viii GENERAL CONSIDERATIONS ON THE TEXTURES. 



" plastic force," and is known also by various other names. But in reality 

 the process of assimilation produces two different effects on the matter 

 assimilated : first, the nutrient material, previously in a liquid or amorphous 

 condition, acquires determinate 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 operation 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 the most part differ from those appear- 

 ing 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 con- 

 ditions 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 terra. 

 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. 

 Mechanical irritation, as scratching with a sharp point, or slightly pinching 

 with the forceps, electricity obtained from a piece of copper and a piece of 

 zinc, or from a larger apparatus if necessary, and the sudden application of 

 cold, are the stimuli most commonly applied. Heat, when of certain 

 intensity, is apt to cause permanent shrinking of the tissue, or "crispation," 

 as it has been called, which, though quite different in nature from vital 

 contraction, might yet be mistaken for it ; and the same may happen with 

 acids and some other chemical agents, when employed in a concentrated 

 state ; in using such stimulants, therefore, care should be taken to avoid this 

 source of deception. 



3. Vis Nervosa. The stimulus which excites contraction may be applied 

 either directly to the muscle, or to the nerves entering it, which then com- 

 municate the effect to the muscular fibre, and it is in the latter mode that 

 the voluntary or other mental stimuli are transmitted to muscles from the 

 brain. Moreover, a muscle may be excited to contract by irritation of a 

 nerve not directly connected with it. The stimulus, in this case, is first 

 conducted by the nerve irritated, to the brain or spinal cord ; it is then, 

 without participation of the will, and even without consciousness, transferred 

 to another nerve, by which it is conveyed to the muscle, and thus at length 

 excites muscular contraction. The property of nerves by which they 

 convey stimuli to muscles, whether directly, as in the case of muscular 

 nerves, or circuitously, as in the case last instanced, is named the " vis 

 nervosa. " 



4. Sensibility. We become conscious of impressions made on various 

 parts of the body, both external and internal, by the faculty of sensation ; 



