HAND-BOOK OF PHYSIOLOGY. 



CHAPTER I. 



THE GENERAL AND DISTINCTIVE CHARACTERS OF LIVING 



BEINGS. 



HUMAX PHYSIOLOGY is the science which treats of the \ife of man 

 of the way in which he lives, and moves, and has his being. It teaches 

 how man is begotten and born; how he attains maturity; and how he 

 dies. 



Having, then, man as the object of its study, it is unnecessary to speak 

 here of the laws of life in general, and the means by which they are car^ 

 ried out, further than is requisite for the more clear understanding of 

 those of the life of man in particular. Yet it would be impossible to 

 understand rightly the working of a complex machine without some 

 knowledge of its motive power in the simplest form; and it may be well 

 to see first what are the so-called essentials of life those, namely, which 

 are manifested by all living beings alike, by the lowest vegetable and the 

 highest animal before proceeding to the consideration of the structure 

 and endowments of the organs and tissue belonging to man. 



The essentials of life are these, Birth, Growth and Development, 

 Decline and Death. 



The term birth, when employed in this general sense of one of the 

 conditions essential to life, without reference to any particular kind of 

 living being, may be taken to mean, separation from a parent, with a 

 greater or less power of independent life. Taken thus, the term, 

 although not defining any particular stage in development, serves well 

 enough for the expression of the fact, to which no exception has yet been 

 proved to exist, that the capacity for life in all living beings is obtained 

 by inheritance. 



Growth, or inherent power of increasing in size, although essential 

 to our idea of life, is not confined to living beings. A crystal of common 

 salt, or of any other similar substance, if placed under appropriate condi- 

 VOL. I. 1. 



