432 THE PINEAL GLAND. 



The sides of the thalamencephalon become very early 

 thickened to form the optic thalami, which constitute the most 

 important section of the thalamencephalon. They are separated, 

 in Mammalia at all events, on their inner aspect from the 

 infundibular region by a somewhat S-shaped groove, known as 

 the sulcus of Munro, which ends in the foramen of Munro. They 

 also become in Mammalia secondarily united by a transverse 

 commissure, the grey or middle commissure, which passes 

 across the cavity of the third ventricle. This commissure is 

 probably homologous with, and derived from, a commissural 

 band in the roof of the thalamencephalon, placed immediately 

 in front of the pineal gland which is well developed in Elasmo- 

 branchii (fig. 254). 



The roof undergoes more complicated changes. It becomes 

 divided, on the appearance of the pineal gland as a small 

 papilliform outgrowth (the development of which is dealt with 

 separately), into two regions a longer anterior in front of the 

 pineal gland and a shorter posterior. The anterior region 

 becomes at an early period excessively thin, and at a later 

 period, when the roof of the thalamencephalon is shortened by 

 the approach of the cerebral hemispheres to the mid-brain, it 

 becomes (vide figs. 250 and 255, did 3, and 254) considerably 

 folded, while at the same time a vascular plexus is formed in 

 the pia mater above it. On the accomplishment of these 

 changes it is known as the tela choroidea of the third ventricle. 



In the roof of the third ventricle behind the pineal gland 

 there appear in Elasmobranchii, the Sauropsida and Mammalia 

 transverse commissural fibres, forming a structure known as the 

 posterior commissure, which connects together the two optic 

 thalami. 



The most remarkable organ in the roof of the thalamen- 

 cephalon is the pineal gland, which is developed in most Verte- 

 brates as a simple papilliform outgrowth of the roof, and is at 

 first composed of cells similar to those of the other parts of the 

 central nervous system (figs. 250, 252, 254 and 255, pn or pin). 

 In the lower Vertebrata it is directed forwards, but in Mammalia, 

 and to some extent in Aves, it is directed backwards. 



In Amphibia it is described by Gotte (No. 296) as being a product of the 

 point where the roof of the brain remains latest attached to the external skin. 



