662 DR JAMES W. DAWSON ON 



the impression that not one single vessel but the branches of a blood-vessel stem 

 are the starting-point of the process. Serial sections have frequently given definite 

 proof, as the area was followed up, that it broke up into individually distinct smaller 

 areas, which were related to the component branches of the previous central vessel. 

 These sections, again, showed the gradual fusion of such areas around the ramifica- 

 tions of a vessel, and marginal strands of myelinated fibres still left between each 

 primary focus. It must not be forgotten, however, that the whole vessel territory 

 need not be attacked, and, on the other hand, that the area of ramification of non 

 end-arteries has no sharp definition. The peri-ventricular areas, which we trace to 

 the involvement of the terminal branches of the central arteries, which spread out 

 on the ventricular surface, are a striking illustration of the implication of the area 

 of ramification of end-arteries, and an instance equally marked is the demyelination 

 of the surface layers of the cortex corresponding to the area of supply of the smaller 

 cortical vessels. 



The difficulty of understanding how a " noxa," e.g. a circulating toxin, in the 

 circulation could assert itself in the immediate area of a large vessel, for a limited 

 distance — so far as it had not to do with an actual disease of the vessel wall itself — 

 rather than in the region of the capillaries which are distributed everywhere, has 

 led numerous writers to fall back upon the assumption of a special predisposition of 

 these areas either in the form of congenital defects or acquired " loci minoris 

 resistantiae." Such a predisposition was also held to explain the frequent symmetry 

 of the areas, but this is again more readily understood if we admit, as the histo- 

 logical evidence favours, that it is the area of distribution of the vessels, which may 

 naturally be of very varying extent, that is affected. Such a view, however, 

 removes only a portion of the difficulty. If certain areas of distribution, large or 

 small, are affected, what factors determine the exact areas chosen ? For this no 

 satisfactory explanation has been suggested, but certain analogies may be given. 



In peri-axial neuritis in man, the result of chronic lead-poisoning, not only is 

 the radial nerve picked out, but certain discontinuous areas are primarily affected in 

 a manner very closely resembling the discontinuous myelin sheath degeneration in 

 disseminated sclerosis. Gombault and Stransky have also experimentally produced 

 similar discontinuous areas in subacute lead-poisoning in animals. In these cases 

 the toxin circulating in the blood is known to affect only certain irregularly 

 distributed areas. The selective action of toxi-infective agents on particular parts 

 of the nervous system may be further illustrated by the action of the virus of 

 syphilis and that of rabies ; by the effect of arsenic and that of the neurotoxin 

 formed by the diphtheria bacillus, on certain nerves ; and further, by the tetanus 

 toxin, which like strychnine, acts mainly on the synapses of the reflex arcs of the 

 brain stem and spinal cord, reducing their. resistance. 



Further, the combined degenerations found in pernicious anaemia, cancerous 

 cachexia, diabetes, and the sclerosis found in chronic ergot poisoning, pellagra, 



