The next drug is cyclopentolate hydrochloride, which is a cycloplegic that acts on the muscle of the eye – importantly, the ciliary muscle. I have diagrams of cross-sections of the eye, for anyone who might like a copy, taken from Dr. Patrick Trevor-Roper's lecture notes on ophthalmology. It can be seen that the ciliary muscle is in the ciliary body and suspends the lens within the vitreous humour. This drug is totally different from tropicamide, which acts on the sphincter and dilator muscles of the iris, although they look similar. It is an important drug which is close in action and chemical composition to atropine and has similar side-effects. One of the problems is that these side-effects are frequently systemic.