222 HUMAN PHYSIOLOGY. 



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 toward 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 sensi- 

 bility, 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-fibers themselves are in- 

 sensitive 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 the circular space before a fixed 

 eye within which reflections of 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 embedded. As to the manner in which the objective 

 stimuli light and color, so called are transformed into nerve impulses, 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 special- 

 ized 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 impressed 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 the 

 sclerotic coat; the superior and inferior muscles rotate the eye around a hori- 

 zontal axis; the external and internal rotate it around a vertical axis. 



The superior oblique muscle, having the same origin, passes forward to the 

 inner and upper angle of the orbital cavity, where its tendon passes through 

 a cartilaginous pulley; it is then reflected backward and inserted into the 

 sclerotic just behind the transverse diameter. Its function is to rotate the 

 eyeball in such a manner as to direct the pupil downward and out-ward. 



The inferior oblique muscle arises at the inner angle of the orbit, and then 



