124 THE CHEMISTRY OF NERVES. [BOOK i. 



greater part must exist in the medulla, and form nearly the whole 

 of the medulla) is a very complex body indeed, especially so if the 

 cholesterin exists in combination with the lecithin, or cerebrin (or 

 protagon). Being so complex it is naturally very unstable, and in- 

 deed, in its instability resembles proteid matter. Hence probably 

 the reason why the medulla changes so rapidly and so profoundly 

 after the death of the nerve. It seems moreover that a certain 

 though small quantity of proteid matter forms part of the medulla, 

 and it is possible that this exists in some kind of combination with 

 the complex fat ; but our knowledge on this point is imperfect. 



The presence in such large quantity of this complex fatty 

 medulla renders the chemical examination of the other consti- 

 tuents of a nerve very difficult, and our knowledge of the chemical 

 nature of, and of the chemical changes going on in the axis-cylinder, 

 is very limited. Examined under the microscope the axis-cylinder 

 gives the xanthoproteic reaction and other indications that it is 

 proteid in nature ; beyond this we are largely confined to inferences. 

 We infer that its chemical nature is in a general way similar to that 

 of the cell-substance of the nerve cell of which it is a process. We 

 infer that the chemical nature of the cell-substance of a nerve 

 cell, being of the kind which is frequently called 'protoplasmic,' 

 is, in a general way, similar to that of other ' protoplasmic ' cells, 

 fox instance of a leucocyte. Now where we can examine con- 

 veniently such cells we find, as we have said, 30, the proteid 

 basis of the kind of cell-substance which is frequently spoken of 

 as ( undifferentiated protoplasm/ though it has certain special 

 features, resembles, in a broad way, the proteid basis of that ' dif- 

 ferentiated protoplasm/ which we have called muscle substance. 

 Hence we infer that in their broad chemical features the axis- 

 cylinder of a nerve fibre and the cell-body of a nerve cell resemble 

 the substance of a muscle fibre ; and this view is supported by the 

 fact that both kreatin and lactic acid are present as ' extractives/ 

 certainly in the central nervous system, and probably in nerves. 

 The resemblance is of course only a general one; there must be 

 differences in chemical nature between the axis-cylinder which 

 propagates a nervous impulse without change of outward form 

 and the muscle fibre which contracts ; but we cannot at present 

 state exactly what these differences really are. 



After the fats of the medulla (and the much smaller quantity 

 of fat present in the axis-cylinder), the proteids of the axis-cylinder, 

 and the other soluble substances present in one or the other, or 

 gathered round the nuclei of the neurilemma, have by various 

 means been dissolved out of a nerve fibre certain substances still 

 remain. One of these in small quantity is the nuclein of the 

 nuclei : another in larger quantity is the substance neurokeratin 

 which forms as we have seen a supporting framework for the 

 medulla, and whose most marked characteristic is perhaps its 

 resistance to solution. 



