HANDBOOK OF PHYSIOLOGY, 



CHAPTER I 

 THE PHENOMENA OF LIFE 



PHYSIOLOGY is the science which treats of the various processes or changes 

 which take place in the organs and tissues of the body during life. These 

 processes, however, must not be considered as by any means peculiar to the 

 human organism, since, putting aside the properties which serve to distin- 

 guish man from other animals, the changes which go on in the tissues of man 

 go on in much the same way in the tissues of all other animals as long as they 

 live. Furthermore, it is found that similar changes proceed in all living 

 vegetable tissues; they indeed constitute what are called vital phenomena, 

 and are those properties which mark out living from non-living material. 



The lowest types of life, whether animal or vegetable, are found to con- 

 sist of minute masses of a substance generally known under the name of 

 protoplasm. Each such living mass is called a cell, so that these minute 

 elementary organisms are designated unicellular. 



Just as in the lowest types of life the morphological unit of organization 

 is the cell, so also are the most complex organisms composed of cells. The 

 tissues and organs of the higher plants and animals are great aggregates of 

 differentiated cells. The phenomena of life are exhibited in cells, whether 

 existing alone or developed into the organs and tissues of animals and plants. 

 It must be at once evident that a correct knowledge of the nature and activi- 

 ties of the cell forms the very foundation of physiology; cells being, in fact, 

 physiological no less than morphological units. 



The prime importance of the cell as an element of structure was first 

 established by the researches of the botanist Schleiden, and his conclusions, 

 drawn from the study of vegetable histology, were at once extended by Theo- 

 dor Schwann to the animal kingdom. The earlier observers defined a cell 

 as a more or less spherical body limited by a membrane, and containing a 

 smaller body termed a nucleus, which in its turn incloses one or more still 

 smaller bodies or nucleoli. Such a definition applied admirably to most vege- 

 table cells, but the more extended investigation of animal tissues soon 

 showed that in many cases no limiting membrane or cell wall could be 

 demonstrated. 



