DUEA MATER AND SINUSES. 27 



that its inner surface is smooth and polished ; this is due 

 to a serous coat which is the parietal part of the arachnoid, 

 a serous membrane investing the brain and reflected upon 

 the dura mater, and which will be more particularly spoken 

 of hereafter. The dura mater is therefore a nbro-serous 

 membrane. By separating the hemispheres of the brain 

 it will be seen that a process of the dura mater penetrates 

 like a septum between them : this is called the falx cerebri. 

 Narrow in front, it is attached to the crista galli of the 

 ethmoid bone ; broader posteriorly, it is continuous with a 

 horizontal expansion of the dura mater lying between the 

 cerebrum and cerebellum, called the tentorium ; its superior 

 border is attached along the vertex of the skull, and the in- 

 ferior, which is concave and free, nearly reaches the corpus 

 callosum of the brain. The falx sometimes presents perfo- 

 rations of various sizes, or a rarefaction of its fibres, which 

 give it a lace-like appearance ; more rarely a solution of 

 continuity, sufficient to allow the hemispheres of the brain 

 to come in contact with each other, has been noticed. 



The superior longitudinal sinus occupies the convex 

 border of the falx, and may be laid open with the scissors ; 

 it extends from the crista galli to the internal occipital pro- 

 tuberance, is triangular in shape, and perforated by n umerous 

 small veins ; in its interior, transverse fibrous bands, called 

 chordae Willisii, cross it here and there, and occasionally 

 Pacchionian glands are found within the sinus. It termi- 

 nates posteriorly at the torcular Herophili. The inferior 

 longitudinal sinus occupies the concave border of the falx ; 

 this terminates in the straight sinus of the tentorium, 

 which also continues to the torcular Herophili, the common 

 centre of several sinuses, the situation of which corres- 

 ponds to the internal occipital protuberance. The straight 

 sinus receives the venae Galeni, which come from the inte- 

 rior of the brain. 



The brain must now be removed. The head should be allowed to 

 depend as much as possible, and the operation is to be commenced by 

 the division of the anterior attachment of the falx ; this is to be raised 

 and thrown backward, but not detached posteriorly; in reflecting this, 

 several veins, entering the superior longitudinal sinus, will necessarily 

 be divided. 



The examination of the arteries at the base of the brain is not 

 always easy in the subject which is used for the study of other parts 

 in the same region. If the brain is too soft to be examined with 

 benefit, it is much better to leave the arteries in connection with the 

 skull. The softened cerebral matter left attached to them may be. 



