152 THE BRAIN AND ITS MENINGES. 



length of the cerebrum. These ventricles contain a very- 

 small quantity of fluid ; which is perfectly clear. 



The cavities communicate with each other by a canal situ- 

 ated behind the foi'nix ; which is no more than another name 

 given to a portion of the corpus callosum, which projects 

 within the lateral ventricles. Upon laying open the ventricles, 

 the vascular web, or plexus choroides, is seen. The lateral 

 ventricles are prolonged anteriorly by curved projecting 

 hollows, communicating with the olfactory sinuses. Under 

 the more forward portion of the corpus callosum is a semi- 

 transparent substance ; being composed chiefly of two layers 

 of the serous membrane of the ventricles ; it is called the 

 septum lucidum. By the sides of the septum are two bulging 

 bodies ; which when divided present a streaked appearance ; 

 and are therefore termed the corpora striata. Behind these 

 are other and lighter coloured substances ; whose edge, 

 waving or forming a curved line, is named the tenia 

 semicircularis. The wdiite substances themselves are called 

 the hippocampi majores ; and are very thin. Upon lifting 

 up the fornix, or white arch which connects the hippocampi 

 wdth the anterior wall of the ventricles, will be seen a com- 

 munication between the two ventricles ; it being perhaps 

 better discovered directly between the two choroid plexuses ; 

 spoken of as the foramen of Munro. The plexus choroides 

 are enveloped in a fold of the serous membrane of the ven- 

 tricles ; within this, and connecting the two, is a certain 

 portion of cellular tissue or pia mater. 



By cutting directly down the centre of the brain the 

 student will gain the third ventricle ; and will behind it 

 behold a small dark coloured body called the pineal gland. 

 In front of the third ventricle is a small hole leadinir down- 

 wards, spoken of as the foramen commune : by gently in- 

 serting a probe in the direction of, but under the pineal 

 gland, a communication will be discovered leading to the 

 fourth ventricle under the cerebellum ; which is termed the 

 iter a tertio ad quartum ventriculum. Raise the cere- 

 bellum ; and directly behind the cerebrum lies the fourth 

 ventricle : just above the opening which leads to it are four 

 small bodies called the nates and testes, or corpora quadri- 

 gemini ; and guarding the opening from the third to the 

 fourth ventricle is a portion of pendulous membrane, spoken 

 of as the velum interpositum. 



