GLANDULE PACCHIONII OR ARACHNOIDAL VILLI. 



191 



the elevations found on the outer surface of the dura mater and within the 

 longitudinal sinus, in no instance take origin in those positions, but that they are 

 grape-like bodies which are attached more deeply, and in their growth have 

 invaginated the dura mater. Their precise origin and nature were long the subject 

 of conflicting opinions, but it has been satisfactorily shown by Luschka that they are 

 villi of the arachnoid. On each side of the sinus, and communicating with it, are 

 large venous spaces in the dura mater (lacunae laterales, see p. 184) ; into these the 

 villi project even in new-born animals, and those which appear to perforate the dura 



Fig. 134. SECTION OF THE UPPER PART OF THE BRAIN AND MENINGES TO SHOW THE RELATIONS OP 

 THE ARACHNOIDAL VILLI. (Key and Retzius.) Magnified. 



c.c., corpus callosum ; /, falx cerebri ; s.a., subarachnoid space, pervaded by a network of fine 

 trabeculse ; from it the fungiform villi are seen projecting into the lateral lacunae of the dura mater. 

 Some are projecting into the superior longitudinal sinus, *. 



mater and appear on the surface have their inner parts in these spaces. Each villus 

 is covered by a membrane, continuous with the arachnoid. Outside this is another 

 fine membranous sheath, derived from the dura mater, and the interval between the 

 two is continuous with the subdural space. Within the villus is a spongy trabecular 

 tissue, continuous with the subarachnoid tissue, and of similar structure (Key and 

 Retzius). 



Fluid injected into the subarachnoid space passes freely into the Pacchionian 

 bodies, and is found after a time to filter through their walls and thus to get into 

 the subdural space, although there does not appear to be any open communication 

 between the interior of these bodies and the prolongation of the subdural space 

 which surrounds them. Moreover, if the injection is continued it can be driven 

 even into the interior of the venous sinuses and lacunas which are found in connection 

 with them, especially into the superior longitudinal sinus, into which the arachnoidal 

 villi project. So that these villi seem to afford a means of passage of the cerebro- 

 spinal fluid from the subarachnoid space into the venous sinuses, when the fluid 

 pressure in the subarachnoid space becomes from any cause increased above 

 the normal. 



BLOOD-VESSELS OP THE BRAIN AND SPINAL CORD. 



Blood supply of the spinal cord. The arteries of the spinal cord are (1) the 

 anterior spinal, double above where it is derived from the vertebrals, but single and 

 median below where it is reinforced by a series of small vessels derived from the 

 vertebral, intercostal, lumbar, and other arteries, and passing to the cord along 



