THE ARACHNOIDEA. 559 
When the interior of the tube of spinal dura mater is inspected, the series of 
apertures of exit for the roots of the spinal nerves is seen. These are ranged in 
pairs opposite each intervertebral foramen. 
Viewed from the inside of the tube of dura mater, each of the two roots of a spinal nerve is 
seen to carry with it a special and distinct sheath. When examined on the outside, however, the 
appearance is such that one might be led to conclude that both roots are enveloped in one sheath 
of dura mater. This is due to the fact that the two sheaths are firmly held together by inter- 
vening connective tissue. The two tubular sheaths remain distinct as far as the ganglion on the 
posterior root, and then blend with each other. 
Subdural Space.—The dura mater and the arachnoid mater are closely applied 
to each other, and the capillary interval between them is termed the subdural 
space.. It contains a minute quantity of fluid, which is just sufficient in amount to 
moisten the opposed surfaces of the two bounding membranes. 
The subdural space in no way communicates with the subarachnoid space. The fluid which 
it contains is led into the venous blood-sinuses around the Pacchionian bodies, and thus gains 
exit. The subdural space is carried outwards for a very short distance on the various nerves 
which are connected with the brain and the spinal cord, and it has a free communication with 
the lymph paths present in these nerves. In the case of the optic nerve the sheath of dura 
mater is carried along its whole length, and with it the subdural space is likewise prolonged to 
the back of the eyeball. 
| THE ARACHNOIDEA. 
The arachnoid mater is a very thin membrane, remarkable for its delicacy and 
transparency, which envelopes both the brain and the cord between the dura mater 
and the pia mater. The cranial part of the arachnoid mater or the arachnoidea 
encephali, except in the case of the great longitudinal and the Sylvian fissures, does 
Pacchionian body 
Lacuna lateralis 
SUP LONGITUDINAL 
SINUS gee 
Subdural space— y 7 aera eee 
Arachnoidea 
Subarach- 
noid space 
and tissue 
Pia mate: 43 
Falx cerebri 
Fs 
1 
oy, 
LJ 
=, 
Cerebral 
convolution 
Fic. 414.—Dr1acram to show the relations of the membranes of the brain to the cranial wall and the cerebral 
convolutions, and also of the Pacchionian bodies to the superior longitudinal sinus and the lateral lacune. 
not dip into the sulci on the surface of the brain. In this respect it differs from 
the pia mater. It bridges over the inequalities on the surface of the brain. 
Consequently, on the basal aspect of the encephalon it is spread out in the form of 
a very distinct sheet over the medulla, the pons Varolii, and the hollow which lies 
in front of the pons, and in certain of these regions it is separated from the brain- 
surface by wide intervals. 
The spinal part of the arachnoid mater or arachnoidea spinalis, which is directly 
continuous with the cranial arachnoidea, forms a loose wide investment for the 
7 
