The same advantage in a greater degree is obtained by vaccination, even in the exceptional instances in which it fails to render the person altogether insusceptible to the disease. The great advantage which inoculation secured was counterbalanced in great measure by the fact that it always maintained small-pox rife throughout the whole country, and that consequently all who either had neglected inoculation, or young children on whom, on account of their tender age, it had not yet been practised, were more than ever exposed to constant risk of infection. This very real danger led to the almost unanimous welcome which the practice of vaccination received towards the end of the last century, since it was hoped that by it not only would the risk attending small-pox be lessened, and the disease when it did occur be even milder in character than inoculated small-pox, but that small-pox itself would eventually be extirpated. These anticipations have not hitherto been fully realised; but the good effected by vaccination has been such as to render it, in the opinion of nearly everyone qualified to form an opinion on the subject, one of the greatest boons ever conferred on the human race. Small-pox, like other eruptive fevers, has the peculiarity of occurring for the most part only once in a person's life. We do not know in the least on what this protecting influence depends. We know the fact, but are the less able to offer an explanation, since there are other constitutional diseases, such as gout and rheumatism, in which the local symptoms are equally the outcome of previous constitutional disorder, where exactly the opposite rule obtains, and in which their occurrence does but increase the liability to their return. The protective power is apparently possessed by the mild form of the disease communicated by inoculation as much as by the severer form of small-pox which is contracted by direct contagion or infection. This knowledge has been applied in the treatment of some of the diseases of animals, and it has been found in the case of the so-called small-pox in sheep (a disease which, however, is quite distinct from human small-pox) that while one in two of the animals who contracted it in the ordinary way died, death took place in only three per cent, or not one in thirty, of those in whom it was produced by inoculation; and the inoculated sheep were thereby safeguarded from subsequent attacks as completely as the others.