LIFE: ITS NATUEE, OEIGIN AND MAINTENANCE 23 



ment of all the cells composing the organism. The further course of 

 evolution produced many changes of form and ever-increasing complexity 

 of the cavity thus produced by simple invagination. Some of the cell- 

 aggregates settled down to a sedentary life, becoming plant-like in appear- 

 ance and to some extent in habit. Such organisms, complex in form but 

 simple in structure, are the Sponges. Their several parts are not, as in 

 the higher Metazoa, closely interdependent : the destruction of any one part, 

 however extensive, does not either immediately or ultimately involve 

 death of the rest: all parts function separately, although doubtless 

 mutually benefiting by their conjunction, if only by slow diffusion of 

 nutrient fluid throughout the mass. There is already some differentiation 

 in these organisms, but the absence of a nervous system prevents any 

 general co-ordination, and the individual ceils are largely independent of 

 one another. 



Our own life, like that of all the higher animals, is an aggregate life ; 

 the life of the whole is the life of the individual cells. The life of some 

 of these cells can be put an end to, the rest may continue to live. This 

 is, in fact, happening every moment of our lives. The cells which cover 

 the surface of our body, which form the scarf-skin and the hair and nails, 

 are constantly dying, and the dead cells are rubbed off or cut away, their 

 place being taken by others supplied from living layers beneath. But the 

 death of these cells does not affect the vitality of the body as a whole. 

 They serve merely as a protection or an ornamental covering, but are 

 otherwise not material to our existence. On the other hand, if a few cells, 

 such as those nerve-cells under the influence of which respiration is 

 carried on, are destroyed or injured, within a minute or two the whole 

 living machine comes to a standstill, so that to the bystander the patient is 

 dead ; even the doctor will pronounce life to be extinct. But this pro- 

 nouncement is correct only in a special sense. What has happened is 

 that, owing to the cessation of respiration, the supply of oxygen to the 

 tissues is cut off. And since the manifestations of life cease without this 

 supply, the animal or patient appears to be dead. If, however, within a 

 short period we supply the needed oxygen to the tissues requiring it, all 

 the manifestations of life reappear. 



It is only some cells which lose their vitality at the moment of < 

 so-called ' general death.' Many cells of the body retain their individual 

 life under suitable circumstances long after the rest of the body is dead. 

 Notable among these are muscle-cells. Me William showed that the 

 muscle-cells of the blood-vessels give indications of life several days after 

 an animal has been killed. The muscle-cells of the heart in mammals 

 have been revived and caused to beat regularly and strongly many hours 

 after apparent death. In man this result has been obtained as many as * 

 eighteen hours after life has been pronounced extinct (Kuliabko) ; in 

 animals after days have elapsed. Waller has shown that indications of life 

 can be elicited from various tissues many hours and even days after 



