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 distinguish 

 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 consist 

 of minute masses of a jelly-like substance, which is generally known under the 

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

 minute elementary organisms are designated unicellular. 



Not only is it true that the lowest types of life are made up of cells, but it 

 has also been shown that the tissues of which the most complex organisms are 

 composed consist of cells. 



The phenomena of life are exhibited in cells, whether existing alone or de- 

 veloped into 'he organs and tissues of animals and plants. It must be at once 

 evident that a correct knowledge of the nature and activities 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 nudeoli. Such a" definition applied admirably to most veg- 

 etable cells, but the more extended investigation of animal tissues soon showed 

 that in many cases no limiting membrane or cell-wall could be demonstrated. 



The presence or absence of a cell-wall, therefore, was then regarded as 

 quite a secondary matter, while at the same time the cell-substance came 

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