S92 



THE NERVOUS SYSTEM 



rior part of the longitudinal fissure and slightly into the lateral (Sylvian) fissure. 

 Otherwise it fills in the inequalities of surface of the encephalon, its outer surface 

 forming a sheet enveloping the whole and bridging over the sulci and the deeper 

 grooves between the gross divisions. Upon the summits of the gyri it is more closely 

 applied to the pia mater, and there its reticulum becomes so dense that the two 

 membranes almost appear as one. The sulci, occupied by the looser reticulum, form a 

 continuous system of channels filled more abundantly by the cerebro-spinal fluid. 



The arachnoid does not fold in between the cerebellum and medulla oblongata, and 

 at the base of the brain it ensheathes the olfactory bulbs and tracts, and its outer sur- 

 face forms a continuous sheet stretching from one temporal lobe to the other and bridg- 

 ing over the interpeduncular fossa and the inequalities of surface in the region of the 

 optic chiasma and the stems of the lateral fissures. Obviously, therefore, the sub- 

 arachnoid cavity between its outer surface and the pia mater is of considerable depth 

 in certain localities. These localities comprise the subarachnoid cisterna;. These 

 make the cavity at the base of the brain especially large, and make possible a 

 'water-bed' which serves to protect the brain from injurious contact with the bones. 

 The following cisternse are distinguished: 



(1) The cisterna basalis lies at the base of the cerebrum and is divided by the 



FIG. 663. DIAGRAM SHOWING THE RELATIONS OF THE PIA MATER, THE ARACHNOID, AND THE 

 SUBARACHNOID CAVITY TO THE BRAIN. 



Pia mater SUSAXACffJfOID CAVITY 



Arachnoid 



THIRD VENTRICLE 

 IXFCNDIBULUU 



CISTERNA BASALIS 

 CISTERXA PONTIS 



FOURTH VEXTIilCLE 



... </;/.>/;- 

 BELLI l-)l El> I -LLABIS 



FUR A .VK\ OF 

 MAGEKDIE 



optic chiasma into two parts (a) the cisterna chiasmatis and (6) the cisterna inter- 

 peduncular is. 



(2) The cisterna pontis is situated about the pons, especially in its basilar sulcus 

 and the transverse fissures of either border, and is continuous in front with the 

 cisterna basalis and behind with the subarachnoid cavity of the medulla. 



(3) The cisterna superior lies in the angle between the splenium of the corpus 

 callosum, the superior surface of the cerebellum, and the upper surface of the mesen- 

 cephalon, and is connected ventrally, around the cerebral peduncles, with the cisterna 

 basalis. 



(4) The cisterna cerebello-medullaris (cisterna magna) is the cavity between 

 the lower surface of the cerebellum and the upper surface' of the medulla oblongata 

 ,t is continuous below with the spinal subarachnoid space. The fluid in this cavity 

 is directly continuous with that in the fourth ventricle by way of the foramen of 

 Magendie (median aperture of the fourth ventricle). 



Pacchionian Bodies (Arachnoid Granulations). In certain situations, more 

 particularly on the vertex of the brain, along the margins of the longitudinal fissure 

 particularly in the frontal region, and to a much less extent upon the upper surface of 

 the vermis of the cerebellum, the subarachnoid tissue proliferate* and forms numer- 

 ous small, ovoid or spherical nodules, the Pacchionian bodies. Each body or arach- 



