DISSECTION OF THE HEAD AND NECK 239 



Examine the membrane (arachnoidea spinalis) which bounds 

 the cavum subdurale internally. Note that this is what many 

 anatomists call the " visceral layer" of the arachnoid; below, 

 at the summit of the conus terminal is (second sacral), it is re- 

 flected upon the dura mater to form the " parietal layer " of the 

 arachnoid; the " arachnoid cavity" of various writers is sit- 



Dura mater spinalis 



.Conus medullaris 



Arachnoidea spinalis 



Cavum subdurale 



Filum terminate 



..7T^:Cavum subarachnoideale 



. Junction of arachnoidea and dura 



Filum terminale 



Arachnoidea spinalis 



x Cavum subdurale 



Dura mater spinalis 



Disposition of the arachnoidea spinalis at the inferior extremity of the medulla spinalis. The 

 sections are schematic (longitudinal and transverse). The pia mater is in red. The transverse section 

 passes through the cauda equina. (From Poirier et Charpy, Traite d'Anat. hum., Paris, 1899, t. iii., 

 1, p. 118, Fig. 83.) 



uated between the parietal layer and the visceral layer, being the 

 cavum subdurale of the present nomenclature. It is a serous 

 cavity like the pleura. Can you separate the " parietal layer" 

 from the dura! Do you see any blood-vessels in the " visceral 

 layer"? Are there any calcified plates in the latter? Note the 



