THE BRAIN. 155 



(2) The posterior subarachnoid space is larger than 

 the anterior. It is formed at the base of the cerebellum 

 by the arachnoid membrane which extends from the pos- 

 terior part of the cerebellum to the upper surface of the 

 medulla. It communicates with the spinal arachnoid space 

 below, with the anterior subarachnoid space around the 

 sides of the medulla, and with the interior of the brain 

 through the foramina of Majendie, Key, and, Retzius. 



The foramen of Majendie is centrally located in the pia 

 which reaches from the under surface of the cerebellum 

 to the upper surface of the medulla, and assists in forming 

 the roof of the posterior part of the fourth ventricle. The 

 opening of Majendie is found close to the medulla and in 

 the middle line. The foramina of Key and Retzius are 

 minute fissures at the extension of the pia into the lateral 

 angles of the fourth ventricle. Other smaller and less 

 important spaces are found (3) Below the falx cerebri and 

 above the corpus callosum, in the great longitudinal fissure. 

 (4) In the fissure of Sylvius. (5) Between the corpora 

 quadrigemina and the anterior extremity of the cerebel- 

 lum. 



The Pacchionian Glands, Arachnoidal Villi. These 

 are up-growths from the arachnoid along the margins, and 

 even into the cavity, of the superior longitudinal sinus. 

 They look like small masses of granulation tissue. They 

 penetrate the dura and produce corresponding depressions 

 in the inner table of the skull. 



The subarachnoid spaces are filled with a lymphatic 

 fluid called the cerebrospinal fluid. This fluid is able to 

 circulate freely from the cord to the exterior of the brain, 

 and even to the interior of the latter through the foramina 

 of Majendie, Key and Retzius. Its usual quantity is about 

 two ounces. 



