456 REPRODUCTION AND DEVELOPMENT 



lose their small degree of pliancy, the tendons and cartilages stiffen, the 

 muscles lose part of their strength and their power of endurance because 

 of the lessened circulation and nutrition. The joints move with more or 

 less difficulty. All these conditions combine to decrease to a minimum 

 bodily movements of a voluntary sort and to lessen the activity of the 

 vegetative smooth muscles. In addition the perfection of adjustment 

 and coordination is lost. When the muscular weakness is considerable, 

 the graceful balancing of youth and middle-age at last gives place to 

 a trembling and uncertain mode of movement characteristic of very 

 advanced age and called decrepitude. 



The senses undergo changes often some time before other alterations 

 are apparent. The accommodation of normal eyes begins to lessen 

 even in infancy, and by fifty years the lens has become so rigid as to 

 necessitate the use of convex lenses in order that near objects may be 

 clearly seen. The general hardening of tendons, etc., has by sixty years 

 often checked the free adjustment-movements of the ossicles of the ears 

 and made slightly less movable the membrana tympani. The sense of 

 touch is regularly blunted and that of pain somewhat so. In general 

 the mental processes are slowed and rest becomes natural in larger pro- 

 portion than formerly, in a way corresponding to the lessened strength. 



Death. Physiologically, death is not development but the cessation of 

 development and of that "continual adjustment" and ceaseless chemical 

 and physical change in which bodily life essentially consists. Metabolism 

 stops and thereupon the former protoplasm is no longer protoplasm 

 but only animal matter liable at once to that retrograde series of changes 

 which ends in distributing the elements of the former organism into other 

 shapes, whether living or dead. 



There are two sorts of death, corresponding to the two orders of units, 

 the cell and the individual animal. .Let us look briefly at cellular death 

 first. 



The essential thing about the death of a protoplasmic cell is the cessa- 

 tion of its metabolism, because it is in that process largely that life inheres. 

 Whether the nucleus or the nucleolus or the centrosome or none of 

 these organs controls the cell's activities we do not as yet know, and hence 

 we can state nothing as to the process of its death. Pathology is that 

 branch of biology which discusses the various steps toward death to 

 which cells are liable, and to the text-books of that science the reader is 

 referred for the facts and theories as to cytological death. The important 

 thing for us here is that cell-death of one sort or another, in one organ 

 or another, is always the cause of the death of the entire individual, the 

 higher unity, just as in turn a race dies only from the death or decay of 

 the individuals composing it. When, as always happens in all animals 

 save unicells, the death of some cells precedes that of others, we have 

 the condition termed by Schultz necrobiosis (death-in-life). Poikilo- 

 thermous animals show this process best, and it is because of it that 

 frogs and turtles and batrachians generally are so useful to humanity 

 for studying some of the conditions of life. Homotherms die all over 



