VITAL PROPERTIES OF THE TEXTURES. 5 



often also employed to signify a single principle, force, or agent, which 

 has been regarded as the common source of all vital properties, and the 

 common cause of all vital actions. 



As ordinan' physical forces, such as mechanical motion, heat, electricity, 

 chemical action, and the like, although differing from each other in specific 

 character and mode of operation, are nevertheless shown to be mutually con- 

 vertible and equivalent, and are lield to be but different modifications of one and 

 the same common force or " energy." so it may in like manner come to be shovm 

 that \-ital action is similarly related to the physical forces as they are related to 

 each other, and is also a manifestation, imder conditions special to the li\-ing 

 economy, of the same common energy. 



1. Assimilatory Property. — Of the vital properties, there is one 

 which is universal in its existence among organised beings, namely, the 

 property, with wliich all such beings are endowed, of converting into 

 their own substance, or " assimilating," alimentary matter. The opera- 

 tion of this power is seen in the continual renovation of the materials 

 of the body by nutrition, and in the increase and extension of the 

 organised substance, which necessarily takes place in growth and repro- 

 duction ; it manifests itself, moreover, in individual textures as well as 

 in the entire organism. It has been called the " assimilative force or 

 property," " organising force," " plastic force," and is known also by 

 various other names. But in reality the process of assimilation pro- 

 duces two different effects on the matter assimilated : first, the nutrient 

 material, previously in a liquid or amorphous condition, acquires deter- 

 minate form ; and secondly, it may, and commonly does, undergo more 

 or less change in its chemical qualities. Such being the case, it seems 

 reasonable, in the mean time, to refer these two changes to the opera- 

 tion of two distinct agencies, and, with Schwann, to reserve the name 

 of " plastic " force for that which gives to matter a definite organic 

 form ; the other, which he proposes to call " metabolic," being already 

 generally named " vital affinity." Respecting the last-named agency, 

 however, it has been long since remarked, that although the products 

 of chemical changes in living bodies for tlie most part diflFer from those 

 appearing in the inorganic world, the difference is nevertheless to be 

 ascribed, not to a peculiar or exclusively vital affinity different from 

 ordinary chemical affinity, but to common chemical affinity operating 

 in circumstances or conditions which present themselves in living 

 bodies only. 



2. Vital Contractility. — When a muscle, or a tissue containing 

 muscular fibres, is exposed in an animal during life, or soon after death, 

 and scratched with the point of a knife, it contracts or shortens itself ; 

 and the property of thus visibly contracting on the application of a 

 stimulus is named " vital contractility," or " irritability," in the 

 restricted sense of this latter term. The property in question may be 

 called into play by various other stimuli besides that of mechanical 

 irritation — especially by electricity, the sudden application of heat or 

 cold, salt, and various other chemical agents of an acrid character, and, 

 in a large class of muscles, by the exercise of the will, or by involuntary 

 mental stimuli. 



The evidence that a tissue possesses vital contractility is derived, of 

 course, from the fact of its contracting on the application of a stimulus. 



