THE MEMBRANES OF THE BRAIN. 641 



attached, the straight sinus being placed at their point of junction. Its anterior 

 border is free and concave, and bounds a large oval opening for the transmission 

 of the crura cerebri. 



The/rte cerebelli is a small triangular process of dura mater received into the 

 indentation between the two lateral lobes of the cerebellum behind. Its base is 

 attached, above, to the under and back part of the tentorium ; its posterior margin, 

 to the lower division of the vertical crest on the inner surface of the occipital 

 bone. As it descends it sometimes divides into two smaller folds, which are lost 

 on the sides of the foramen magnum. 



The diaphragma settee is a horizontal process formed by a reduplication of the 

 meningeal layer of the dura mater. It forms a small circular fold, which con- 

 stitutes a roof for the sella turcica. This almost completely covers the pituitary 

 body, presenting merely a small central opening for the infundibulum to pass 

 through. 



The Arachnoid Membrane. 



The arachnoid (apdyyrj ea?0c, like a spider's web), so named for its extreme 

 thinness, is a delicate membrane which envelops the brain, lying between the pia 

 mater internally and the dura mater externally ; from this latter membrane it is 

 separated by a space, the subdural space. 



It invests the brain loosely, being separated from direct contact with the 

 cerebral substance by the pia mater, and a quantity of loose areolar tissue, the 

 subarachnoidean. On the upper surface of the cerebrum the arachnoid 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 sulci between them. At the base of the brain the arachnoid is thicker, 

 and slightly opaque toward the central part ; it covers the anterior lobes, and 

 extends across between the two temporal lobes so as to leave a considerable 

 interval between it and the brain, the anterior subarachnoidean space (Cisterna 

 pontis) ; it is in contact with the pons and under surface of the cerebellum ; but 

 between the hemispheres of the cerebellum and the medulla oblongata another con- 

 siderable interval is left between it and the brain, called the posterior subarach- 

 noidean space (Cisterna magna). These two spaces communicate together across 

 the crura cerebelli. Other smaller cisternae are found in various positions ; and all 

 communicate freely with one another. The arachnoid membrane surrounds the 

 nerves which arise from the brain, and encloses them in loose sheaths as far as 

 their point of exit from the skull. 



The subarachnoid space is the interval between the arachnoid and pia mater. 

 It is not, properly speaking, a space, for it is occupied everywhere by a spongy 

 tissue consisting of trabecuL'B of delicate connective tissue, which pass from the pia 

 mater to the arachnoid, and in the meshes of which the subarachnoid fluid is con- 

 tained. This so-called space is small on the surface of .the hemispheres ; but at 

 the base of the brain the subarachnoid tissue is less abundant and its meshes 

 larger, where it forms the Cisternse pontis et magna mentioned above. In addition 

 to these two large spaces, a third is formed on the upper surface of the corpus cal- 

 losum, for the arachnoid stretches across from one cerebral hemisphere to the other 

 immediately beneath the free border of the falx cerebri, and thus leaves a space in 

 which the anterior cerebral arteries are contained. Another space is found in the 

 fissure of Sylvius, for the arachnoid stretches across from the anterior to the middle 

 lobe of the brain, without dipping down to the bottom of the fissure, and in this 

 space the middle cerebral artery ramifies. The subarachnoid space communi- 

 cates with the general ventricular cavity of the brain by means of three openings : 

 one of these is in the middle line at the inferior boundary of the fourth ventricle, 

 where an opening in the pia-raatral covering of this cavity, the foramen of Magen- 

 die, exists and permits the passage of fluid from the one space to the other. The 

 other two communications are at the extremities of the lateral recesses of the fourth 

 ventricle, behind the upper roots of the glosso-pharyngeal nerves ; they are named 



41 



