THE NERVOUS SYSTEM 1475 



attachment it contains the occipital sinus and its two divisions. 

 Anteriorly it ends in a free border. 



The diaphragma sellae is a small circular fold, horizontally placed, 

 which forms a roof for the sella turcica or pituitary fossa of the 

 sphenoid tone, and almost entirely covers the pituitary body. At 

 its centre there is a small opening for the passage of the infundi- 

 bulum. 



Venous Sinuses of the Dura Mater. — These are blood-channels 

 or spaces situated bet^veen the two layers of the dura mater, and 

 lined with endothelium. They are as follows : 



Superior longitudinal sinus. Spheno-parietal sinuses (two). 



Inferior longitudinal sinus. Circular sinus. 



Straight sinus. Superior petrosal sinuses (two). 



Lateral sinuses (two). Inferior petrosal sinuses (two). 



Occipital sinus. Transverse or basUar sinus. 



Cavernous sinuses (two). Petro-squamous sinuses (two). 



The superior longitudinal sinus is situated in the median line 

 within the superior convex border of the falx cerebri. It extends 

 from the crista galli of the ethmoid bone to the internal occipital 

 protuberance, where, as a rule, it turns sharply to the right, and 

 opens into the right lateral sinus. It sometimes, however, turns 

 to the left, and opens into the left lateral sinus. Its posterior 

 extremity is dilated, and forms the torcular Herophlli, which usually 

 occupies a depression on the right side of the internal occipital 

 protuberance, and is connected with the dilatation at the posterior 

 extremity of the straight sinus by a transverse vessel. It increases 

 in size as it passes backwards, and its shape is triangular, the base 

 being directed towards the cranial vault. The apex is directed 

 downwards, and in this region the sinus is crossed by a number 

 of delicate fibrous bands, called the chordae Willisii. Opening into 

 the sinus on either side there are venous spaces, called lacunae 

 laterales, which are situated within the dura mater, and projecting 

 into these lacunse from below, or into the sinus itself, there are 

 several Pacchionian bodies, covered by the endothelial lining. 

 The sinus receives the superior cerebral veins and some of the 

 meningeal veins of the falx cerebri. The former open into it from 

 behind forwards, so that the blood-flow in these veins is opposed 

 to the current of b.'ood in the sinus, which is from before back- 

 wards. 



The superior longitudinal sinus frequently communicates an- 

 teriorly with the veins of the roof of the nose through the foramen 

 ccBciim, and it communicates with the veins of the scalp by means 

 of an emissary vein, which passes through the parietal foramen 

 of each side, when that is present. 



The inferior longitudinal sinus (vein) is of small size, and is. 

 situated in the lower free border of the falx cerebri over its posterior 

 two-thirds. Its direction is backwards, and it opens into the 

 front part of the straight sinus at the anterior margin of the ten- 

 torium cerebelli. It is circular, and increases in size as it passes 



