THE ARACHNOID 971 



is thin and transparent, and may be easily demonstrated by injecting a stream of 

 air beneath it by means of a blowpipe; it passes over the convolutions without 

 dipping down into the fissures between them, but does pass into the sylvian and 

 intercerebral fissures and is prolonged upon the nerves as a sheath. At the base 

 of the brain the arachnoid is thicker, and slightly opaque toward the central part; 

 it covers the orbital surface of the anterior lobes and extends across between the 

 two temporal lobes so as to leave a considerable interval between it and the brain, 

 the cisterna basalis . 



The subarachnoid space (cavum subarachnoideale) (Fig. 722) is the interval 

 between the arachnoid and pia. It is not only on the surface, but dips between 

 the convolutions. It is not, properly speaking, a space, for it is occupied every- 

 where by a spongy tissue consisting of trabeculse of delicate connective tissue 

 covered with endothelium, which pass from the pia to the arachnoid, and in the 

 meshes of which the subarachnoid fluid is contained. This so-called space is 

 small on the surface of the cerebrum; but at the base of the brain the subarachnoid 

 tissue is less abundant and its meshes larger. 



----**:-S-. R A N I U (VJ 



C ^ .' : .-.-.:.....-. .1^1 



RACHNOID 

 PIA 



CONVOLUTION 

 OF BRAIN 



FIG. 722. The subarachnoid space. (Schematic.) (Poirier and Charpy.) 



In certain regions the arachnoid and pia are farther apart than was previously 

 indicated, and these spaces are called subarachnoid cisternae (cisternae subarack- 

 noidalis}. The largest space is the continuation of the posterior part of the sub- 

 arachnoid space of the spinal cord. It is called the postcisterna (cisterna cere- 

 bellomedullaris}. It is a space formed by the arachnoid passing across the back 

 and under portions of the medulla oblongata and cerebellum. It communicates 

 with the fourth ventricle by three foramina. The largest opening is the foramen 

 of Magendie (metapore; apertura medialis ventriculi quarti). It is in the middle 

 line of the tela choroidea. At the end of each recessus lateralis of the fourth 

 ventricle there is also an opening, and each opening is called the foramen of Luschka 

 or of Key and Retzius (apertura lateralis ventriculi quarti). The cisterna pontis is 

 the continuation upward of the anterior part of the subarachnoid space of the 

 cord. About the medulla oblongata it is continuous with the postcisterna, so 

 this important nerve centre is surrounded by a large subarachnoid space. The 

 crural cisterna or cisterna basalis (cisterna inter peduncular is} is formed by the arach- 

 noid extending between the two temporal lobes, and contains the arteries forming 

 the circle of Willis. The anterior subarachnoid space (cisterna pontis, interpedun- 

 cularis et chiasmatis] includes the cisterna pontis, the cisterna basalis, and the 

 cisterna of the optic chiasm. There is a cisterna between the inferior edge of 

 the falx cerebri and the superior surface of the corpus callosum which contains 

 the anterior cerebral arteries, a cisterna in the sylvian fissure (cisterna sylviana) 



