26 TEXT-BOOK OF PHYSIOLOGY 



sodium, and calcium chlorids and phosphates, are almost invariable and 

 essential constituents. 



MANIFESTATIONS OF CELL LIFE 



Growth, the Maintenance of Nutrition, and Reproduction. All 



cells exhibit three fundamental properties of life viz., growth, the mainte- 

 nance of their nutrition, and reproduction. Growth is an increase in size. 

 When newly reproduced all cells are extremely small, but in consequence of 

 their organization and the character of their surrounding medium, they 

 gradually grow until they attain the size characteristic of the adult state. 



Nutrition may be defined as the sum of the processes concerned in the 

 maintenance of the physiologic condition of the cell and includes both growth 

 and repair. So long as this is accomplished, the cells and the tissues which 

 are formed by them continue to exhibit their functions or their characteristic 

 modes of activity. Both growth and nutrition are dependent on the power 

 which living material possesses of not only absorbing nutritive material from 

 the surrounding medium, the lymph, but of subsequently assimilating it, 

 organizing it, transforming it into material like itself and endowing it with 

 its own physiologic properties. 



In the physiologic condition the living material of the cell, the bioplasm, 

 is the seat of a series of chemic changes which vary in degree from moment to 

 moment in accordance with the degree of functional activity, and on the 

 continuance of which all life phenomena depend. Some of these chemic 

 changes are related to or connected with the molecules of the living material, 

 while others are connected with the food material supplied to them. Of 

 the chemic changes occurring within the molecules some are destructive, 

 dissimilative or disintegrative in character, whereby the molecule is in part 

 eventually reduced through a series of descending chemic stages to simpler 

 compounds which, apparently of no use in the cell, are eliminated from it. 

 It is, therefore, said that the living material undergoes molecular disintegra- 

 tion as a result of functional activity. To these changes the term katabolism 

 is also applied. Other of these changes are constructive, assimilative or 

 integrative in character, whereby a part at least of the food material furnished 

 by the blood-plasma is transformed through a series of ascending chemic 

 stages into living material, and whereby it is repaired and its former physio- 

 logic condition restored. It is, therefore, said that the living material under- 

 goes molecular integration as a preparation for functional activity. To these 

 changes the term anabolism is also applied. During the course of its physio- 

 logic activities the cell bioplasm produces materials of an entirely different 

 character which vary with the cell, such as fat, glycogen, mucigen, pigments, 

 ferments, etc., which are generally spoken of as metabolic products. 



Living material has also a temperature varying in degree in different 

 species of animals as well as in different parts of the same animal. Here as 

 elsewhere the temperature is due to heat liberated from organic compounds 

 through disruption and subsequent oxidation to simpler compounds. 

 Though some of the heat liberated may come from the tissue molecules, the 

 larger part by far comes from the food molecules sugar, fat, and protein, 

 constituents of the fluids circulating in the tissue spaces. These foods carry 

 into the body potential energy, ultimately derived from the sun. When they 



