PRESIDENT’S ADDRESS. 33 
adopt to repel the attack, and the knowledge is now extensively utilised 
to assist our defence. For this purpose protective serums or anti- 
toxins, which have been formed in the blood of other animals, are 
employed to supplement the action of those which our own cells 
produce. It is not too much to assert that the knowledge of the 
parasitic origin of so many diseases and of the chemical agents which 
on the one hand cause, and on the other combat, their 
Parasitic symptoms, has transformed medicine from a mere art 
nature of : =e : : 
disskaen. practised empirically into a real science based upon 
experiment. The transformation has opened out an illimit- 
able vista of possibilities in the direction not only of cure, but, more 
important still, of prevention. It has taken place within the memory 
of most of us who are here present. And only last February the world 
was mourning the death of one of the greatest of its benefactors—a 
former President of this Association ?7—who, by applying this know- 
ledge to the practice of surgery, was instrumental, even in his own 
lifetime, in saving more lives than were destroyed fn all the bloody 
wars of the nineteenth century ! 
The question has been debated whether, if all accidental modes of 
destruction of the life of the cell could be eliminated, there would 
remain a possibility of individual cell-life, and even of 
aparepanee aggregate cell-life, continuing indefinitely ; in other words, 
Are the phenomena of senescence and death a natural and 
necessary sequence to the existence of life? To most of my audience 
it will appear that the subject is not open to debate. But some 
physiologists (e.g., Metchnikoff) hold that the condition of senescence 
is itself abnormal ; that old age is a form of disease or is due to disease, 
and, theoretically at least, is capable of being eliminated. We have 
already seen that individual cell-life, such as that of the white blood- 
corpuscles and of the cells of many tissues, can under suitable con- 
ditions be prolonged for days or weeks or months after general death. 
Unicellular organisms kept under suitable conditions of nutrition have 
been observed to carry on their functions normally for prolonged periods 
and to show no degeneration such as would accompany senescence. 
They give rise by division to otHers of the same kind, which also, under 
favourable conditions, continue to live, to all appearance indefinitely. 
But these instances, although they indicate that in the simplest forms 
of organisation existence may be greatly extended without signs of 
decay, do not furnish conclusive evidence of indefinite prolongation of 
life. Most of the cells which constitute the body, after a period of 
growth and activity, sometimes more, sometimes less prolonged, 
eventually undergo atrophy and cease to perform satisfactorily the 
27 Lord Lister was President at Liverpool in 1896. 
1912. 2 
