The flowers of Viola are complex in longitudinal section. Compare this photograph to the line drawing in your textbooks by Zomlefer and Judd et al. Note that the lowermost petal has a nectar spur and that the 2 lowermost anthers produce nectar glands that extend into the nectar spur. Also see the next two images.
Photo taken by K. R. Robertson and D. L. Nickrent of plants cultivated at the University of Illinois.