486 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



they act in such a way that sex is in a certain sense self-regulating — the 

 scarcity of one sex tends to the greater production of individuals of that sex ; 

 this is instanced by the fact mentioned above that after the destruction of 

 males by Mar relatively more males are born than previously. 



E. Epochs in the Physiological Life of the Individual. 



Fertilization begins, somatic death ends, the physiological life of the indi- 

 vidual. Between these two events the life- processes go on gradually, and, 

 with the exception of birth, are marked by few abrupt changes. It is some- 

 times convenient to divide the individual life into a number of successive 

 stages, as follows : the embryonic period, the fetal period, infancy, childhood, 

 youth or adolescence, maturity, and old age or senescence. Such a division, 

 however, is not physiologically exact, the stages are not sharply limited, and 

 the terms are employed in very different senses by different writers. Between 

 fertilization and birth the functions originate and are developed gradually. 

 At birth the environment of the individual is abruptly changed, organic 

 connection with the mother suddenly ceases, and profound physiological 

 changes occur. At this time, or shortly after it, the individual is capable 

 of performing all the functions of adult life with the exception of reproduc- 

 tion, the functions needing, however, to be exercised and improved before 

 they are at their best. From birth to maturity, therefore, the physiological 

 history is mainly a history of progressive modifications of function — modi- 

 fication-, indeed, of great importance, but secondary to the primary fact of 

 function itself. The same may be said of the period of old age, with the dif- 

 ference that here the modifications of function are retrogressive. In the present 

 book, devoted mainly to the physiology of the adult at the time of maturity, 

 little can be said of the origin and development of function in the embryo; 

 the modifications of function at different periods of life have been discussed in 

 connection with the various functions themselves ; certain topicsof special physio- 

 logical significance have, however, been left for brief treatment in this chapter. 



Growth of the Cells, the Tissues, and the Organs. — All growth, 

 whether of the cells, the tissues, or the organs, is the result of no more than 

 three processes, viz. multiplication of cells, enlargement of cells, and deposition 

 of intercellular substance, the first two processes being the most potent of all. 

 Increase in the number of cells is largely, although not wholly, an embryonic 

 phenomenon ; increase in the size of cells and deposition of intercellular sub- 

 stance are especially important from the later embryonic period through the 

 time of birth and up to the cessation of the body-growth. The periods of 

 growth of the several tissues differ ) in view of this it is quite impossible to 

 designate any period except that of death at which the growth of the tissues 

 wholly terminates. Detailed statistics of the growth of organs are wanting. 



Growth of the Body before Birth. — The most obvious result of growth 

 of the cells, the tissues, and the organ-, is growth or increase in size of the 

 body. Growth of the body continue- actively from the beginning of the seg- 

 mentation of the ovum up to about the age of twenty-five years, and results in 



