414 PINEAL GLAND THALAMI OPTICI. 



roof of tlie fourth ventricle, and called the valve of Vieus- 

 sens. 



This valve is streaked with gray matter is covered by 

 the superior vermiform process, and has the fourth pair of 

 nerves arising from it. A tube or canal is traced beneath 

 the tubercula quadrigemina, leading from the third to the 

 fourth ventricle, beneath the valve of Vieussens, called the 

 aqueduct of Sylvius. The four tubercles, though separated 

 l^y a transverse and vertical fissure, have but one common 

 base. They are covered by a vascular membrane the 

 velum interpositum. The structure of these bodies is white 

 without, and gray within. 



Only two of these tubercula are seen in fish and reptiles, 

 and are viewed as the optic lobes, or the organs which re- 

 ceive and recognize the impressions of light, color, &c. 



The pineal gland, (Fig. 116,) is situated upon the nates 

 and is surrounded by the velum interpositum, a reflection 

 of pia mater. It is an oblong, reddish body, composed of 

 cineritious matter, and containing calcareous particles or 

 gritty matter called acervulus, which on analysis is found 

 to consist principally of phosphate and carbonate of lime. 

 The posterior part of this gland is soft and called cona- 

 rium. It is united to the thalami by two delicate processes, 

 the pedunculi, which proceed forward on the inner mar- 

 gins of the thalami, and join the descending pillars of 

 the fornix. A variety of opinions have been indulged in 

 reference to the use of this gland. Des Cartes believed it 

 to be the seat of the soul, arid Majendie that it closed the 

 aqueduct of Sylvius, and thus cut off the communication 

 between the third and fourth ventricles, while another and 

 more plausible view makes it a commissural body. 



Thalami optici, or posterior ganglia, (Fig. 117.) These 

 ganglia are very prominent in the interior of the brain. 

 They are situated upon the upper surface of the crura cere- 

 bri, and can be most readily got at by separating the 

 hemispheres, and turning aside the corpus callosum and 

 fornix, which cover them. The thalami are large, oval 

 bodies, placed side by side in the longitudinal direction, 



