THE ARACHNOID VILLl 



973 



holes in the inner layer of the dura, which open into large venous spaces situated 

 in the tissues of the membrane, on either side of the longitudinal sinus and com- 

 municating with it. In their onward growth the villi push the outer layer of the 

 dura before them, and this forms over them a delicate membranous sheath. In 

 structure they consist of spongy trabecular tissue, covered over by a membrane, 

 which is continuous with the arachnoid. The space between these two coverings, 

 derived from the dura and arachnoid respectively, corresponds to and is con- 

 tinuous with the subdural space. The spongy tissue of which they are composed- 



FRONTAL 

 LOBE 



MIDDLE CEREBRAL 

 ARTERY 



ANTERIOR CHOROID 

 ARTERY 



ISLAND OF REIL 



EXT GENICULATE 

 BODY 



ANT GENICULATE 

 BODY 



MIDDLE CORNU 

 OF LATERA 

 VENTRICLE 



ANTERIOR CEREBRAL 

 ARTERY 



ANTERIOR 



COMMUNICATING 

 ARTERY 



NTERNAL 

 CAROTID 



POSTERIOR 

 COMMUNICATING 

 RTERY 



BASILAR 



ARTERY 



CORPORA 

 QUADRIGEMINA 



OCCIPITAL 

 LOBE 



FIG. 724. The anterior cerebral and choroid arteries. (Spalteholz.) 



is continuous with the trabecular tissue of the subarachnoid space; so that fluid 

 injected into the subarachnoid space finds its way into the Pacchionian bodies, 

 and through their coverings filters into the superior longitudinal sinus. They are 

 supposed to be a means of getting rid of an excess of cerebrospinal fluid when 

 its quantity is increased above normal, or for replenishing the cerebrospinal 

 fluid from the blood plasma when needed. Another means of getting rid of 

 cerebrospinal fluid is absorption by the lymph spaces of the cranial nerves, which 

 possess sheaths of arachnoid up to the points at which they emerge from the skull. 



