Which structure is located between the choroid and root of the iris?

The ciliary body is the structure located between the choroid and the root of the iris. It plays a crucial role in the eye’s function, notably in the process of accommodation, which allows the eye to focus on objects at different distances. The ciliary body is composed of two parts: the pars plicata, which is involved in the production of aqueous humor, and the pars plana, which is more posterior. This body is essential for anchoring the iris and controlling the shape of the lens, thus impacting vision.

The other options, such as the iris sphincter and the ciliary muscle, are related to the eye's anatomy and function but serve different purposes. The iris sphincter primarily regulates the size of the pupil and controls the amount of light entering the eye. The ciliary muscle, on the other hand, is responsible for changing the shape of the lens during the accommodation process but is not located directly between the choroid and the root of the iris. The pars plicata, while part of the ciliary body, specifically refers to the anterior portion where the aqueous humor is produced, rather than the overall structure that encompasses the ciliary body itself.