134 ESSENTIALS OF PHYSIOLOGY. 



refractive media of the eye, especially by the lens. After removal of 

 the lens for cataract, visual sensations may be excited by rays of as 

 short wave-length as 313 million ths of a millimetre. The ultra-violet 

 rays are described as actinic, because they can be detected by their 

 effect on silver salts, for example in a photographic plate. 



The ultra-red rays, on the other hand, are not absorbed by the 

 refractive media of the eye, but are invisible because they do not 

 form an adequate stimulus for the end-organs of the retina. They 

 are heat rays, and can be detected by means of a thermometer. 



The impulses excited in the retina pass to the occipital lobes of 

 the cerebral hemispheres by the tracts already described (p. 90), 

 those from the left half of each retina reaching the left occipital lobe, 

 and those from the right halves passing to the right lobe. Each tract 

 has cell stations in the thalamus and the external geniculate body. 

 The fovea of the retina is represented bilaterally in the brain. 



The impulses conveyed by the optic tracts reach the visuo-sensory 

 area of the cortex, and by means of association fibres are transmitted 

 to the visuo-psychic area. They are then passed on to the great 

 association areas, and, with the aid of the memory of previous visual, 

 tactile, and other sensations, visual judgments are formed, as regards 

 size and distance, shape, depth, and other properties of the objects 

 seen. In connection with the production of visual sensations we have 

 to consider the time required to excite a sensation, the duration of 

 the effect of a stimulus, the result of variations in the degree of 

 stimulation, the effect of adaptation, contrast and fatigue, and, finally, 

 the nature of the sensation. 



A certain interval must elapse between the application of a stimulus 

 to the rods and cones and the production of sensation. This latent 

 period has not been measured, but it has been ascertained that the 

 reaction time for sight is rather longer than that for hearing or for 

 stimulation of the skin. By reaction time is meant the time taken 

 to make a voluntary movement in response to a given stimulus. This 

 includes time taken in the receptor organ, in the afferent nerve fibres 

 with their intermediate cell stations, in the sensory and psychic areas 

 of the cortex, in the association area and the motor area, and, finally, 

 in the efferent path and in the effector organ. The reaction time for 

 sight is usually about one-fifth of a second, for hearing about one- 

 seventh, and for skin stimulation rather less. 



More definite information is available as to the duration of the 

 effect of a stimulus. If a bright electric light is looked at for a few 

 seconds and then the eyes are closed, the image of the light will persist 

 for a short time and then fade away. This is known as a positive 



