704 THE NERVOUS SYSTEM. 



situations : 1. Upon the outer surface of the dura mater, in the vicinity of the 

 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 simply enlarged normal villi 

 of the arachnoid. In their growth they perforate the dura mater, and are thus 

 found on its outer surface, and when of large size they cause absorption of the 

 bone, and come to be lodged in pits or depressions on the inner table of the skull. 

 The manner in which they perforate the dura mater is as follows : At an early 

 period of their growth they project through minute holes in the inner layer of the 

 dura mater, which open into large venous spaces situated in the tissues of the 

 membrane on either side of the longitudinal sinus and communicating with it. 

 In their onward growth the villi push the outer layer of the dura mater before 

 them, and this forms over them a delicate membranous sheath. In structure they 

 consist of trabeculse of connective tissue covered over by a layer of endothelium. 

 The spongy tissue of which they are composed is continuous with the trabecular 

 tissue of the subarachnoid space. 



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 Dura Mater. The processes of the dura mater, sent inward 

 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 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 is an arched lamina of dura mater, elevated in the 

 middle and inclining downward toward the circumference. It covers the upper 

 surface of the cerebellum, and supports the occipital lobes of the brain, and 

 prevents them pressing upon the cerebellum. It is attached, behind, by its convex 

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

 there encloses the lateral sinuses ; in front, to the superior margin of the petrous 

 portion of the temporal bone, enclosing the superior petrosal sinuses ; and at the 

 apex of this bone the free or internal border and the attached or external border 

 meet, and, forming two processes, cross one another and are continued forward, to 

 be attached to the anterior and posterior clinoid processes respectively. 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 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 Arachnoid Membrane. 



The arachnoid (dpd%vy e?ooc, like a spider's web), so named from its extreme 

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



