PINEAL GLAND. FOURTH VENTRICLE. 419 



covered in partly by the pia mater and partly by the velum interposi- 

 tum, and the nates form the base of support of the pineal gland. 



PINEAL GLAND. The pineal gland is a small reddish grey body of 

 a conical form (hence its synonym conarium), situated on the anterior 

 part of the nates and invested by a duplicature of pia mater derived 

 from the under part of the velum interpositum. The pineal gland, 

 when pressed between the fingers is found to contain a gritty matter 

 (acervulus) composed chemically of phosphate and carbonate of lime 

 and is sometimes hollow in the interior. It is connected to the brain 

 by means of two medullary cords called peduncles and a thin lamina 

 derived from the posterior commissure ; the peduncles of the pineal 

 gland are attached to the thalami optici and may be traced along the 

 upper and inner margin of those bodies to the crura of the fornix with 

 which they become blended. From the close connection subsisting be- 

 tween the pia mater and the pineal gland and the softness of texture 

 of the latter, the gland is liable to be torn away in the removal of the 

 pia mater. 



Behind the corpora quadrigemina is the cerebellum, and beneath the 

 cerebellum the fourth ventricle. The student must therefore divide 

 the cerebellum down to the fourth ventricle, and turn its lobes aside to 

 examine that cavity. 



FOURTH VENTRICLE. The fourth ventricle (sinus rhomboidalis) 

 is the ventricle of the medulla oblongata, upon the posterior surface of 

 which, and of the pons Varolii, it is placed. 1 1 is a lozenge-shaped cavity, 

 bounded on each side by a thick cord passing between the cerebellum 

 and corpora quadrigemina, called the processus e cerebello ad testes, and 

 by the corpus resttforme. It is covered in behind by the cerebellum, 

 and by a thin lamella of medullary substance, stretched between the 

 two processus e cerebello ad testes, termed the valve of Vieussens.* 



That portion of the cerebellum which forms the posterior boundary of 

 the fourth ventricle, presents four small prominences or lobules, and a 

 ihin layer of medullary substance, the velum medullare posterius. Of 

 the lobules two are placed in the middle line, the nodulus and uvula, 

 the former being before the latter; the remaining two are named 

 amygdalae, or tonsils, and are situated one on either side of the uvula. 

 They all project into the cavity of the fourth ventricle, and the velum 

 medullare posterius is situated in front of them. The valve of Vieussens 

 or velum medullare anterius is an extremely thin lamella of medullary 

 substance, prolonged from the white matter of the cerebellum to the 

 testes, and attached on each side to the processus e cerebello ad testes. 

 This lamella is overlaid for a short distance by a thin, transversely- 

 grooved lobule of grey substance (linguetta laminosa) derived from the 

 anterior border of the cerebellum, and its junction with the testes is 

 strengthened by a narrow slip given off by the commissure of those 

 bodies, the freenulum veli medullaris anterioris. The anterior wall, or 



* Raymond Vieussens, a great discoverer in the anatomy of the brain and 

 nervous system. His " Neurographia Universalis " was published at Lyons, 

 iii 1685. 



