G76 



THE XERVOUS SYSTEM 



greater part of which, however, is lodged in the meshes of the subarachnoid tissue, 

 and in the ventricles of the brain (page 678). This space separates the arachnoid 

 from the dura mater, except where it is crossed by the veins of the brain passing 

 to the cranial sinuses, by the Pacchionian bodies, and by the cranial nerves at tlieir 

 points of exit from the skull. 



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, Avith 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 Avhich perforate the dura 

 mater and cause absorption of the bone in their vicinity. Most of these bodies are 



Fig. 406. — Coronal Section' through the Great Longitudinal Fissure, showing the 



Meninges. (Key aud Retzius. ) 



SUB A HA CHSOID SPA CE 



Superior lomjititdinul sinus 



PACCHIOXIAX BODY 



CORPiS CALLOSIM 



lodged in irregular pits in thecalvaria; others project into the superior longitudinal 

 sinus. They may also occur in other situations. 



The subarachnoid tissue consists of very fine trabeculae, clotlied with endo- 

 thelial cells, which pass from the arachnoid to the pia 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 r(^gions where the distance between the araclnioid and pia niater is increased as 

 subarachnoid spaces. It should be understood, however, that in these larger 

 spaces tlie subarachudid tissue is less abundant and the meshes are larger than in 

 the regions where the arachnoid and the pia mater are more ap])roximated. 



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, page 718) 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 fourtli ventricle by means of three small o]>enings; the principal of 

 these (foramen of Majendie) is in the middle line of the tela choroidea inferior 

 immediately above the obex (page 718). The two others (foramina of Key and 



