508 NERVOUS SYSTEM. 



superior longitudinal sinus, being received into little depressions on the inner 

 surface of the calvarium. 2. On the inner surface of the dura mater. 3. In the 

 superior longitudinal sinus. 4. On the pia mater near the margin of the 

 hemispheres. 



These bodies are not glandular in structure, but consist of a fibre-cellular matrix 

 originally developed from the pia mater ; by their growth, they produce absorption 

 or separation of the fibres of the dura mater; in a similar manner they make their 

 way into the superior longitudinal sinus, where they are covered by the lining 

 membrane. The cerebral layer of the arachnoid in the situation of these growths 

 is usually thickened and opaque, and adherent to the parietal portion. 



These bodies are not found in infancy, and very rarely until the third year. 

 They are usually found after the seventh year ; and from this period they increase 

 in number as age advances. Occasionally they are wanting. 



PROCESSES OF THE DUKA MATER. 



The processes of the dura mater, sent inwards into the cavity of the skull, are 

 three in number, the falx cerebri, the tentorium cerebelli, and the falx cerebelli. 



The falx 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 convex, 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, so named from its tent-like form, is an arched lamina of 

 dura mater, elevated in the middle, and inclining downwards towards its cir- 

 cumference. 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 occi- 

 pital 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 pos- 

 terior 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 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. 



ARACHNOID MEMBRANE. 



The arachnoid (apa^, f?6oj, l/'ke 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 this 

 membrane the smooth and polished surface which it presents ; it is also reflected 

 over those processes which separate the hemispheres of the brain and cerebellum. 



1 Kolliker denies this ; and states, 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. 



