THE PIA MATER 673 



sinus wall. On each side of the superior sagittal sinus there is a number of 

 irregular spaces in the clura mater which communicate with the sinus either by 

 a small aperture or a narrow channel. These spaces are called the lacunas laterales, 

 and certain of the meningeal veins and some of the diploic veins open into them. 

 Granulations push themselves into the lateral lacunae from below in such a manner 

 that they receive a complete covering from the layer of dura mater which forms 

 the sinus floor. Nor does the bone escape. As the granulations enlarge they 

 cause absorption of the cranial wall, and small pits are hollowed out on its internal 

 surface for their reception. It must be clearly understood, however, that in such 

 cases the granulation is separated from the bone by the following : (1) A con- 

 tinuation round the granulation of the subdural space ; (2) the thinned floor of 

 the lateral lacuna ; (3) the lumen of the sinus ; and (4) the greatly thinned upper 

 wall of the sinus. 



The granulations have a special function to perform. Through them fluid can 

 pass from the subarachnoid space into the venous sinuses with which they stand 

 in connexion. Wheaever the pressure of blood in the sinuses is lower than that 

 of the fluid in the subarachnoid space and the ventricles of the brain, the cerebro- 



Granulationes araclmoideales Mouth of a vein 



^--saigas Bone 



FIG. 598. MEDIAN SECTION THROUGH THE CRANIAL VAULT IN THE FRONTAL REGION. ENLARGED. 

 Displays a portion of the superior sagittal sinus and the arachnoideal granulations protruding into it. 



spinal fluid filtrates through the granulations into the blood-sinuses. This is not 

 the only way that subarachnoid fluid may obtain exit. The subarachnoid space 

 is carried outwards for a short distance on the nerves in connexion with their 

 arachnoideal sheaths, and communicates with the lymph channels of the nerves. 

 This connexion is more complete in the case of the olfactory, the optic, and 

 the acoustic nerves than in other nerves. A very free communication is said to 

 exist between the subarachnoid space and the lymph-vessels of the nasal mucous 

 membrane. 



THE PIA MATER. 



The pia mater forms the immediate investment of the brain and spinal 

 medulla. It is a delicate and very vascular membrane. 



Pia Mater Encephali. The pia mater which covers the brain is finer and 

 more delicate than that which clothes the spinal medulla. It follows closely all 

 the inequalities on the surface of the brain, and in the case of the hemisphere 

 it dips into each sulcus in the form of a fold which lines it completely. On the 

 cerebellum the relation is not so intimate ; it is only into the larger fissures that it 

 penetrates in the form of folds. 



The larger blood-vessels of the brain lie in the subarachnoid space. The finer 

 twigs ramify in the pia mater before they proceed into the substance of the brain. 

 As they enter they carry with them sheaths derived from the pia mater. When 

 a portion of the membrane is raised from the surface of the encephalon, numerous 

 fine processes are withdrawn from the cerebral surface. These are the blood-vessels 

 with their sheaths, and they give the deep surface of the pia mater a rough and 

 flocculent appearance. 



As the pia mater is carried over the inferior part of the roof or posterior wall 



the fourth ventricle of the brain it receives the name of the tela chorioidea 



44 



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