106 THE CHEMISTRY OF NERVES. [BOOK i. 



nervous system and of spinal nerves (of which fat by far the 

 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 

 cholesteriu 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 knowledg6 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 as yet limited. Examined under the microscope the axis-cylinder 

 gives the xanthoproteic reaction and other indications that it is 

 largely proteid in nature. From nervous matter, and especially 

 from the grey matter of the brain and spinal cord, there may by 

 appropriate methods be extracted certain proteids similar to those 

 found in leucocytes and other cells ( 29 ; namely, a nucleo-albu- 

 min and one or more globulins ; these are probably constituents 

 both of the nerve cells and of the axis-cylinders which are pro- 

 cesses of cells. Since kreatiu and a lactic acid are present among 

 the 'extractives ' of nervous tissue, we may infer that in a broad 

 way the chemical changes in nerves are similar to those in 

 muscles. Beyond this we can say very little. 



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 a supporting framework for the medulla, and whose 

 most marked characteristic is perhaps its resistance to solution. 



In the ash of nerves there is a preponderance of potassium 

 salts and phosphates but not so marked as in the case of muscle. 



67. The nervous impulse. The chemical analogy between 

 the substance of the muscle and that of the axis-cylinder would 

 naturally lead us to suppose that the progress of a nervous im- 

 pulse along a nerve fibre was accompanied by chemical changes 

 similar to those taking place in a muscle fibre. Whatever changes 

 however do or may take place are too slight to be recognized by 

 the means at our disposal. We have no satisfactory evidence 

 that in a nerve even repeated nervous impulses can give rise to 

 an acid reaction or that the death of a nerve fibre leads to such a 

 reaction. The grey matter of the central nervous system it is true 



