Langdon, The Arachnoid of the Brai)i. 207 



region, removing the bone only. The following features 

 were then easily demonstrated in successive order: i. Peri- 

 osteal layer of dura, traceable to its continuity with the 

 sutural " ligament." 2. The dura proper (subserous connec- 

 tive tissue), forming the walls of sinuses and carrying the 

 nutrient vessels for, 3, The parietal layer of arachnoid, a thin 

 pellicle separable with the handle of the scalpel. 4. Space 

 between parietal and visceral layers of arachnoid, or the 

 arachnoid cavity proper. 5. Visceral layer of arachnoid 

 passing over sulci, etc. 6. Subarachnoid space. 7. The 

 pia mater. 8. The convolutions. 



Second dissection: Footus at term. — This was practically 

 a repetition of the first, except as to region, the frontal bone 

 being removed instead of the parietal. 



Third dissection: Adult, negro, aged about thirty-five, 

 brain and membranes normal. — The dura covering vertex 

 and forming falx cerebri and tentorium was found to be 

 inseparably united with the parietal arachnoid; at the base 

 of the skull, however, and especially in the region of the 

 sella turcica and orbital plates, the two membranes are quite 

 freely separable with an ordinary scalpel, and the arachnoid 

 could be stripped off' in places. This separation also was 

 more marked at the points of exit of the larger cranial nerves 

 — e.g., the optic. The following diagram [A) will show at 

 at a glance these points in the parietal region of the new- 

 born infant — the only change in adult life being fusion of the 

 vertical parietal arachnoid with the subserous dura, a condi- 

 tion in every way similar to the conditions which exist in the 

 pericardium. At the base of the skull, however, the separa- 

 tion is readily appreciable in the adult, as already stated. 



Considering the nature of the sinuses — as simply dilated 

 veins — and the fact that the inner dura is the necessary 

 medium for vascular supply of the parietal arachnoid, it 

 would seem in every point of view proper to consider the 

 inner dura as homologous with the subserous connective 

 tissues elsewhere. 



