BastiaNj on the so-called Pacchionian Bodies. 93 
either vessels or nerves in them. Some of the statements at 
present in vogue concerning the histology of the Pacchionian 
bodies are very lax : thus^ by Todd and Bowman^ they are 
stated to be " whitish granules composed of an albuminous 
material found amongst the vessels of the pia mater/' which 
in their increase push the arachnoid before them ; whilst in 
another placet each of these bodies is said by Dr. Todd ^' to 
consist of a mass of minute granules enclosed in a mem- 
branous sac.^'' 
Luschka is_, I believe, the only writer who mentions the 
existence of epithelium upon the surface of these bodies ; he 
describes it, however, as scanty, and met with only in 
isolated patches. It is often not easily recognised without 
the aid of reagents, but when small growths of this kind, or 
portions of larger ones, are mounted in equal parts of water 
and acetic acid for microscopical examination, the structures 
swell and become more transparent, whilst a pretty uniform 
covering of roundish or elliptical cells can generally be dis- 
covered on the surface, lying close together, though not in 
contact (fig. 2) . The use* of the same reagent renders visible 
bodies precisely similar on the surface of the arachnoid itself, 
though they are somewhat difficult of detection when por- 
tions of it are immersed in water alone previous to examina- 
tion. The examination of other serous membranes, such as 
the peritoneum, pleura, and pericardium, showed bodies of a 
similar nature on their surface, and it was after careful 
scrutiny of them that I convinced myself that these appa- 
rently distinct, elliptical, nucleated cells were in reality only 
the nucleolated nuclei of a pavement epithelium, just such as 
has been described by Henle, and similar to what I have 
represented in fig. 3. The containing cell on the arachnoid 
and its outgrowths must be extremely delicate and fragile, 
since, though scrapings with a knife from their surface yield 
crowds of nuclei, separate as well as aggregated, yet I have 
never succeeded in detecting the same tesselated arrange- 
ment of their containing cells as I have seen elsewhere, 
though I have occasionally seen isolated nuclei contained in 
a delicate and almost invisible cell (fig. 4), and all analogy 
would lead us to believe that the nature of the epithelium 
was the same on the arachnoid as on other serous mem- 
branes, more especially when bodies precisely similar to 
what are undoubtedly the nuclei of these are found on the 
former. The epithelium is generally best seen on the 
younger growths, or else on those older ones which have 
* 'Physiolog. Anat. of Man,' vol. i, p. 255. 
f ' Cycl. of Anat. and Physiol.,' loc. cit. 
