Abstract: In this paper the theoretical analysis for equivalent elastic modulus of binary system having closely arranged fibrous structure is given. The quantitative expressions were drived for equivalent elastic modulus of the system in terms of the elastic modulus of each individual component. The analysis shows that velocity of ultrasonic beam parallel to the fibers is always greater than that of perpendicular ultrasound, not depending on relative magnitude of elastic modulus of the components in the system.
The theoretical results were further used to discuss sound velocities in animal muscular tissues. According to the conception of histology, muscular tissue can be considered as a binary system including fibers and their （as well as their bundle's） membranes. Analysis has estimated that the parallel velocity appears to be about 12 m/s greater than perpendicular one. This theoretical prediction is well proved by experimental data of ours and other authors. Therefore the physical mechanism responsible to the anisotropy of ultrasonic velocity for muscular tissue has been revealed.