The muscularis externa  of the esophagus consists of two thick layers of muscle, an inner circular layer and an outer longitudinal layer (although many of the muscle bundles may be arranged obliquely).  In the upper third of the esophagus, the muscularis externa is composed entirely of skeletal (voluntary) muscle.  In the middle third, skeletal muscle bundles are mixed with smooth muscle, the proportion of which gradually increases until only smooth muscle is found in the lower third of the organ.  Since image slide is from the upper portion of the esophagus, the muscularis externa contains only skeletal muscle.  External to the muscularis is the adventitia, which consists of fibrous connective tissue that blends into surrounding structures.