CHAP, vii THE STAGES OF LIFE AND DEATH 261 



Of all the morphological and functional changes which occur 

 during the various natural periods in the course of life, the most 

 important is undoubtedly that by which man acquires the power 

 of reproduction described in Chapters IV. and V. It exercises in 

 both sexes the greatest influence on every part of the organism, 

 as well as on the generative system. Just as birth draws a line 

 of demarcation between intra-uterine and extra-uterine life, so 

 puberty separates infancy, childhood, and adolescence from youth 

 and virility or maturity, and the cessation, absolute or relative, 

 of the generative capacity, which in woman is clearly marked by 

 the menopause, separates maturity from green old age and from 

 advanced age or decrepitude which eventually ends in death. 



The greatest philosophers and physicians have treated of the 

 ages of man in their works ; we may mention amongst others 

 Aristotle in his De juventute et senectute\ Bacon in his Historia 

 vitae et mortis (1623), in which he follows closely the course 

 of human existence in its phenomena of growth and decay ; 

 Ettmiiller in his dissertation De vitae periodis (1725); G. A. 

 Testa, the physician of Ferrara, in his classic treatise entitled 

 Elementi di dinamica animate (1783); Esparron, the pupil of 

 Bichat, in his Essai sur les dges de I'homme (1803). In this 

 chapter we shall sum up as briefly as possible the most inter- 

 esting teachings of modern science on the physiology of the stages 

 of life and death. 



I. For an account of the first phases of the development of the 

 germ, which are known as embryonic life and last until the third 

 month from conception, we refer the reader to the preceding 

 chapter, and to special works on embryology for everything con- 

 cerning the genesis of the organs and the development of the 

 foetus, as the uterine product is termed from the third month of 

 gestation to birth. We shall confine ourselves to describing the 

 chief functions of intra-uterine life, which may be summed up 

 as consisting essentially of foetal circulation, respiration, and 

 nutrition, knowledge of which is of the greatest theoretical and 

 practical value. 



So long as the blood-vessels are not developed in the embryo, 

 all its vegetative functions are carried out by means of currents 

 of plasma which originate in the secretions of the uterine mucosa, 

 pass into the blastodermic vesicle, and are used for the develop- 

 ment of the tissues. After the formation of the blood-vessels of 

 the embryo, two forms of circulation may be distinguished, each 

 of which is adapted to the mode of life of the embryo and the 

 foetus, the way in which it breathes and is nourished, during two 

 different periods of development. The first circulation ie carried 

 on between the heart of the embryo and its blood-vessels, which 

 are still few in number and are united with the vascular net- 



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