34 LIFE: ITS NATUEE, OEIGIN AND MAINTENANCE 



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 

 diseased na ure symptoms, has transformed medicine from a mere art 



practised empirically into a real science based upon 

 experiment. The transformation has opened out an illimitable 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 * who, by applying this knowledge to the practice of 

 surgery, was instrumental, even in his own lifetime, in saving more 

 lives than were destroyed in 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 cells could be eliminated, there would 



remain a possibility of individual cell-life, and even of 

 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 

 functions which are allotted to them. And when we consider the body 

 as a whole, we find that in every case the life of the aggregate consists 

 of a definite cycle of changes which, after passing through the stages 

 of growth and maturity, always leads to senescence, and finally 

 * Lord Lister was President at Liverpool in 1896. 



