Bastian, on the so-called Pacchionian Bodies, 
91 
much greater area. lu cases in whicli there has been lodge- 
ment only, but no perforation, by the Pacchionian bodies, the 
aperture through the inner table is often sharp and clearly 
defined. 
Such being the appearances and positions occupied by these 
bodies, and arising in all cases as they do from the arachnoid 
membrane, it comes to be a matter almost of accident whether 
they are found on the surface of this membrane, on the ex- 
ternal surface of the dura mater, or projecting into the longi- 
tudinal sinus. Those springing up on a portion of the 
surface of the arachnoid not in very intimate contact with 
the dura mater, have room and are enabled to grow without 
contracting adhesions ; whilst those commencing on a portion 
closely in contact with the upper membrane (over the top of 
a convolution, for instance, instead of over a sulcus) penetrate 
between its fibres in the manner before stated, and if the 
portion of dura mater against which they impinge chances to 
be the wall of the sinus, they project into it instead of appear- 
ing on the outer surface of the dura. These latter in their 
grow^th push aside the reticulated fibrous bundles of the dura 
mater till they come in contact with the firm elastic mem- 
brane constituting the inner wall of the sinus. This they are 
unable to perforate, and instead, it slowly gives way before 
them, so that at last they come to project into the sinus in 
much the same way as an organ does into a serous sac. I 
have fully satisfied myself as to the nature of this covering 
over those bodies projecting into the sinuses, not only by 
recognising its microscopical similarity of structure to that 
which can be dissected off from other parts of the sinus, but 
also from the fact that I have frequently succeeded, by exert- 
ing considerable traction with a forceps upon a portion of 
adherent arachnoid outside the sinus, in pulling some of these 
bodies out, and leaving the little membranous caps with 
which they were surrounded — constituting the "propria et 
tenuissima membrana,^^ in which Pacchionius described them 
as being enclosed. The bodies projecting into the horizontal 
portions of the lateral sinuses, which are much less frequently 
met with than those in the longitudinal, would appear to rise 
from the arachnoid covering the posterior border of the cere- 
bellum, and not from that on the lower surface of the 
posterior extremities of the cerebral hemispheres, since I 
have found them situated only along the lower angle of the 
base of the sinus."^ 
* Since this was written I have twice detected small Pacchionian bodies on 
the arachnoid over the posterior border of tlie cerebellum, as above suggested. 
VOL. XIV. g 
