1480 A MANUAL OF ANATOMY 



It does not dip into the fissures, except in the case of the great longi- 

 tudinal fissure, its general course being over the gyri and other 

 eminences and depressions of the encephalon. It is conspicuous at 

 the base of the encephalon in the region of the inter-peduncular 

 space, pons Varolii, and medulla oblongata. Its outer surface is 

 practically in close contact with the mner surface of the dura 

 mater, the extremely slight interval containing a very little 

 lubricating serous fluid, and being known as the subdural space. 

 Between the arachnoid and the pia mater there is the interval 

 known as the subarachnoid space. The membrane furnishes 

 sheaths to the various cranial nerves. 



Subarachnoid Space. — This space, as stated, is situated between 

 the arachnoid and the pia mater. It is crossed by delicate trabeculse 

 of connective tissue, which pass between the two membranes in a 

 reticular manner. The meshes of this reticulum contain the cerebro- 

 spinal fluid. 



The subarachnoid space communicates with the ventricles of 

 the brain by means of five openings. Three of these communicate 

 with the fourth ventricle, namely, the foramen of Magendie, 

 and the foramina of Luschka, the former being situated in the 

 median line of the roof of the ventricle a little above the calamus 

 scriptorius, and the latter being situated each in the roof of the 

 lateral recess of the ventricle. The other two openings communicate 

 with the descending cornua of the lateral ventricles, each being 

 situated at the extremity of the corresponding cornu. In some 

 situations — as, for example, over the gyri — the arachnoid and pia 

 mater are in close contact ; but in other localities the two mem- 

 branes are more or less widely separated by intervals, called cisternse. 

 These are principally three in number, namely, the cistema magna, 

 cisterna pontis, and cistema basalis. 



The cisterna magna lies between the posterior part of the inferior 

 surface of the cerebellum and the medulla oblongata. It is of large 

 size, the arachnoid, which here passes from cerebellum to 

 medulla, being widely separated from the pia mater. It is con- 

 tinuous through the foramen magnum with the posterior part of the 

 subarachnoid space of the spinal cord. 



The cisterna pontis is situated on the ventral aspect of the pons 

 Varolii. Interiorly it is continuous with the anterior part of the 

 subarachnoid space of the spinal cord, and in the region of the 

 medulla oblongata it is continuous posteriorly with the cisterna 

 magna. It contains the basilar artery. 



The cisterna basalis is situated in front of the pons Varolii, in 

 which situation the arachnoid extends over the interpeduncular 

 space from one temporal lobe to the other. It contains the arteries 

 which form the circle of Willis. This cisterna is prolonged outwards 

 on each side into the stem of the fissure of Sylvius, each of these pro- 

 longations containing the middle cerebral artery. Anteriorly it 

 extends in front of the optic commissure into the great longitudinal 

 fissure over the upper surface of the corpus callosum, this prolonga- 

 tion containing the anterior cerebral arteries. 



The subarachnoid fluid can be drained away in two directions. 



