CEREBRO-SPINAL FLUID 



993 



the arterial and venous pressures. The idea that the cranial contents 

 constitute a fixed quantity, without the power of contraction or 

 expansion, can no longer be maintained (Dixon and Halliburton). 

 A graphic record of the rate of secretion of the cerebro-spinal fluid 

 in the dog may be obtained by inserting a hollow needle through the 

 occipito-atlantoid ligament into the great subarachnoid cistern, 

 and allowing the liquid to fall upon a drop-counter writing on a 

 drum (Fig. 399). It is not always possible, however, to be certain 

 by this method that the rate at which the fluid escapes represents 



Fig. 399. Influence of Extract of the Posterior Lobe of the Pituitary upon the Flow 

 of Cerebro-Spinal Fluid through a Hollow Needle inserted into the Cisterna 

 Magna through the Occipito-Atlantoid Ligament in a Dog. The animal was 

 anaesthetized by a constant method, insufflation of ether into the trachea. The 

 uppermost curve is respiration; the next, drops of cerebro-spinal fluid; the next, 

 arterial blood pressure; the fourth, signal line showing the point at which 50 mg. 

 of a dried extract of posterior lobe was injected into a vein. The signal line is 

 also the zero of blood pressure. The bottom trace is the time in seconds (Weed 

 and Cushing). 



accurately the rate at which it is formed. A more exact method 

 appears to be the introduction of the needle into the third ventricle 

 (Fig. 400). 



Cerebro-spinal fluid can easily be obtained in man by lumbar 

 puncture with a hypodermic needle sufficiently long to enter the 

 subarachnoid space in the spinal canal. The point usually selected 

 for the puncture is between the fourth and fifth lumbar vertebrae. 

 The normal pressure of the fluid is such that it trickles out by drops, 

 but in disease it is sometimes so high that it spurts out in a steady 

 stream. An examination of the fluid, especially for leucocytes or 

 bacteria, is of great diagnostic value in certain conditions. Nor- 

 mally it is a thin, clear, watery fluid, faintly alkaline in reaction 



63 



