REMOVAL OF THE BRAIN 



101 



the superior sagittal sinus, into which many of them protrude 

 (Fig. 31). As a general rule, they are most evident towards 

 the posterior part of the parietal region. At first sight they 

 appear to be protrusions from the dura mater, but that is not 

 the case. They spring from the arachnoid and the subarach- 

 noid tissue, and are normal erlargements of processes of 

 the arachnoid (Figs. 30, 31). 



Two Layers of the Dura Mater. Having noted the 

 preliminary details from an examination of the outer surface 

 of the dura mater, the student is in a position to understand 



Lateral lacuna 



Arachnoideal 

 granulation 



Superior sagittal sir 



Blood vessels 



Grey cortex 



of a gyrus 



Arachnoideal granulation 

 Lateral lacuna 



Dura mater 



Pia mater 

 Subarachnoid space 



Arachnoid 



Falx cerebri 



FIG. 31. Diagram of a frontal section through the middle portion of the 

 cranial vault and subjacent brain to show the membranes of the brain 

 and the arachnoideal granulations. 



that the membrane does not belong entirely to the brain. 

 It performs a double function : (i) it acts as an internal 

 periosteum to the bones forming the cranial cavity; and (2) 

 it gives support to the different parts of the brain. Conse- 

 quently, it consists of two strata, which, in most localities, are 

 firmly adherent, but they can usually be easily demonstrated in 

 the dissecting-room. The two strata may very appropriately 

 be termed the endocranial and the supporting layers. Along 

 certain lines the two layers separate from each other. In 

 some places they separate to form blood channels, termed sinuses 

 of the dura mater, for the passage of venous blood ; in other 

 places they separate not only to form blood channels but also 

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