46 PHYSIOI.OGY. 



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merely that of those fluids which require no change or elaboration ; 

 when elaboration is demanded, a vital action becomes necessary. 



Vital 'properties of the tissues. — The vital properties are those 

 which belong only to living organized products, and they manifest 

 themselves by a change in their molecules, resulting from a stimulus 

 applied. This change may be evident from a visible alteration in 

 the tissue stimulated, or it may show itself through a secondary in- 

 fluence exerted upon some other texture or organ, with which the 

 stimulated tissue may be in connexion. 



These properties exist in two tissues, viz., in muscle, and in nerve. 

 When seated in muscle, it is called contractility, and is characteris- 

 tic of that tissue. When in nerves, it is manifested in three ways : 

 first, by its inducing contraction in the muscle supplied by it ; se- 

 condly, by exciting contractions in muscles not supplied by it, through 

 a change effected in the nervous centre with which it is connected ; 

 thirdly, by its exciting sensation. 



The same stimuli which are capable of developing muscular con- 

 tractility, will produce these effects in nerves, and the parts well 

 supplied with nerves are said to be highly endowed with sensibility, 

 while the nerves concerned in ministering to sensation are called 

 sensitive. 



From these it will be seen that tico vital properties are described 

 by Physiologists, to wit, sensibility and contractility. The first 

 being resident in nervous tissue, the second in muscular. A vital 

 property, however, is one possessed hy every living organized being, 

 vegetable '^iS well as animal. As vegetables have no nervous system, 

 they cannot possess sensibility, and as this latter is a function of one 

 particular part of the nervous system, viz., the brain, the term irri- 

 tability or excitability will, perhaps, better express the property. 

 Plants are irritable, but not sensible, inasmuch as they have no con- 

 sciousness. Muscles, also, when separated from the body, are irri- 

 table, but not sensible. Hence irritability, excitability, contractility, 

 or incitability , as the property has been variously termed, may be 

 considered as the only vital property that is possessed by all living 

 organized beings. 



These properties are entirely dependent on the nutrition of their 

 respective tissues; they quickly vary with the state of that function, 

 and when it ceases in death, they vanish with it. 



