THE BRAIN AND ITS MENINGES 225 



ing part of the sinus runs downwards and slightly forwards to 

 a point three-quarters of an inch below and behind the centre 

 of the meatus. Very often, however, it approaches nearer to 

 the meatus, and in this event it may be exposed during opera- 

 tions on the tympanic (mastoid) antrum. 



The Cavernous Sinuses lie one on each side of the body 

 of the sphenoid and are separated by the hypophysis (pituitary 

 body), but they communicate freely with one another. An- 

 teriorly, the cavernous sinus receives the ophthalmic veins, 

 and posteriorly the blood is drained into the transverse sinus 

 by the superior petrosal, and into the commencement of the 

 internal jugular vein by the inferior petrosal sinus. The cavern- 

 ous sinus communicates with the pterygoid venous plexus 

 through the foramen ovale and the foramen of Vesalius. Be- 

 tween the endothelial wall of the sinus and the serous layer cf 

 the dura mater which supports it laterally, the third, fourth, 

 and sixth cerebral nerves and the ophthalmic division of the fifth 

 pass forwards to the orbit (p. 207), and a similar position is 

 occupied by the internal carotid artery. Any or all of these 

 structures may be involved by (i) tumours of the hypophysis 

 (pituitary body) which extend in a lateral direction, (2) throm- 

 bosis of the sinus, (3) fractures of the skull in this region. 



The Occipital Sinus (p. 227) lies in the attached border 

 of the falx cerebelli, and runs upwards to join the right or left 

 transverse sinus near its commencement. 



The Arachnoid and the Pia Mater are two membranes 

 which are applied to the surface of the brain more closely than 

 the dura mater. The pia mater dips into all the fissures, but the 

 arachnoid only does so in the case of the longitudinal fissure 

 and the lateral fissure (of Sylvius). In certain areas the two 

 membranes are separated by the subarachnoid cisterns. Of 

 these the most important are situated on the basal surface 

 of the brain, and in basal meningitis they become filled with 

 purulent exudates, which may press on the neighbouring 

 cerebral nerves (second to the eighth inclusive). 



The Ventricular System of the Brain and Spinal 

 Medulla. The central canal is a small channel which extends 

 upwards through the whole length of the spinal medulla into 

 the medulla oblongata, where it expands to form the fourth 

 ventricle. From the upper part of the fourth ventricle the 

 cerebral aqueduct (of Sylvius} passes through the mid-brain to 

 open into the third ventricle, which in its turn communicates 



15 



