98 ESSENTIALS OF PHYSIOLOGY. 



which can be extracted either from the choroid plexus itself or from 

 brain tissue by means of normal saline or other fluids ; injection of 

 this extract into the blood stream is followed by an increased flow of 

 cerebro-spinal fluid, which may be collected by means of a cannula. The 

 hormone contained in the extract is not destroyed by boiling and is 

 soluble in alcohol. It is not a protein, but must have a relatively large 

 molecule, since it will not pass through a Chamberland filter. 



That the fluid is not formed by filtration is shown by experiments in 

 which a fall of blood pressure is accompanied by an increased produc- 

 tion of the fluid. For example, if one vagus nerve is divided and the 

 peripheral portion stimulated, the blood pressure may fall to zero both 

 in the carotid artery and in the cranial veins, while at the same time 

 the pressure of the cerebro-spinal fluid shows a very marked rise. The 

 explanation of this rise is found in the diminished supply of oxygen 

 to the brain and the local accumulation of carbonic acid. The pressure 

 exhibits a considerable rise if the air breathed is either deficient in 

 oxygen or contains an excess of carbonic acid for example, if it contains 

 5 per cent, of the latter gas. In this connection it is interesting to notice 

 that the cerebro-spinal fluid may contain in normal circumstances 53 to 

 61 volumes per cent, of carbonic acid, 40 volumes of which are in firm 

 combination, whereas it only contains 0*1 to 0'3 volume per cent, of 

 oxygen. 



The production of the fluid is also increased, but to a less extent, 

 by injections of cholesterol, pilocarpine, or atropine, and by the adminis- 

 tration of anaesthetics. It is diminished by increased ventilation of 

 the lungs, or by an increased amount of oxygen in the air breathed. If 

 there is a sufficient supply of oxygenated blood, the pressure of the 

 cerebro-spinal fluid varies with the blood pressure ; it rises therefore 

 with the administration of adrenalin, though this substance has no effect 

 on the amount of the secretion. 



Changes in the pressure of the cerebro-spinal fluid affect the respira- 

 tory, vaso-motor, and cardio-inhibitory centres, a moderate rise of 

 pressure having a stimulating effect and a greater rise paralysing the 

 centres. When the pressure is raised to 300-400 mm. Hg, respiration 

 ceases in a few seconds, a little later the cardio-inhibitory centre is 

 paralysed, and later still the vaso-motor centre. When the pressure 

 is relieved, the vaso-motor centre recovers first and the respiratory 

 centre last. 



