334 CIRCULATION. 



liquid, the cephalo-rachidian fluid, which is capable of pass- 

 ing from the surface of the brain to the spinal canal, and 

 communicates with the fluid in the ventricles. 1 This he has 

 conclusively demonstrated to be situated, not between the 

 layers of the arachnoid, as was supposed by Bichat, but be- 

 tween the inner layer of this membrane and the pia mater. 

 The communication between the cranial cavity and the spinal 

 canal is very free. This was demonstrated by exposing the 

 dura mater of the brain and of the cord, making an opening 

 in the membranes of the cord, so as to allow the liquid to 

 escape (which it does in quite a forcible jet), when pressure 

 on the membranes of the brain not only accelerated the flow, 

 but pressed out a quantity of the liquid after all that would 

 escape spontaneously had been evacuated. 



It is easy to see one of the physiological uses of this liquid. 

 When the pressure of blood in the arteries leading to the 

 brain is increased, or when there is an obstacle to its return 

 by the veins, more or less congestion takes place, and the 

 blood forces the liquid from the cranial into the spinal cavity ; 

 the reverse taking place when the supply of blood to the brain 

 is diminished. The functions of all highly organized and 

 vascular parts seem to require certain variations in the sup- 

 ply of blood ; and there is no reason to suppose that the 

 brain, in its varied conditions of activity and repose, is any 

 exception to this general rule, though the physiological con- 

 ditions of its vascularity are not easily studied. 



In some late experiments by Mr. Durham on the physi- 

 ology of sleep, the comparative vascularity of the meninges 

 of the brain at different times has been studied in animals, by 

 removing a portion of the skull with a trephine, and supply- 

 ing its place by a watch-glass cemented to the edges of the 

 bone with Canada balsam. In these experiments, the author 

 demonstrates that the vessels are much more congested dur- 



1 MAGENDIE, Journal de Physiologic, 1825, tome v., p. 27 et seq. } and 1827, 

 tome vii., p. 66 et seq. Sur un Liquide qui se trouve dans le Crane et le Canal 

 Vertebral de FHomme et des Animaux, Mammifcres. 



