THE NERVE-TISSUE. 389 



of the nerve-fibre is essentially of an albuminous nature. They are 

 without doubt intimately concerned in the specific function of the 

 nerve-tissue, but of the part which they take in such function 

 nothing whatever is known. It is interesting to note that the gray 

 matter of the brain, as also of the spinal cord and groups of gan- 

 glionic cells outside of the central nervous system, always present 

 an acid reaction, while the white matter of the brain and cord and 

 the peripheral nerves is always neutral or slightly alkaline. The 

 substance which produces the acid reaction of the gray matter is 

 apparently the common, optically inactive lactic acid, and it is note- 

 worthy that in the nerve-tissue also a lactic acid is encountered in 

 those portions which are especially rich in albumins. But while the 

 acid reaction of muscle- tissue becomes manifest only after death, it 

 can be readily shown that in the case of the brain and spinal cord 

 this is normal during life. Whether or not other substances besides 

 lactic acid contribute to the acid reaction of the gray matter has not 

 been definitely established. But it is quite likely that this is the 

 case, as in the presence of lactic acid a transformation of diphos- 

 phates to monophosphates would of necessity occur. Bibra and 

 Miiller, moreover, claim to have obtained traces of formic acid from 

 the aqueous extract of the gray matter. Paralactic acid has not 

 been found in nerve-tissue. 



Neurokeratin. This substance, which was first isolated by 

 Kuhne, forms the greater portion of the supporting tissue of the 

 central nervous system, and is likewise found in the medullary 

 fibres, where it constitutes the axilemma and outer sheath of the 

 medullary substance. According to some observers, moreover, it 

 forms a fine reticulated network in the latter. 



Neurokeratin belongs to the group of the keratins, which are 

 found widely distributed among the tissues of epiblastic origin. In 

 the invertebrate animals, in which medullary fibres are not found 

 and chitinous substances largely enter into the composition of the 

 outer skeleton of the body, it is accordingly represented by a 

 neurochitin. 



Neurokeratin is insoluble in water, ether, alcohol, in dilute solu- 

 tions of the alkaline hydrates, in gastric juice and pancreatic juice. 

 To isolate the substance from nerve-tissue, this is accordingly ex- 

 tracted with alcohol and ether, to remove the myelin substances. 

 The remaining material is freed from albumins and other albuminoids 

 by digestion with gastric juice, and is then treated with a dilute solu- 

 tion of sodium hydrate, which dissolves the nucleins. The keratin 

 then remains. From the other keratins, which may be obtained 

 from hair, nails, horns, etc., neurokeratin differs especially in its rela- 

 tively small amount of sulphur, and the large amount of carbon and 

 hydrogen and the smaller quantity of nitrogen which it contains. 

 This is shown in the following table, which is taken from Ham- 

 marsten : 



