Pupillary response is a physiological response that varies the size of the pupil,via the trigeminal cranial nerve, either resulting in constriction (miosis) or dilation (mydriasis, expansion), via relaxation or contraction of the iris dilator muscle. The response can have a variety of causes, from an involuntary reflex reaction to exposure or inexposure to light — in low light conditions a dilated pupil lets more light into the eye — or it may indicate interest in the subject of attention or indicate sexual stimulation. The pupils contract immediately before someone falls asleep. A pupillary response can be intentionally conditioned as a Pavlovian response to some stimuli.
The latency of pupillary response (the time in which it takes to occur) increases with age. Use of central nervous system stimulant drugs and some hallucinogenic drugs can cause dilation of the pupil.
Human anatomy (gr. ἀνατομία, "dissection", from ἀνά, "up", and τέμνειν, "cut") is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and microscopic anatomy. Gross anatomy (also called topographical anatomy, regional anatomy, or anthropotomy) is the study of anatomical structures that can be seen by the naked eye. Microscopic anatomy is the study of minute anatomical structures assisted with microscopes, which includes histology (the study of the organization of tissues), and cytology (the study of cells). Anatomy, human physiology (the study of function), and biochemistry (the study of the chemistry of living structures) are complementary basic medical sciences that are generally together (or in tandem) to students studying medical sciences.
In some of its facets human anatomy is closely related to embryology, comparative anatomy and comparative embryology, through common roots in evolution; for example, much of the human body maintains the ancient segmental pattern that is present in all vertebrates with basic units being repeated, which is particularly obvious in the vertebral column and in the ribcage, and can be traced from very early embryos.