734 THE BRAIN. 



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

 the falx cerebri, is developed a large venous sinus, the superior longitudinal 

 sinus. This, triangular in section, increasing in calibre from before back- 

 ward, 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 forward, that is to say, against the 

 direction of the blood stream. Along the lower 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 interposi- 

 tum to the transverse fissure, where the veins of Galen join the inferior longi- 

 tudinal 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 reser- 

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

 passing on each side along the convex border of the tentorium and gather- 

 ing veins from the cerebellum and hind regions, as well 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 emissary 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 longi- 

 tudinal 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 valves. This arrangement is correlated to the 

 peculiar surroundings of the brain, which is not, like other organs, protected 

 merely by skin or other extensible or elastic tissue, but is encased by a fairly 

 complete inextensible envelope, the skull. As a consequence of this, when 

 at any time an extra quantity of blood is sent from the heart to the brain 

 room must be made for it by the increased exit of the fluids already present. 

 For any pressure on the brain-substance beyond a certain limit is injurious 

 to its welfare and activity, as is seen in certain maladies, where blood pass- 

 ing by rupture of the bloodvessels out of its normal channels remains effused 

 on the surface of the brain or elsewhere, and thus taking up the room of the 

 proper brain-substance leads, by " compression," as it is called, to paralysis, 

 loss of consciousness, or death. Some room may, as we have seen ( 608), be 

 provided by the escape of cerebro-spinal fluid from the skull. But, within 

 the limits of the normal cerebral circulation, the characteristic venous sinuses 

 especially serve to regulate the internal pressure ; they form temporary reser- 

 voirs from which a comparatively large quantity of blood can be rapidly dis- 

 charged from the cranium, the flow from the sinuses being greatly assisted 

 by the low or negative pressure obtaining in the veins of the neck at each 

 inspiratory movement of the chest. 



611. The supply of blood to the brain seems at first sight not to cor- 

 respond to the importance of this the chief organ of the body. In the rabbit 

 it would appear that hardly more than one per cent, of the total quantity of 

 the blood of the body is present at any one time in the brain, a quantity but 

 little more than half that which is found in the kidneys ; and while the 



