THE BRAIN AND ITS MENINGES. 151 



called the brain. Of its membranes or meninges, the dura 

 mater is the most external ; performing the double office of 

 a support to the brain, and an internal pericranium to the 

 bones, unto which it strongly adheres by numerous filaments. 

 It forms several folds or prolongations ; such as the falx 

 which divides the cerebral lobes : other expansions form 

 sinuses to receive the blood returned from the interior of the 

 brain ; and it is prolonged to line the orbits, and likewise 

 cover the spinal marrow. The memhrana arachnoidea ap- 

 pears a very fine serous membrane, in connexion with the 

 internal surface of the dura mater ; and after investing every 

 vein or artery that runs to or from the organ, it is reflected 

 over the free surface of the brain itself. The arachnoid is in 

 contact with nothing but itself; and here, as in every other 

 case, a serous membrane forms a closed sac, and facilitates 

 motion. With regard to its forming a closed sac ; the arach- 

 noid accompanies the spinal marrow out of the cranium ; but 

 is not reflected over the nerves which emerge from that canal, 

 or are given off by the brain. With respect to its faciU- 

 tating motion ; the brain, during life, constantly moves, and 

 may be seen to pulsate in the forehead of an infant. 



The arachnoid is held close to the surface of the brain, 

 over the convolutions of which it is reflected. 



The pia mater is nothing more than cellular tissue ; which, 

 entering into the substance of the brain, holds its convolu- 

 tions together ; as well as forms the sheath of every nerve 

 that emerges from the organ. 



The substance of the hrain, by a natural division, divides 

 itself into two portions (see Plate V. Fig 1 . a,k). The 

 cerebrum is the first and largest portion (see Plate V. Fig 

 1 . a, a) ; its external or cortical part is of a pale dull red 

 colour; and is indented by convolutions. The interior and 

 larger portion, called the medullary, is white ; and is after 

 death more tenacious than the cortical substance. On 

 separating the hemispheres {Plate V. Fig 1 . a, a) the 

 corpus callosum appears of a glistening white tint. The 

 centrum ovale is nothing more than the greatest quantity 

 of medullary matter which can be exposed at a single cut ; 

 w^hich must be made just above the corpus callosum, re- 

 moving the top of the hemispheres. Two cavities of an 

 oblong form, one upon either side of the corpus callosum, 

 are called the lateral ventricles ; and extend throughout the 



