object of removing a tumour. A glance at the trans- 

 verse section (Fig. 279) will show the relations of the tela choroidea with 

 its contained vessels to the transverse fibres and fimbria; of the fornix 

 and also the connection of the latter with the body of the corpus callosum. 

 Overlapping the fimbria of the fornix on each side is the choroid plexus, 

 which projects into the lateral ventricle. The size, vascularity, and 

 density of the plexus varies considerably in different individuals. In the 

 specimen drawn the lateral ventricle is of moderate size, but in cases of 

 obstruction to the aqueduct of Sylvius the ventricle may be greatly dis- 

 tended or, if emptied, the walls may be collapsed. On either side of the 

 pineal body is the pulvinar of the optic thalamus. This is separated from 

 the pineal body, superior colliculus, and superior brachium by pia mater 

 containing blood-vessels. Lateral to the pulvinar is the internal capsule, 

 passing between the caudate and lenticular nuclei and coursing down- 



