Pathologically rabies and tetanus (see page 168) are closely allied diseases, and the recent remarkable additions to our knowledge of the latter disease only make the similarity more evident. There are in rabies three chief sets of _post-mortem_ signs. First, and by far the most important, are the changes in the nervous system. Here we find patches of congestion in the brain, and breaking down of the axis cylinders of the nerves. The stomach, in the second place, exhibits hæmorrhagic changes, not unlike acute for arsenical poisoning. Thirdly, the salivary glands show a degenerative change in a breaking down of their secreting cells. Roux has pointed out that in life the saliva of a mad dog becomes virulent three days before the appearance of the symptoms of disease. We may now turn to the method of treatment which was introduced by Pasteur. Before his time cauterisation of the wound was the only means adopted. If more than half an hour has elapsed since the bite, cauterisation is of little or no avail. The basis for Pasteur's treatment was the difference in virulence obtainable in spinal cords infected with rabies. Pasteur found that drying the cord led to a lessening of its virulence, just as certain other conditions increased its virulence. Next he established the fact that subcutaneous injection of a weak virus, followed up with doses of ever-increasingly virulent cords, immunised dogs against infection or inoculation of fully virulent material. From this he reasoned that if he could establish a standard of weakened virulence he would have at hand the necessary "vaccine" for the treatment of the disease. [Illustration: SUSPENDED SPINAL CORD In drying jar containing Calcium Chloride]