VITAL PROPERTIES OF THE TISSUES. 91 



growth, and nutrition; and certain ranges of temperature are subse- 

 quently essential to the excitement and maintenance of all the nutri- 

 tive processes. Food, water, oxygen, and other chemical agents, are 

 essential to the manifestation of plastic power. Light also exercises 

 a stimulating influence upon this property; and, when the organism 

 is once formed and complete, the internal stimulus of the blood, and 

 that of the animal heat, also excite and support the formative and 

 assimilative processes. Lastly, the purely nervous and mental stimuli, 

 originating in the nervous centres, likewise modify the formative and 

 assimilative processes. 



The uses of the three so-called vital properties of the animal tissues, 

 may be thus briefly summed up. The use of contractility is to pro- 

 duce all the varieties of independent motion proper to certain parts of 

 the living frame. The use of the excitability and conductility of the 

 nervous tissue, is shown in the control of the movements of the body, 

 both involuntary and voluntary, and in the various forms of sensation, 

 and their consequences, such as emotion, thought, and will. Lastly, 

 the office of the organizing power, i. e., of the conjoined assimilative 

 and plastic powers, is the formation, development, and growth, of all 

 the individual tissues, parts, and organs of the body, and their main- 

 tenance in an active living condition. 



In that condition, there occurs a ceaseless internal motion and 

 change of material, involving the constant removal of old, or used and 

 disorganized, matter, and the absorption and conversion, the assimi- 

 lation and organization of new matter, which are the great charac- 

 teristics of a living body. The cessation of these changes constitutes 

 death. 



Indeed, it is this active condition of all the parts of the body, mani- 

 fested through the exercise of the various vito-physical, vito-chemical, 

 and vital properties, called into play by external and internal stimuli, 

 which yields a total result known as "vital action" or "life." The 

 life of an individual animal is the sum of its various actions, the 

 aggregate of its vital phenomena. "Life is organization in action." 

 (Be^clard.) Sometimes, however, the term life has been understood to 

 signify "the mode of action" of living bodies. Again, it is also fre- 

 quently employed to indicate a special agent, principle, or entity, which 

 is considered to be the soured, or cause, of all the vital properties and 

 actions ; but this use of the word is, perhaps, better avoided. The 

 term "vitality" has a somewhat similar signification; but it might 

 rather be restricted to the power or capacity of living. The expres- 

 sion " vital force" indicates a still further step made by our minds in 

 the endeavor to define the causation of vital phenomena. The use of 

 the term "force," in this sense, is hardly to be avoided in physio- 

 logical discussions any more than that of the "force" of gravitation 

 in physical explanations ; but it is, unfortunately, employed in totally 

 dissimilar senses. A vital "force" is as unknown to us as the force 

 which causes gravitation or attraction; and we can only infer or 

 assume its existence, as the cause of certain properties in living things: 

 we cannot know it. Besides this, we use the term " force" very dif- 

 ferently, when we speak of the vital force of the whole system, in 



