Mydriasis iris sphincter, composed of circularly arranged muscle fibers, and the iris dilator, composed of radially arranged muscle fibers. The sphincter is innervated by (signaled by nerves of) the parasympathetic nervous system, and the dilator by the sympathetic nervous system. Sympathetic stimulation of the adrenergic receptors causes the contraction of the radial muscle and subsequent dilation of the pupil. Conversely, parasympathetic stimulation causes contraction of the circular muscle and constriction of the pupil. The mechanism of mydriasis depends on the agent being used. It usually involves either a disruption of the parasympathetic nerve supply to the eye (which normally constricts the
Sum of normally distributed random variables more general case we find that the closest point on the line to the origin is located a (signed) distance away, so that The same argument in higher dimensions shows that if then Now we are essentially done, because So in general, if then In the event that the variables "X" and "Y" are jointly normally distributed random variables, then "X" + "Y" is still normally distributed (see Multivariate normal distribution) and the mean is the sum of the means. However, the variances are not additive due to the correlation. Indeed, where ρ is the correlation. In particular, whenever ρ