924 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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- 

 fications, 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 topics of 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 organs, is growth or increase in size of the 

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

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

 an increase in all dimensions and in weight. In determining the extent of 

 growth, the two most convenient and most commonly used measurements are 

 those of length, or height, and weight. For the embryo the following table 

 has been compiled by Hecker : l 



Table showing the Average Length and Weight of the Human Embryo at 



Different Ages. 



Month. Length of embryo in centimeters. Weight of embryo in grams. 



Third . 4 to 9 11 



Fourth 10 to 17 57 



Fifth 18 to 27 284 



Sixth . 28 to 34 634 



Seventh 35 to 38 1218 



Eighth 39 to 41 1569 



Ninth 42 to 44 1971 



Tenth 45 to 47 2334 



1 C. Hecker : Monatsschri/t fur Oeburtskunde und Frauenkrankheiten, xxvii., 1866. 



