The arachnoid is a thin delicate membrane, which presents a well-defined limiting surface towards the dura mater, but on its deep or pia-matral surface passes insensibly into the subarachnoid tissue.
The arachnoid does not dip into the fissures of the cerebrum and cerebellum, with the exception of those fissures which contain processes of dura mater. Thus it is carried into the great longitu- dinal fissure for a depth corresponding to the falx cerebri, and it passes for a short distance into the fissure of Sylvius around the fold of dura mater which projects from the lesser wing of the sphenoid.
On each side of the superior longitudinal sinus groups of small lobulated bodies, the so-called Pacchionian glands, project from the surface of the arachnoid ; these are enlargements of the normal villi of that membrane which perforate the dura mater and cause absorption of the bone in their vicinity. Most of these bodies are lodged in irregular pits in the calvaria ; others project into the superior longitu- dinal sinus. They may also occur in other situations.
The subarachnoid tissue consists of very fine trabeculse, clothed with endo- thelial cells, which pass from the arachnoid to the i3ia mater. Thus, a sub-arachnoid space, in the proper sense of the word, does not exist ; it is convenient, however, to retain the term to designate the region which is occupied by the sub- arachnoid tissue, and to speak of the accumulations of this tissue which are formed in regions where the distance between the arachnoid and pia mater is increased as subarachnoid spaces. It should be understood, however, that in these larger spaces the subarachnoid tissue is less abundant and the meshes are larger than in the regions where the arachnoid and the pia mater are more approximated.
The largest of these spaces is the cisterna magna, which is a continuation of the posterior subarachnoid space of the spinal cord. This space appears triangular in sagittal section. It is bounded in front by the layer of pia mater (tela choroidea inferior) which closes in the lower part of the roof of the fourth ventricle, and above by the inferior vermiform process of the cerebellum. It extends laterally as far as the outer margins of the tonsillar lobes. It communi- cates with the fourth ventricle by means of three small openings ; the principal of these (foramen of Majendie) is in the middle line of the tela choroidea inferior immediately above the obex (page 758). The two others (foramina of Key and Eetzius) are at the extremities of the recessus laterales of the fom^th ventricle, behind the upper roots of the gios so -pharyngeal nerves. Another large space, the cisterna pontis, is continued from the anterior subarachnoid space of the cord, and extends forwards as far as the commencement of the great longitudinal fissure, and laterally to the inner margins of the temporo-sphenoidal lobes. The basilar artery and the circle of Willis are placed in this space. The cisterna pontis communicates freely around the medulla oblongata with the cisterna magna - thus the medulla is encircled by a wide subarachnoid space.
A large space is also to be found between the lower edge of the falx cerebri (where the arachnoid passes across from one cerebral hemisphere to the other) and the upper surface of the corpus callosum. This space contains the trunks and larger branches of the anterior cerebral arteries. Another considerable space exists in the fissure of Sylvius, and in this the middle cerebral artery ramifies. Thus, with the exception of the cisterna magna, all these spaces serve for the accommodation of large arteries. There is also a space between the corpora quadri-gemina and the anterior extremity of the superior vermiform process of the cere- bellum, and through this the veins of Galen pass to terminate in the straight sinus.