380 LINING MEMBRANE OF THE VENTRICLES. 



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 restiforme. 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 thin 

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

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

 mer being before the latter ; the remaining two are named amygdala, 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 sub- 

 stance (linguetta laminosa) derived from the anterior border of the cere- 

 bellum, and its junction with the testes is strengthened by a narrow slip 

 given off by the commissure of those bodies, thefranulum veli medullans 

 anterioris. The anterior wall, or floor of the fourth ventricle is formed 

 by two slightly convex bodies, processus teretes or posterior pyramids, 

 separated by a longitudinal groove which is continuous inferiorly with the 

 fissura longitudinalis posterior of the spinal cord. The processus teretes 

 are crossed transversely by several white and grey fasciculi (linece trans- 

 verses) the origin of the auditory nerves. And upon the lower part of the 

 floor of this ventricle is an impression resembling the point of a pen, and 

 hence named calamus scriptorius ; the lateral boundaries of the calamus 

 are the processus clavati of the posterior median columns of the spinal 

 cord. Move, the fourth ventricle is bounded by the corpora quadrige- 

 mina and aqueduct of Sylvius ; and below by a layer of pia mater and 

 arachnoid, called the valve of the arachnoid. It is by rupture of this lat- 

 ter that a communication is established between the ventricles of the brain 

 and the sub-arachnoidean space. Within the fourth ventricle and lying 

 against the uvula and tonsils are two small vascular fringes formed by the 

 pia mater, the choroid plexuses of the fourth ventricle. The fourth ven- 

 tricle is lined by grey matter derived from the interior of the spinal cord, 

 the gr.ey matter being partly concealed by a thin expansion of white sub- 

 stance. 



LINING MEMBRANE OF THE VENTRICLES. 



The lining membrane of the ventricles is a serous layer distinct from the 

 arachnoid ; it lines the whole of the interior of the lateral ventricles, and 

 is connected above and below with the attached border of the choroid 

 plexus, so as to exclude all communication between the ventricles and 

 the exterior of the brain. From the lateral ventricles it is reflected through 

 the foramen of Monro on each side, into the third ventricle, which it in- 

 vests throughout. From the third it is conducted into the fourth veiiiricle, 

 through the iter a tertio ad quartum ventriculum, and lines its interior, to- 



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

 ystem. His " Neurographia Universalis" was published at Lyons, in 1685. 



