PRACTICAL EXERCISES 957 



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). 



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 

 to litmus, and with a specific gravity of about 1004 to 1007. It 

 contains the ordinary salts, but more potassium than sodium, unlike 

 other body fluids; a very small amount of protein (globulin) 

 usually about o-i per cent. and a little dextrose (Nawratzki). 

 Its composition is evidently different from that of ordinary lymph. 

 Only a few lymphocytes are present in health, but in some diseases 

 (as in general paralysis of the insane, tabes, and cerebro-spinal 

 syphilis) a marked increase occurs. In acute cerebro-spinal menin- 

 gitis numerous polymorphonuclear leucocytes are found, which are 

 absent from the normal fluid. 



The depression of the freezing-point (A) usually lies between 

 -0-60 and 0-65 C. In a case of hydrocephalus it was -0-65 C. 

 Normally, cerebro-spinal fluid is somewhat hypertonic to the blood- 

 serum. In injury of the cribriform plate of the ethmoid bone and 

 also in some cases where there is no traumatic injury, the fluid 

 escapes from the nose, and the rate of its formation can thus be 

 ascertained. In one case it was found to be as much as 2 c.c. to 

 nearly 4 c.c. in ten minutes. 



PRACTICAL EXERCISES ON CHAPTER XVI. 



i. Section and Stimulation of the Spinal Nerve-Roots in the Frog. (a) 

 Select a large frog (a bull-frog, if possible). Pith the brain. Fasten 

 the frog, belly down, on a plate of cork. Make an incision in the middle 

 line over the spinous processes of the lowest three or four vertebrae, 

 separate the muscles from the vertebral arches, and with strong scissors 

 open the spinal canal, taking care not to injure the cord by passing the 

 blade of the scissors too deeply. Extend the opening upwards till two 

 or three posterior roots come into view. Pass fine silk ligatures under 

 two of them, tie, and divide one root central to the ligature, the other 

 peripheral to it. Stimulate the central end, and reflex movements will 

 occur. Stimulate the peripheral end: no effect is produced. Now cut 

 away the exposed posterior roots and isolate and ligature two of the 

 anterior roots, which are smaller than the posterior. Stimulate the 

 central end of one : there is no effect. Stimulation of the peripheral end 

 of the other causes contractions of the corresponding muscles. 



(6) Stimulation of the roots may be repeated on the mammal, using 



