The weight required to produce the pressure that must be exerted to bend the hair is determined by means of a balance. If, for instance, this weight is 0.525 g and the hair is 0.07 mm thick, the pressure value of the hair tested is stronger the stimulus applied to the cornea. A series of hairs of increasing pressure value is used in order to find which produces a positive corneal reflex. In the case of a narcotic effect a hair with a fairly high pressure value will obviously produce such a reflex. Therefore, the sensitivity on the cornea will be tested with the hairs before the cannabis is given, and then compared with the sensitivity after various doses and intervals of time.
Cannabis produces a characteristic intoxication pattern in dogs.
As any quantitative estimate of the active constituents of cannabis demands the use of many animals, a method in which dogs are used will encounter serious difficulties. The method using rabbits would appear to be more promising. In testing the corneal reflex in rabbits with the irritant hairs used in the von Frey method, the sensitivity of the cornea can be determined quantitatively. In any pharmacological method used for quantitatively determining the active constituents of a drug, comparison with a standard preparation is indispensable.