124 



PRACTICAL ANATOMY. 



Is the pia mater also intact, so as not to permit communication between the sub- 

 araclinoid fluid and the ventricular fluid in the ventricular cavities of the brain ? 



No ; the pia mater has many slits, by which it permits the fluid in the sub- 

 arachnoid space to communicate with other lymph spaces. The main ones are 

 in the pia mater covering the fourth ventricle. They are three in number : Key, 

 Retzius, Magendie. (Fig. 84.) 



The arachnoid is covered externally with endothelial cells. It is joined to 

 the pia by fibrous trabecuhe, called the subarachnoid tissue. At the base of the 

 brain it is separated from the pia mater by the anterior and posterior subarach- 

 noid spaces. The anterior is between the temporo-sphenoidal lobes of the 

 cerebrum and in front of the pons Varolii ; the posterior subarachnoid space is 

 between the medulla and cerebellum. These two spaces contain the greater 



BRAIN 



HKI.H'ATK aOBA&ACBSOID Tlfxrn I-IIXXKITIM: TIIK AR4CHXOID 

 Til TIIK l>l.\ lt.\TKII IIVKK TIIK I'llXVHXITY I If TIIK KRAIX 



Pia mater 

 Dura mater 



Arachnoid membrane 

 until I' I! A I. SI'AI'J-; 



ASTHHIOR sl'BARAi'H- 



\nll> *l'.lfK 



N \fjin .v/Mf'A' 

 A or KKY IX THK PIA MA 7 



flll/AMKX IIP MM1KXIIIK IX THK 

 I'll MATKK 



nHAMKX i if iif.rr.irs ix TIIK 



I'l \ MATER 



\ 



FIG. 84. SCHEMA lie. 



To show ihe location of the foramina of Key and Retzius in the pia mater, whereby the subarachnoid 

 fluid in the subarachnoid space communicates with the fluid in the fourth ventricle. 



part of the subarachnoid fluid. The posterior space communicates with the 

 fourth ventricle of the brain through the pia mater by the foramina of Key and 

 Retzius. (Fig. 84.) 



Where is the cercbro-spinal fluid found ami u'/uit arc its uses ? 



The subarachnoid or cerebro-spinal fluid is found in the subarachnoid spaces 

 of the brain and cord and in the ventricular cavities of the brain. Its use is to 

 mechanically protect the nerve centres from shock, and to fill up space, as fat 

 does in some other parts of the body. Its amount is estimated at less than 

 two ounces. The cerebro-spinal fluid of the brain and cord communicate. 



Explain the special features of the subaracluwid space. 



As shown in figure 84, the greater part of the space between the arachnoid 

 membrane and the pia mater is occupied by delicate connective tissue. It is im- 



