374 CHOKOID PLEXUSES. 



optici by two small rounded cords, called peduncles, and is very im- 

 properly called a gland. 



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. 



The Fourth ventricle is the ventricle of the medulla oblongata, 

 upon the posterior surface of which it is placed. It is an oblong 

 quadrilateral cavity, bounded on each side by a -thick cord passing 

 between the cerebellum and corpora quadrigemina, called the pro- 

 cessus e cerebello ad tesles, and by the corpus restiforme. It is covered 

 in behind by the arch of the cerebellum, which forms three remarka- 

 ble projections into its cavity, named, from their resemblance, uvula 

 and tonsils: and by a thin lamella of white substance, stretched 

 between the two processus e cerebello ad testes, termed the valve of 

 Vieussens.* This layer is easily broken down, and requires that 

 care be used in its demonstration. In front the fourth ventricle is 

 bounded .by the posterior surface of the medulla oblongata ; above 

 by the corpora quadrigemina, and the termination of the iter e tertio 

 ad quartum ventriculum ; and below by a layer of pia mater and one 

 of arachnoid, passing between the under surface of the cerebellum 

 and the medulla oblongata, called the valve of the arachnoid. 



We observe within the fourth ventricle the choroid plexuses, the 

 calamus scriptorius, and the linece IransverscB. 



The Choriod plexuses resemble in miniature those of the lateral 

 ventricles : they are formed by the pia mater, and lie against that 

 part of the cerebellum called uvula and tonsils. 



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

 gray substance, which is continuous with that contained within the 

 spinal cord. This gray substance is separated into two bands by 

 a median fissure, which is continuous with the calamus scriptorius. 

 The two bands are considered by Mr. Solly as the two posterior 

 pyramids; and he has observed in their structure such an arrange- 

 ment of fibres as induces him to name them the " posterior ganglia 

 of the medulla oblongata" in opposition to the corpora olivaria, which 

 he describes as the " anterior ganglia of the medulla" 



The Calamus scriplorius is a groove upon the anterior wall, or 

 floor, of the fourth ventricle. Its pen-like appearance is produced 

 by the divergence of the posterior median columns, the feather by 

 the lineaa transversas. At the point of the pen is a small cavity lined 

 with gray substance, and called the ventricle of Arantius. 



The LinecB transverscB are irregular transverse lines upon the an- 

 terior wall of the ventricle, which in some degree resemble the 

 plume of the pen. They are the filaments of origin of the auditory 

 nerve. 



The existence of a communication between the fourth ventricle 



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

 system. His " Neurographia Universalis" was published at Lyons, in 1G85. 



