THE MEMBRANES OF THE BRAIN. 



1205 



variable but usually short distance beyond their emergence from the dural sac. The 

 arachnoid sheath is especially well marked along the optic nerve, which it follows as 

 far as the eyeball, and completely subdivides the space between the pial and dural 



Fig. 1039. 



Pacchionian body 

 Dura, reflected medially 



Cerebral vein 



Artery in 

 pia 



Cerebral vein 



Pacchionian body 



Portion of superior surface of right hemisphere covered by pia and arachnoid ; dura has been partly separated 

 and reflected towards mid-line to expose Pacchionian bodies and cerebral veins, which are seen entering superior 

 longitudinal sinus. 



sheaths into a subdural and a subarachnoid perineural compartment, directly contin- 

 uous with the corresponding intracranial spaces. 



As previously noted, the cerebro-spinal fluid secreted within the ventricles 

 escapes through the openings in the roof of the fourth ventricle— foramen of Magendie 

 and the foramina of Luschka (page iioo) — into the subarachnoid space. After 

 filling the cisterna magna and the other large spaces on the basal surface of the brain 

 and surrounding the spinal cord, the fluid finds its way into the smaller spaces on 

 the exterior of the cerebrum. In this manner the entire mass of nervous tissue is 

 enveloped by a more or less extensive cushion of fluid which, particularly at the base 

 of the brain, is well adapted to protect the enclosed delicate structures from undue 

 concussion. Since the cerebro-spinal fluid is being continuously secreted, it is evi- 

 dent that some adequate means of escape must be provided to insure, under normal 

 conditions, the maintenance of intra- 

 cranial and intracerebral pressure Fig. 1040. 

 within due limits. The paths by which 

 this is accomplished include: (i) the 

 extension of the subarachnoid space 

 along the nerve-trunks, and (2) the 

 villous projections of arachnoid tissue, 

 the Pacchionian bodies, along the 

 course of the dural blood-sinuses. 



The Pacchionian bodies (graii' 

 ulationes arachnoidales) are numerous 

 cauliflower-like excrescences of the 

 arachnoid, for the most part small but 

 occasionally reaching a diameter of 

 5 mm. or over, which lie on the outer 

 surface of the membrane along the 

 course of the dural venous sinuses. 

 Their favorite site is on either side of 

 the superior longitudinal sinus, where 



they occur in groups, although they occur in smaller number and size in connec- 

 tion with other sinuses, as the lateral, cavernous and straight. They consist 

 entirely of arachnoid tissue and contain no blood-vessels. Although lying mostly 

 at the side of the longitudinal sinus with which they are then indirectly related 

 through the lateral diverticula, the laaaice laterales or blood-lakes, in some instances 





Diagram showing relations of Pacchionian bodies to 

 blood-spaces and dura; if, bone, 5, longitudinal sinus; 

 jC, lacunas; /", Pacchionian bodies; V, cerebral vein 

 emptying into lacuna; SD, subdural space; dura is blue 

 and pia is red, intervening tissue is arachnoid; A. 



