The ornamental design for a toothbrush having short inner bristles, as shown and described.
FIG. 1 is a perspective view of a toothbrush having short inner bristles showing our new design;
FIG. 2 is an enlarged detail view of the brush head thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof; and,
FIG. 7 is a top plan view thereof.
A bottom view thereof is omitted since it does not contain ornamental features. The broken lines, which define the bounds of the claimed design, form no part thereof. The circle drawn in larger-dashed broken lines in FIG. 1 illustrates the bounds of the enlarged view of FIG. 2, and forms no part of the claimed design.