168 PHYSIOLOGY OF NERVE [CH. XIV. 



of degenerated nerve-fibres show when after being hardened in 

 Miiller's fluid they are treated with Marchi's reagent, a mixture 

 of Miiller's fluid, and osmic acid. Healthy nerve-fibres are not 

 blackened by this reagent, because the more rapidly penetrating 

 chromic acid of the Miiller's fluid has already supplied the unsaturated 

 oleic acid radical in the lecithin and other phosphatides with all the 

 oxygen they can take up. But when the nerve is degenerated, the 

 oleic acid is either increased in amount, or so liberated from its 

 previous combination in the lecithin molecule, that it is then able 

 also to take oxygen from osmic acid and reduce it to a lower black 

 oxide. In the later stages of degeneration the Marchi reaction is 

 not obtained, because the fat globules have then been absorbed. 



In certain diseases of the central nervous system, such as general 

 paralysis of the insane, degeneration occurs on a large scale, and the 

 products of the chemical disintegration of the cerebral tissue have 

 been sought for in the blood, but with more profitable results in the 

 cerebro-spinal fluid. This fluid under those circumstances shows an 

 excess of protein which is mainly nucleo-protein ; cholesterin can 

 also be usually detected in the fluid, and so also can choline or some 

 similar base which originates from the decomposition of phosphatides. 

 Although many physiologists have taken up the choline question 

 and the methods for identifying this base, it must be admitted that 

 the tests hitherto devised are not absolutely conclusive, for sufficient 

 of the base cannot be collected for a complete analysis. The base 

 which is present if not choline is a nearly related substance, perhaps 

 a derivative of choline, and according to the latest researches the 

 questionable material is trimethylamine, which we have already 

 seen is a cleavage product of choline. 



Cerebro-spinal fluid. This plays the part of the lymph of the 

 central nervous system, but it differs considerably from all other 

 forms of lymph. It is a very watery fluid, containing besides some 

 inorganic salts similar to those of the blood a trace of protein matter 

 (globulin) and a small amount of sugar. Normally it contains 

 neither nucleo-protein, cholesterin, or choline, and practically no 

 cells. Colourless corpuscles, however, occur in it in inflammatory 

 conditions. There is no doubt that the fluid is a true secretion, and 

 that the cells which secrete it are the cubical epithelial cells which 

 cover the choroid plexuses. The choroid structure may indeed be 

 spoken of as the choroid gland ; only it differs from other glands in 

 having the secreting epithelium on its outer surface. Injection of an 

 extract of the choroid plexuses into the circulation causes a very 

 rapid increase in the flow of the cerebro-spinal, fluid which can be 

 collected from a cannula thrust into the subcerebellar space, or 

 into the lumbar region of the spinal canal. 



