600 THE BLOOD-VASCULAR SYSTEM. 



reservoirs for the blood. These are the veins of the diploe ; they Can only be 

 displayed by removing the outer table of the skull. 



In adult life, as long as the cranial bones are distinct and separable, these 

 veins are confined to the particular bones ; but in old age, when the sutures are 

 united, they communicate with each other and increase in size. These vessels 

 communicate, in the interior of the cranium, with the meningeal veins and with 

 the sinuses of the dura mater, and on the exterior of the skull with the veins of 

 the pericranium. They are divided into the frontal, which opens into the supra- 

 orbital vein by an aperture in the supra-orbital notch ; the anterior temporal, 

 which is confined chiefly to the frontal bone, and opens into one of the deep 

 temporal veins, after escaping by an aperture in the great wing of the sphenoid ; 



Frc. 32f>. Veins of the Diploe as displayed hy the removal of the outer table of the skull. 



the posterior temporal, which is confined to the parietal bone, and terminates in 

 the lateral sinus by an aperture at the posterior inferior angle of the parietal 

 bone ; and the occipital, the largest of the four, which is confined to the occipital 

 bone, and opens either into the occipital vein or internally into the lateral sinus 

 or torcular Herophili. 



The Cerebral Veins. 



The Cerebral Veins are remarkable for the extreme thinness of their coats in 

 consequence of the muscular tissue in them being wanting, and for the absence 

 of valves. They may be divided into two sets : the superficial, which are placed 

 on the surface, and the deep veins, which occupy the interior of the organ. 



The Superficial Cerebral Veins ramify upon the surface of the brain, being 

 lodged in the sulci between the convolutions, a few running across the convolu- 

 tions. They receive branches from the substance of the brain and terminate in 

 the sinuses. They are named, from the position they occupy, superior, median, 

 and inferior cerebral veins. 



The Superior Cerebral Veins, eight to twelve in number on each side, return 

 the blood from the convolutions on the superior surface of the hemisphere; they 

 pass forward and inward toward the great longitudinal fissure, where they receive 

 the median cerebral veins ; near their termination they become invested with a 

 tubular sheath of the arachnoid membrane, and open into the superior longitudi- 

 nal sinus in the opposite direction to the course of the current of the blood. 



The Median Cerebral Veins return the blood from the convolutions of the mesial 



