578 NERVOUS SYSTEM. 



The/a/x cerebri, so named from its sickle-like form, is a strong arched process 

 of the dura mater, which descends vertically in the longitudinal fissure between 

 the two hemispheres of the brain. It is narrow in front, where it is attached 

 to the crista galli process of the ethmoid bone; and broad behind, where it is 

 connected with the upper surface of the tentorium. Its upper margin is con- 

 vex, and attached to the inner surface of the skull as far back as the internal 

 occipital protuberance. In this situation it is broad, and contains the superior 

 longitudinal sinus. Its lower margin is free, concave, and presents a sharp 

 curved edge, which contains the inferior longitudinal sinus. 



The tentorium cerebelli is an arched lamina of dura mater, elevated in the 

 middle, and inclining downwards towards the circumference. It covers the 

 upper surface of the cerebellum, supporting the posterior lobes of the brain, 

 and preventing their pressure upon it. It is attached, behind, by its convex 

 border, to the transverse ridges upon the inner surface of the occipital bone, 

 and there incloses the lateral sinuses; in front, to the superior margin of the 

 petrous portion of the temporal bone, inclosing the superior petrosal sinuses, 

 and from the apex of this bone, on each side, is continued to the anterior and 

 posterior clinoid processes. Along the middle line of its upper surface, the 

 posterior border of the falx cerebri is attached, the straight sinus being placed 

 at their point of junction. Its anterior border is free and concave, and presents 

 a large oval opening for the transmission of the crura cerebri. 



The falx 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 pos- 

 terior 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. 



ARACHNOID MEMBRANE. 



The Arachnoid (apa**^ *^f. tike a spider's web), so named from its extreme 

 thinness, is the serous membrane which envelops the brain, and is then reflected 

 on the inner surface of the dura mater. Like other serous membranes, it is a 

 shut sac, and consists of a parietal and a visceral layer. 



The parietal layer covers the inner surface of the dura mater, 1 and gives that 

 membrane its smooth and polished surface ; it is also reflected over those pro- 

 cesses which separate the hemispheres of the brain and cerebellum. 



The visceral layer invests the brain more 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 convolu- 

 tions without dipping down into the sulci between them. At the base of the 

 brain the arachnoid is thicker, and slightly opaque towards the central part; it 

 covers the anterior lobes, and is extended across between the two middle lobes, 

 so as to leave a considerable interval between it and the brain, the anterior sub- 

 troehnendean space it is closely adherent to the pons and under surface of the 

 cerebellum; but between the hemispheres of the cerebellum and the medulla 

 oblongata another considerable interval is left between it and the brain, called 

 the posterior subarachnoidean space. These two spaces communicate together 

 across the crura cerebri. The arachnoid membrane surrounds the nerves which 

 arise from the brain, and incloses them in loose sheaths as far as their point of 

 exit from the skull, where it becomes continuous with the parietal layer. 



1 l\illiker denies this; and stales, that the inner surface of the dura mater is covered with 

 pavement epithelium, but has no other investment which can be regarded as a parietal layer of 

 the arachnoid. 



