, ii.] THE BRAIN. ll:;;i 



therefore does riot correspond to functional divisions, for \\hil. th, 

 middle cerebral supplies a large part of th i does 



ii"t supply the whole of it, and does supply parts outbid, it. 

 Though the small arteries as they run in the pia mater on Ui 

 surface of the cortex anastomose tn-. -Iy, there i little 



:ina-tomosi> between the small arteries which leaving the pia 

 mater dip down into the substance of the brain; henc wh.-n 

 these latter arteries are blocked, the nutrition of the part of t)i 

 cortex supplied by them is apt to be impaired. 



698. The venous arrangements of the brain have very special 

 characters. 



Along the upper convex border of the sickle-shaped fold of dura 

 mater, the falx cerebri, is developed a large venous sinn 

 superior longitudinal sinus. This, triangular in section, incr- 

 in calibre from before backwards, is a sinus, not a vein; its walls 

 are formed of nothing but connective tissue lined with epithelium, 

 muscular elements being entirely absent. Though its channel is 

 broken by bridles of connective tissue passing across it, it possesses 

 no valves, and indeed these are absent from all the sinuses and 

 veins of the brain. Most of the blood returning from the cortex 

 and subjacent white matter is carried into this sinus by veins, the 

 mouths of which are for the most part directed forwards, that is to 

 say, against the direction of the blood stream. Along the lnw.-r 

 concave border of the falx is a similar sinus, the inferior longitudinal 

 sinus, which however is small and into which relatively few veins 

 open. 



From the deeper parts of the brain, and especially from the 

 choroid plexus, blood is conveyed by the veins of Galen along the 

 velum interpositum to the transverse fissure, where the veins of 

 Galen join the inferior longitudinal sinus to form the straight sinus. 

 This, running along the line formed by the intersection of the 

 vertical falx with the (more or less) horizontal tentorium, joins the 

 end of the superior longitudinal sinus to form the reservoir or 

 cellar, called the torcular Herophili, from which the lateral sinus, 

 passing on each side along the convex border of the tentorium 

 and gathering veins from the cerebellum and hind regions, as \\vll 

 as from the base of the brain, delivers the blood into the internal 

 jugular vein. 



It should be added that veins from the nose and, through the 

 ophthalmic veins, from the face join the veins and sinuses of the 

 brain, and that the so-called emissory veins pass through the 

 cranium from the scalp to the superior longitudinal and lateral 

 sinuses. 



The channels for the venous blood of the brain are therefore 

 not veins but sinuses, not so much tubes for maintaining a uniform 

 current as longitudinal reservoirs, which while affording an easy 

 onward path can also be easily filled and easily emptied, and in 

 which the blood can move to and fro without the restrictions of 



