732 THE BRAIN. 



solution is introduced into the subarachnoid cavity at the lower end of the 

 spinal cord very little resorption takes place so long as the pressure remains 

 as low as about 10 c.c. of mercury ; as the pressure is increased beyond this 

 resorption quickly increases. But it may be doubted whether the resorptiou 

 of added fluid is a fair test of the escape of fluid naturally present ; and tli 

 experiment is of value rather as showing simply that there are means of 

 escape than as affording a measure of the rate of escape. Besides, the 

 immediate effects of applying pressure at the caudal end of the spinal cord 

 are not the same as those of applying the pressure within the skull. 



The rate of possible escape is not without importance as regards the 

 mechanical importance of the cerebro-spinal fluid. Thus it has been urged 

 that when an extra quantity of blood is driven into the skull, any injurious 

 intercranial compression is prevented, not only by the transference of a cor- 

 responding quantity of cerebro-spiual fluid through the foramen of Majendie 

 from the cranium into the spinal canal, the walls of which are less rigidly 

 complete, but also by the direct escape of the fluid from the cavity of the 

 skull along the cranial nerves in the manner described. It has also been 

 urged that the fluid at the base of the skull, in the large subarachuoid spaces 

 of which it gathers in larger quantity than elsewhere, acts as a sort of pro- 

 tective water cushion to the delicate cerebral substance, and that, in general, 

 the presence of the fluid is mechanically useful to the welfare of the brain, 

 removal of the fluid by aspiration being said to lead to hemorrhage from the 

 pia mater and to various nervous disorders. But our knowledge as to the 

 part which the fluid plays is at present very imperfect ; and its very peculiar 

 chemical characters suggest that it has some chemical functions. 



THE VASCULAR ARRANGEMENTS OF THE BRAIN AND SPINAL CORD. 



609. The bloodvessels reach the nervous structures by means of the 

 pia mater. In the spinal cord arteries coming from the vertebral, inter- 

 costal, and other arteries, and travelling along the nerve-roots join the pia 

 mater, and then through the fissures and septa reach all parts of the cord ; 

 but, as we have previously remarked, the capillary network is much denser, 

 and, therefore, the blood-supply much greater in the gray than in the white 

 matter. The veins, also gathered up along the septa and fissures into the 

 pia mater, those coming from the gray matter forming, before they reach the 

 external pia mater, a conspicuous longitudinal vein on each side of the poste- 

 rior gray commissure, pass from the pia mater to the large venous sinuses 

 of the dura mater and so to adjoining veins. 



In the brain two important features of the distribution of the arteries 

 deserve special attention. In the first place, the quadruple supply by the 

 right and left vertebral and internal carotid arteries is made one by remark- 

 able anastomoses forming the circle of Willis. The right and left vertebral 

 arteries entering the vertebral canal at the level of the sixth cervical ver- 

 tebra and running forward toward the brain, join beneath the ventral surface 

 of the bulb to form the single median basilar artery. This, after giving off 

 branches to the bulb, cerebellum, and pons, divides into the right and left 

 posterior cerebral arteries. Each internal carotid entering the skull reaches 

 the base of the brain in the region of the floor of the third ventricle, and, 

 passing ventral to and athwart the optic tract, gives off the large and impor- 

 tant middle cerebral artery along the fissure of Sylvius, and, then, turning 

 forward and toward the median line, passes dorsal to the optic nerve to end 

 in the anterior cerebral artery. Just, however, as it gives off the middle 

 artery, it sends backward, inclining to the middle line, a relatively large 



