THE HISTOLOGY OF DISSEMINATED SCLEROSIS. 661 



which acts injuriously on the nutrition of the nerve fibres. Dixon Mann thinks 

 that disseminated sclerosis is due to an unknown autogenous toxin, which probably 

 acts by setting up changes in the ultimate vascular supply of the part affected. 



(4) The Mode of Action of the Causal Agent. 



It must have been noted that as this chapter has progressed we have got 

 deeper into the realm of theory. In discussing the nature and the origin of the 

 process there were certain definite, though restricted, data to go upon : in regard 

 to the final causal factor there were few data, but the hypothesis of a latent 

 organism or a circulating toxin rested on certain analogies : and when we come 

 to discuss the mode of action of this causal agent, and the further questions which 

 this consideration presents, it is seen that in relation to some at least of these 

 questions recourse has been taken still more to analogy. Yet it is necessary, at 

 least, to state what these questions are, and, at most, to indicate briefly any 

 reasonable explanation that has been put forward to answer them. 



The principal questions which present themselves in this section may be 

 stated thus : 



1. Why and how is the process so irregularly distributed and at first circumscribed ? 



2. What is the cause of its further advance ? 



3. What factors cause modifications in its mode of action % 



4. By what route, blood or lymph channels, is it conveyed to the nervous 



tissues ? 



1. Its Irregular Distribution and Circumscription. 



If we could trace the disease to the entry of corpuscular elements, such as bacteria, 

 or, thrombi infected by such, or emboli, there would be no further difficulty in 

 accounting for circumscribed vascular areas being irregularly attacked. If it is 

 not, however, a question of the immediate action of bacteria at an identical point, 

 but probably of toxins circulating in the blood, how is it that these agents in 

 solution in the blood choose such irregularly distributed spots 1 



From the frequent presence of a central larger vessel in the area it has been 

 assumed that this vessel, by some alteration in its walls, limited to a certain 

 definite stretch of its longitudinal course, was responsible for the sclerotic area. 

 Strumpell and Muller, who opposed the view of the role ascribed to the vessels 

 in the process, naturally found in this an argument in their favour, for they found 

 it difficult to perceive how toxins could diffuse from such a large vessel instead of 

 following the capillary area of ramification. It seems to me that Muller has missed 

 the significance of the vessels largely because he has preferred" the cord areas, which 

 do not allow these relations to be so readily recognised ; but, on the other hand, it 

 is probable, as has been frequently shown in the histological study, that it is not a 

 question of one central vessel but of many transverse and oblique vessels, giving 



TRANS. ROY. SOC. EDIN., VOL. L, PART III (NO. 18). 92 



