VITAL PROPERTIES AND FUNCTIONS OF CELLS. 29 



Size of cells : The size of cells varies much. Some of the 

 bacteria are only about 1 micrornillimetre l in diameter ; these 

 are very small cells. The diameter of ordinary vegetable 

 cells ranges from 20 to 100 //; while some plant-cells are 

 fairly visible to the naked eye, nearly 1 mm. in diameter. 

 The mammalian ovum is a very large cell, about 0.2 mm. 

 (200 //) in diameter. Human red blood-corpuscles are about 

 7 or 8 IJL in diameter, and leukocytes 7 to 15 //. Human vol- 

 untary muscle-cells range from 10 to 40 fj. in diameter, and 2 

 to 4 cm. in length. The ordinary cells of human tissues 

 range from about 5 to 40 fj. in diameter. 



Vital properties and functions of cells : The study of func- 

 tion pertains especially to the science of physiology ; in his- 

 tology it is, however, necessary to pay some regard to func- 

 tions, since function and structure are intimately related, and 

 a consideration of the former throws light on the latter, as a 

 full understanding of the latter is essential to an adequate 

 understanding of the former. The function expresses the 

 meaning of the structure. 



The properties and manifestations of life and organisms in 

 general reside in the individual cells. The functions and ac- 

 tivities of cells are the same as those of life itself. These 

 activities are usually divided into three classes, nutrition, sensi- 

 bility, and motion. 



Nutrition broadly comprehends the ingestion of food-ma- 

 terial ; its elaboration and assimilation into the organic struct- 

 ure ; the rejection of waste ; the formation of the structural 

 organization ; the preserving of it intact ; development to 

 maturity ; reproduction ; decline ; the passage through a 

 definite life-cycle. 



Independent free-living cells, as the unicellular organisms, 

 leukocytes, etc., carry on all these vital operations alone 

 nourish themselves through their life-cycle, manifest irri- 

 tability, and possess the power of quasi-spontaneous move- 

 ment. The specialized cells of differentiated organisms each 

 devote their respective energies to some particular part of the 

 work of life, instead of each performing all the vital functions. 



x The linear unit employed in microscopic measurements is the micro- 

 millimetre, or micron, briefly designated by the Greek letter /*; it is J^QQ th 

 of a millimetre, or about 2 sibo tn of an 



