i 



CEREBRAL AND CEREBELLAR 623 



Internally it communicates with the opposite (iinus, ami posteriorly it ends in the 

 sui)erior and inferior petrosal sinuses. It also receives veins from the inferior sur- 

 face of the frontal lobe of the brain, and some of the middle cerebral veins. 

 Through the Vesalian vein, which runs in a minute foramen in the spinous pro- 

 cess of the sphenoid bone, the sinus communicates with the pterygoid plexus 

 of veins; through the venous plexus around the intraosseous portion of the 

 internal carotid, with the internal jugular vein; and through small veins which 

 leave the cranium l>y the foramen ovale and foramen lacerum medium, with 

 tlie pterygoid and |)haryng(>al |)l<'xuses. 



(11) The spheno-parietal sinus, or sinus alae parvse, runs in a slight groove 

 on the under surface of the lesser wing of the sphenoid bone. It originates in one 

 oi the meningeal veins near the apex of the lesser wing, and, running inwards, 

 passes through the sphenoidal fold of dura mater above the third nerve into the 

 front part of the cavernous sinus. It generally receives the fronto-sphenoidal veins 

 from the diploe. 



The petro-squamous sinus is occasionally present. It lies in a groove along 

 the junction of tlie petrous and S(iuamous portions of the temporal bone. It opens 

 posteriorly into the lateral sinus at the spot where the latter enters on its sigmoid 

 course. In front it sometimes, though very rarely, passes through a foramen in the 

 S({uamous portion of the temporal bone between the glenoid cavity and the external 

 auditory meatus into the temporal vein. This sinus is the rudiment of what in 

 early foetal life, before the development of the internal jugvilar vein, was the con- 

 tinuation of the lateral sinus, the blood from the interior of the skull at this i^eriod 

 passing through the above-mentioned foramen into the primitive jugular vein. 



3. THE VEIXS OF THE BPxAIX 



The veins of the brain present the following peculiarities: — (a) They do not 

 accompany the cerebral arteries. (6) Ascending veins do not as in other situations 

 run with descending arteries, but with ascending arteries, and vice versa, (c) The 

 deep veins do not freely communicate, (d) The veins have very thin Avails, no 

 muscular coat, and no valves, (e) The veins opening into the longitudinal, and 

 some of those opening into the lateral sinus pour in their blood in a direction 

 opposite to the current in the sinuses, so impeding the flow in both vein and sinus. 

 (/) The flow of blood in the sinuses is further retarded by the trabecular stretching 

 across their lumen, and in the longitudinal sinus b}' the blood having to ascend, 

 when the body is erect, through the anterior half of its course. 



The veins of the brain may be divided into the cerebral and the cerebellar. 



The Cerebral Veins 



The cerebral veins, like the cerebral arteries, may be divided into the cortical 

 or hemispheral, and the central or ganglionic. 



The cortical, hemispheral or superficial veins ramify on the surface of tlie 

 Itrain and return the blood from tlic cortical substance into the venous sinuses. 

 They lie for the most part in the sulci jjetween the convolutions, Init some pass 

 over the convolutions from one sulcus to another. They consist of two sets: a 

 superior and an inferior. 



(1) The superior cortical veins, some eight to twelve in number on each side, 

 are formed by the union of branches from the convex and median surfaces of the 

 cerebrum. Those from the convex surface pass forwards and inwards towards the 

 longitudinal fissure, where they are joined by the branches coming from the median 

 surface. After receiving a sheath from the arachnoid, they enter oldiquely into 

 the superior longitudinal sinus, ruiming for some distance in its walls. These veins 

 freely eonmiunicate with each other, thus difi"ering from the cortical arteries. They 

 also connnunicate with the inferior cortical veins. They may lie roughly divided 

 into (a) frontal: (7;) paracentral: (r) central; and (d ) occi|)ital. 



(2) The inferior cortical veins ramify on the base of the hemisphere and the 

 lower part of its outer surface. Those on the infi'rior surface of the frontal lobe 



