THE NERVE-TISSUE. 371 



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 noth- 

 ing whatever is known. It is interesting to note, however, that 

 the gray matter of the brain, as also of the spinal cord and groups 

 of ganglionic cells outside of the central nervous system, always pre- 

 sent 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 

 noteworthy 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 even 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 

 diphosphates 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 

 Kiihiie. 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 axi lemma and outer sheath of the 

 medullary substance. According to some observers, moreover, it 

 forms a fine reticulated network in the latter. 



Neurokeratin is an albuminoid and 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 ravelin 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 niicloins. 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 : 



