THE ARACHNOID AND THE PIA MATER. 717 



hemispheres, so as to leave a considerable space beneath it. Superiorly, 

 it is prolonged into the median fissure between the cerebral hemi- 

 spheres as far as the falx cerebri, but does not reach to the bottom of 

 the cleft. 



The arachnoid forms tubular sheaths on the nerves leaving the sheaths on 

 cavity of the cranium which enter the apertures in the dura ruater, ne 

 and then terminate in a free edge ; but around the vessels passing to and vessels. 

 or from the brain, the membrane joins the dura mater. 



The subarachnoid space is filled by a watery fluid named cerebro- Subarach- 

 spinal. The space varies greatly in size at different parts. Over the "aries in 

 convolutions and prominences of the brain the arachnoid approaches extent: 

 the pia mater closely, and the interval between them is very small ; 

 but opposite the sulci and depressions of the surface the space is 

 expanded. The largest cavity (cisterna magna) is between the cere- 

 bellum and medulla oblongata, where the arachnoid is reflected from 

 the one to the other, being widely separated from the pia mater which 

 follows the surfaces. By an aperture in the pia mater at the depth three large 

 of this space the subarachnoid space is placed in communication 

 with the fourth and, ultimately, with the other ventricular cavities 

 of the brain. Another considerable subarachnoid space (cisterna 

 <) exists between the cerebral hemispheres in front of the pons 

 with extensions outwards into the fissures of Sylvius and backwards 

 to the cisterna magna ; and a third extends the whole length of the 

 corpus callosum, in the great longitudinal fissure. 



The PIA MATER closely invests the brain, following all inequalities Pia mater 

 of the surface, and dips into the sulci of the cerebrum and cere- 

 bellum. It also sends a large process, named the velum interpositum, forms velum 

 into the interior of the cerebrum, and from this vascular processes s ^t nni 

 known as the choroid plexuses project into some of the ventricles of the and choroid 

 brain. Two similar fringes, the choroid plexuses of the fourth ventricle, 

 similarly project into that cavity between the cerebellum and medulla 

 oblongata. 



The pia mater consists of a network of vessels, formed by the structure, 

 ramifications of the arteries and veins entering into, or issuing from 

 the nervous substance, the intervals between the vessels being closed 

 by connective tissue so as to form a continuous membrane. From 

 its deep aspect minute and very numerous vessels pass into the brain 

 perpendicularly to the surface ; and these can readily be seen as fine HOW to 

 hair-like processes projecting from the membrane when a portion of vessel? 

 it is stripped from the brain substance under water or when a piece 

 of the freshly removed membrane is floated out in a dish. 



Vessels and nerves. The arachnoid has no vessels, but various Vessels and 

 anatomists have described minute branches of some of the cranial JSbranes. 

 nerves in the membrane. The sources of the vessels of the pia 

 mater are given below, and its nerves, which are probably destined 

 for the vessels, come from several cranial nerves and the sympathetic. 



Dissection. First follow out the arteries at the base (fig. 257, Dissection 

 p. 719), let the brain be upside down, and remove the arachnoid 

 membrane. Having displayed the trunks of the vertebral arteries ( l2 ) 



