THE BRAIN. 385 



Primary Divisions. Secondary Divisions. Cavity. 



F ce e Dhafon r ^^ I *' KSSSSSLSSJSfSto fttauSl?' 



' ( cephalon, or 'tween-brain. 



Mid-brain. 3. Mesencephalon or mid-brain. Aqueduct of Sylvius. 



Hind-brain or rhomb- j 4 " Metencephalon or cerebel- Upper part of fourth 



encenhalon i lum. ventricle. 



(5. Myelencephalon or medulla. Fourth ventricle. 



The brain is thus a hollow multiple organ. Its 

 central cavity is lined with a serous membrane, the 

 ependyma, which secretes a serous fluid, the cerebro- 

 spinal fluid. The outer vascular investments are the 

 meninges, which serve as a delicate packing between 

 the brain wall and the bony vault of the cranium. 



The Meninges. These are three connective-tissue 

 investments to the brain that are practically identical 

 and continuous with those already described inclos- 

 ing the cord. It will be sufficient, therefore, to men- 

 tion the points wherein these membranes slightly 

 differ. 



The dura of the brain forms the periosteum of the 

 investing bones, while each segment of the vertebral 

 column has its own periosteum. Several broad 

 prolongations of the brain dura extend between the 

 different divisions of the brain. These are the ten- 

 torium between the cerebrum and the cerebellum; 

 the falx cerebri which dips into the great fissure be- 

 tween the two lobes of the cerebrum, and the falx 

 cerebelli, which is a small median septum between 

 the cerebellar hemispheres. At the basal skull fora- 

 mina, the dura accompanies the cranial nerves and 

 is continuous with the areolar sheaths of these 

 nerves. 



The pia is composed mostly of areolar tissue and 

 25 



