720 THE CEREBELLUM 



of the lumbar region, the needle being pushed inward between the 

 laminae of the vertebrae. This constitutes the procedure of lumbar 

 puncture^ which is made necessary whenever small quantities of this 

 fluid are to be obtained for chemical and histological analysis. Several 

 cases are also on record, showing that a spontaneous discharge of cere- 

 brospinal liquid may take place at times from the nasal cavity, aver- 

 aging as much as 500 c.c. in 24 hours. 



The fact that this fluid escapes from the cannula with some force 

 snows that it is held under a certain pressure, equaling 5 to 7.3 mm. Hg. 

 The flow decreases later on until only droplets appear. Various symp- 

 toms, such as vertigo, nausea and headache, result if it is allowed to 

 drain off for too long a time. It is also of interest to note that the 

 pressure of this fluid is about equal to that existing in the venous sinu- 

 ses of the cranial cavity. Thus, if salt solution is injected into the 

 subarachnoid space, it will be found to escape with relative ease, one 

 of its channels of escape being the Pacchionian bodies. It seems, 

 therefore, that these protrusions of the arachnoid membrane serve as 

 filters, allowing a quick interchange of pressure between the cerebro- 

 spinal fluid and the blood in the sinuses. A similar interchange may 

 be effected between this fluid and the lymph filling the lymphatics of 

 the nerve roots, but the resistance interposed here seems to be much 

 greater. 



The cerebrospinal fluid is commonly regarded as a true secretory 

 product (Mott) of the epithelial fining cells of the choroid plexus 

 (Luschka). Others, however, consider it as a Ijmiphatic fluid formed 

 by transudation as well as by secretion (Lewandowsky). It cannot be 

 doubted that this fluid possesses a certain independency, because bil- 

 iary pigments frequently appear in the content of the subarachnoid 

 space but not in that of the ventricles; and furthermore, the former 

 often shows certain chemical characteristics which are not displayed 

 by the latter. ^ It has also been found that its flow may be increased 

 by extract of choroid plexus.^ Its formation, however, is slow under 

 ordinary conditions, as has been shown by the experiments of Cavaz- 

 zani,^ which prove that it takes about one hour before an easily recog- 

 nizable salt injected into the general circulatory channels, may be de- 

 tected in the subarachnoid fluid. ^ In a similar way, it has been found 

 that potassium iodid injected into the encephalic arachnoid cavity, ap- 

 pears in the urine only after about twenty minutes. In view of this 

 close connection between the cerebrospinal fluid and the lymph, it 

 seems best to consider the former as being derived from three sources, 

 namely, (a) by secretion into the ventricle from the choroid plexus, (b) 



^ Quincke, Deutsche med. Wochenschrift, 1905. 



2 Thomson, The Cerebrospinal Fluid, New York, 1899. 



3 Halliburton, Proc. R. Soc, London, 1916. 

 ^ Centralbl. fiir Physiol., xiii, 14. 



^ Plant, Rehm and Schottemiiller, Leitfaden zur Unters. der Zerebrospinal- 

 fliissigkeit, Jena, 1913. 



