248 HUMAN PHYSIOLOGY. 



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 pig- 

 mentary 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 

 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 im- 

 pressed 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 in- 

 ternal 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 horizontal axis ; the ex- 

 ternal and internal rotate it around a vertical axis. 



The superior oblique muscle, having the same origin, passes for- 

 ward 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 downzvard and outward. 



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

 and then passes outward and backward, to be inserted into the 

 sclerotic. Its function is to rotate the eyeball and to direct the 

 pupil upward and outward. 



By the associated action of all these muscles, the eyeball is capable 

 of performing all the varied and complex movements necessary for 

 distinct vision. 



The eyelids, bordered with short, stiff hairs, shade the eye and 



