life: its NATUEE, origin^ and maintenance — SCHAFER. 511 



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 plantlike in appearance 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 thi'oughout the mass. There is already 

 some differentiation in these organisms, but the absence of a nervous 

 system prevents any general coordination, and the individual cells 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 indi\ddual 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 Uves. The 

 cells which cover the surface of our body, wliich form the scarf sldn 

 and the hah' and nails, are constantly dying and the dead cells are 

 rubbed off or cut away, their place being taken by others supplied 

 from hving layers beneath. But the death of these cells does not 

 affect the vitahty 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 

 wdthout this supply, the animal or patient appears to be dead. If, 

 however, witliin 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 then' vitality at the moment of 

 so-called "general death." Many cells of the body retain their indi- 

 vidual life under suitable circumstances long after the rest of the body 

 is dead. Notable among these are muscle cells. Mc Williams showed 

 that the muscle cells of the blood vessels give indications of life sev- 

 eral 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 18 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 



