APPENDIX. 547 



when thus separated, its breadth is very unequal, and its contour ill- 

 defined. These filaments are often totally destitute of granular matter. 



" This last species of fibre is very common among the chords com- 

 posing the plexuses of the arachnoid; it is also sometimes situated in the 

 centre of the larger ones, surrounded by the second species of fibre ; this 

 can be detached mechanically, and exhibited separately under the mi- 

 croscope. 



" In the nerves obviously sympathetic, this kind of fibre exists in con- 

 siderable abundance in those branches of the solar and other plexuses 

 which are most remote from the ganglia. 



" Thus far my observations have been confined to the structure of the 

 fibres of the arachnoid, and their supposed use. I will now consider the 

 corpuscular or ganglionic part of this membrane. Some of the plexuses 

 on its cerebral surface have the interstices formed by their interlacing 

 fibres, completely filled up with small roundish corpuscles, about the size 

 of blood- discs ; whilst, in others, these fibres are covered with irregular ly- 

 oval masses of them. On this surface also, in various situations, there 

 are well defined round or oval bodies, having in their centre a granular 

 nucleus surrounded by fibrous tissue, intermixed with more or less cor- 

 puscular matter. Some of these bodies are connected to the fibres of the 

 arachnoid by a very fine thread, others are situated at the conflux of 

 two or more fibrous chords, and their diameter varies from that of two to 

 seven blood-corpuscles. They are generally solitary and not numerous ; 

 but as they have been present in the arachnoid of every human subject 

 which I have examined (a number exceeding twenty), they cannot be 

 regarded as accidental or adventitious. At present I cannot decide as to 

 their nature or office, not having seen anything which they exactly re- 

 semble in other parts of the body ; at any rate, they look more like small 

 ganglia than anything else I have seen. 



" Besides these corpuscles, which, as before stated, exist on the cerebral 

 surface of the arachnoid, I have met with some of a very different cha- 

 racter, situated in its substance, though nearer to the cranial than to the 

 cerebral surface. The most ordinary appearance which these present, 

 when seen by transmitted light, is that of a section of an urinary calculus 

 made through its centre, appearing, like it, to be made up of concentric 

 layers. When viewed by reflected light, these bodies seem to be vesi- 

 cular, and filled with fluid, the quantity of which appears to diminish as 

 the number of layers increases, so that those in which the laminae have 

 extended as far as the centre, are almost flat. Although the most fre- 

 quent form of these bodies is circular, yet some are oval ; occasionally 

 they are connected with a fibre of the arachnoid, in such a manner as to 

 resemble small Pascinian corpuscles. One remarkable fact connected 

 with these bodies is that they occur in the arachnoid of almost every sub- 

 ject which I have had an opportunity of examining, and that no part of 

 the membrane is exempt from them ; generally tfiey are solitary, and very 

 sparingly distributed ; but sometimes they are in clusters. I have found 



