TllK Ah'.\('II\OID 



893 



noid villus consists of a retiforni network of subarachnoid substance and its meshes 

 arc tilled with cerebro-spinal fluid. The Paccliionian bodie> on the vertex of the 

 brain project through I he inner layer of the dura muter, either into the superior 

 sagiltal sinus or into the venous spaces or parasinoidal sinuses which lie at the 

 sides of that sinus, and. as they become larger, they press against the outer layer 

 of the dura mater and produce ovoid depressions on the inner surface of the cranium. 

 They probably facilitate the passage of lymph from the subarachnoid cavity into the 

 bloc id sinuses', and thu> may aid iii relieving pressure within. On the other hand, 

 through them the cerebro-spinal fluid is replenished at need from the blood plasma.. 

 They are not present at birth, but they appear at the tenth year and increase in 

 number and size with advancing age. They are less marked in the female than in 

 the male. 



The spinal arachnoid is in the form of a loose, reticular sac which is most capa- 

 cious about the lumbar enlargement of the spinal cord and about the cauda equina. 

 Like that of the encephalon, the ]*>rtion next to the dura mater alone resembles 

 a membrane, being a loosely onranized feltwork, covered on the side of the subdural 

 cavity by a layer of endothelium common to that cavity. Throughout its length 

 the spinal subarachnoid cavity is relatively wide, and, as in the cranium, contains 

 a fine, spongy, web-like reticulum, numerous threads of which are continuous with 

 the pia mater. This spongy tissue is the inner modification of the arachnoid, and 



Fio. 664. CORONAL SECTION THROUGH THE GREAT LONGITUDINAL FISSURE, SHOWING THE 



MKNINGES. (Key and Retzius.) 



Superior sngiltal sinus 



PA CCltlONlAN BOD Y 



, CORPUS CALLOSUM 



its meshes are occupied by the cerebro-spinal fluid. It is not so abundant as in the 

 cranial subarachnoid cavity. In addition to the delicate threads of this tissue the 

 arachnoid is more firmly attached to the pia mater by three incomplete partitions. 

 The most continuous of these is arranged along the dorsal mid-line and is known as 

 the septum posticum of Schwalbc (subarachnoid septum). This may be described 

 as a linear accumulation of the spongy tissue which pervades the subarachnoid 

 space. It is most incomplete in the upper cervical region, where it becomes merely 

 a line of threads connecting with the pia. It is most complete as a septum in the 

 lower cervical and in the thoracic region, but at best it maintains a spongy char- 

 acter. The other two partitions are formed by the denticulate ligaments, which 

 extend laterally from either side of the spinal cord, connecting the pia and dura 

 mater and involving the arachnoid in passing through it. Within the subarachnoid 

 cavity these form more or less complete septa, though outside the arachnoid they 

 are attached to the dura only at the intervals of their dentations. They belong to 

 the pia mater and will be described with it. The arachnoid is further continuous 

 with the pia by way of the connective-tissue sheaths of the roots of the spinal nerves 

 and the blood-vessels passing through the subarachnoid cavity. 



Vessels and Nerves. The arachnoid has no special blood supply and probably no special 

 nerves other than those supplying the walls of the blood-vessels passing through it. 



