350 AN INTRODUCTION TO ZOOLOGY 



hinder end of the roof and top part of the side wall of the ventricle 

 are thickened to form the trigonum habenula or habenular ganglia. 

 Between these arises the pineal body or epiphysis cerebri, a median 

 unpaired outgrowth consisting of a flattened stalk and an enlarged 

 terminal portion. The cavity of the third ventricle projects a short 

 distance into the stalk as the pineal recess, and so divides it into 

 anterior and posterior laminae. In the anterior lamina runs a 

 transverse band of fibres which connect together the two habenular 

 ganglia, and is known as the habenular or superior commissure. At 

 the hinder end of the posterior lamina where it passes on into a band 

 of tissue lying between the anterior corpora quadrigemina is another 

 transverse band, the posterior commissure, which is functionally 

 related to the optic centres, but anatomatically considered as mark- 

 ing the posterior limit of the third ventricle and, consequently, the 

 thalamencephalon. The pineal body itself is a small reddish pear- 

 shaped mass whose functional significance is not clearly understood, 

 and it should really be considered as forming with the structures 

 immediately surrounding it one pineal complex. In the primitive 

 vertebrate this complex has two potentialities, one glandular and 

 one related to vision, so that in some of the lower vertebrates, e.g. 

 certain Lizards and the Lampreys, we find it represented by a pair 

 of eyelike structures, the pineal eyes. In other vertebrates such as 

 the mammals, this visual potentiality is suppressed, and the 

 glandular function becomes more marked so that in the adult rabbit 

 and sheep, for example, we find that the single pineal body has 

 apparently a glandular significance. 



The optic thalami are two large masses of grey matter forming the 

 main part of the walls of the thalamencephalon. They run forward 

 to the corpora striata, from which, however, they are separated by a 

 slight groove, and at the hinder end each terminates in a swelling, 

 the pulvinar. They are so thick that they almost obliterate the 

 cavity of the ventricle, and, indeed, in the centre they actually meet 

 and adhere in the middle line over an area known as the massa 

 intermedia. This is sometimes referred to as the " middle or grey 

 commissure," an erroneous term, since there is no crossing of fibres 

 at this point, but merely an adhesion. Just behind and slightly 

 lateral to the pulvinar are two ganglionic masses, the internal and 

 external geniculate bodies. The optic chiasma is formed by a large 

 band of fibres from each side running together in the middle line, 

 some of them pass out away from the brain as the optic nerve of the 

 same side, while others cross over and leave the brain with the optic 

 nerve of the other side. The optic tract, as this broad band of fibres 

 is called, passes externally over the lateral surface of the thalamus 

 and terminates in connection with the pulvinar, the external 



