918 



THE NERVOUS SYSTEM 



the nerve-roots, the hgamenta denticulata of the spinal cord, and the occasional delicate tra* 

 beculse passing between the dura mater and the arachnoid. Immediately under the endothelium, 

 the connective-tissue fibres of the arachnoid are woven into a very thin, more or less compact 

 web. This, however, quickly grades into a loose, spongy reticulum which pervades the thick 

 subarachnoid cavity throughout, and the strands of which are directly continuous into the 

 more compact tissue of the pia mater. Thus an inner surface can hardly be claimed. This 

 loose, sponge-Uke arachnoid tissue holds the cerebro-spinal fluid of the subarachnoid cavity, 

 the meshes of the sponge constituting a reticular web of intercommunicating spaces lined by 

 endothehoidal cells covering the strands of the web. The cranial subarachnoid cavity is larger, 

 and the strands of the web are relatively more abundant than in that of the spinal canal. In 

 addition, the cavity is traversed by the spinal and cranial nerves, by the blood-vessels passing 

 to J and from the pia, and, in the spinal canal distinctively, it is traversed by the Hgamenta 

 denticulata and the filum terminale. Through these the arachnoid is further continuous with 

 the pia mater. 



The cranial arachnoid is directly continuous into that of the spinal cord, and 

 in the two localities does not differ as much as does the dura mater. Within the 

 cranium, the arachnoid does not closely follow the surface of the encephalon. It 

 is folded in between the cerebellum and cerebral hemispheres, following the con- 

 tour of the tentorium cerebelli, but it does not dip into the fissures and sulci except 

 the anterior 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 



Fig. 723. — Diagram showing the Relations of the Pia Mater, the Arachnoid, and the 

 Subarachnoid Cavity to the Brain. 



Pia mater Subarachnoid cavity 



Third ventricle 

 Infundibulum 



Arachnoid 



Cisterna basalis 

 Cisterna pontis 



Fourth ventricle 

 Cisterna cerebello- 



medullaris 

 Foramen of 

 Magendie 



that the two membranes almost appear as one. The sulci, occupied by looser 

 reticulum, form a continuous system of channels filled more abundantly by the 

 cerebro-spinal fluid. 



The arachnoid folds in between the cerebellum and medulla oblongata, and 

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

 surface forms a continuous sheet stretching from one temporal lobe to the other 

 and bridging 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 subarachnoid cavity between its outer surface and the pia mater is of 

 considerable depth in certain localities. These localities comprise the subarach- 

 noid cisternal. These occur where the cavity at the base of the brain is especially 

 large, and make possible a 'water-bed' which serves to protect the brain from 

 injurious contact with the bones. 



The following cistcrnin are distinguished (fig. 72.3): — 



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

 into,,two parts — (a) the cisterna chiasmalis and (b) the cisterna interpeduncularis. 



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



