The anatomical arrangement of the autonomic pathways that innervate the vertebrate iris is relatively uniform among the groups studied. Pathways of cranial autonomic (‘parasympathetic’) origin reach the ciliary ganglion of all groups, except possibly amphibians, along the oculomotor nerve and short ciliary root, and postganglionic neurons reach the iris in the ciliary nerves. Nerve fibres from the anterior sympathetic chain ganglia enter the ciliary nerves.
The nature of the autonomic fibres controlling the iris is amazingly variable, and there are no traces of any ‘phylogenetic trends’ in the evolution of the innervation patterns. A survey of the different vertebrate groups yields examples of cholinergic, adrenergic and non-adrenergic, non-cholinergic (NANC) innervations of the sphincter and dilator muscle, that combines striated and smooth types of muscle, α- and β-adrenoceptors and muscarinic and nicotinic cholinoceptors. In addition, the iris sphincter in many fish, amphibians and reptiles, and in the embryonic chicken, 354contains a light-sensitive pigment and constricts in direct response to light.