164 HUMAN PHYSIOLOGY. 



the variations in size of the pupil from variations in the degree of contrac- 

 tion depend upon different intensities of light. If the light be intense the 

 circular fibres contract and diminish the size of the pupil; if the light 

 diminishes in intensity the circular fibres relax and the pupil enlarges. 



Point of most distinct Vision. While all portions of the retina are 

 sensitive to light, their sensibility varies within wide limits. At the macula 

 lutea and more especially in its most central depression, the fovea, where 

 the retinal elements are reduced practically to the layer of rods and cones, 

 the sensibility reaches its maximum. It is at this point that the image is 

 found when vision is most distinct. The macula and fovea are always in 

 the line of direct vision. From the macula towards the periphery of the 

 retina there is a gradual diminution in sensibility and a corresponding decline 

 in the distinctness of vision. In those portions of the retina lying outside 

 the macula, the indistinctness of vision depends not only on diminished 

 sensibility, but also upon inaccurate focusing of the rays. 



Blind Spot. Although the optic nerve transmits the impulses excited 

 in the retina by the ethereal vibration, the nerve fibres themselves are insen- 

 sitive to light. At the point of entrance of the optic nerve, owing to the 

 absence of the rods and cones, the rays of light make no impression. This 

 is the blind spot. As this spot is not in the line of vision, no dark point is 

 ordinarily observed in the field of vision, that circular space before a fixed 

 eye within which objects are perceptible. 



The rods and cones are the most sensitive portions of the retina. A ray 

 of light entering the eye passes entirely through the various layers of the 

 retina and is arrested only upon reaching the pigmentary epithelium in 

 which the rods and cones are imbedded. As to the manner in which the 

 objective stimuli, light and color so-called, are transformed into nerve im- 

 pulses, but little is known. It is probable that the ethereal vibrations are 

 transformed into heat, which excites the rods and cones. These acting as 

 highly specialized end organs of the optic nerve, start the impulses on their 

 way to the brain where the seeing process takes place. As to the relative 

 function of the rods and cones, it has been suggested, from the study 

 of the facts of comparative anatomy, that the rods are impressed only by 

 differences in the intensity of light, while the cones in addition are im- 

 pressed by qualitative differences or color. 



Accessory Structures. The muscles which move the eyeball are six 

 in number; the superior and inferior recti, the external and internal recti, 

 the superior and inferior oblique muscles. The four recti muscles, arising 

 from the apex of the orbit, pass forward and are inserted into the sides of 



