1. Apparatus for representation of expression in a particular tissue-like system, the apparatus comprising: a. a finite-element model of a tissue-like system implemented in a computer, the model having i. a data input for parameter values defining tissue properties for the particular system; ii. a command input for identifying an expression to be represented; and iii. a finite-element output specifying field values, associated with tissue of the system, in finite-element coordinates; b. a graphical representation generator, implemented in a computer and coupled to the finite-element output, for determining a surface of tissue of the system and providing a graphic output defining the surface in world coordinates; and c. an expressive detail generator, implemented in a computer and coupled to the finite-element output, for generating expressive detail by modifying the surface determined by the graphical representation generator, wherein the expressive detail generator employs an anisotropic model of tissue, and a wrinkle generator, the wrinkle generator including: i. an arrangement for computing a field of a strain-related quantity for each of a plurality of finite elements based on the finite-element output; and ii. an arrangement for modifying the surface of tissue as computed in the graphical representation generator in finite-element coordinates before the surface has been mapped into world coordinates; so as to produce tissue surface modifications that are not limited to effects caused by movement of, and volume changes in, any underlying musculature.