The use of the general theory of the mechanics of muscle tissue for constructing a simplified model describing the mechanical behavior of a blood vessel wall containing muscle layers is dicussed. A formalization of the concept of tonus is proposed and the activator concentration in the myofibrils is introduced as a measure of that characteristic. The properties of vessels having a static characteristic with a descending branch are considered, particular attention being given to the case where the descending branch is associated with a strong dependence of the tonus on the state of stress of the vessel wall.