MOVEMENTS OF THE EYEBALL. 807 



white on a black ground. Although the actual dimensions of the two circles are iden- 

 tical, the irradiation of rays from the white circle makes this appear the larger. In a 

 circle with one half black and the other white, the white portion will appear larger, for 

 the same reason. This deception increases sensibly when we look steadily at the object. 

 These phenomena are due to what has been called by physiologists irradiation ; and their 

 explanation is very simple. It is probable that luminous impressions are never confined 

 absolutely to those parts of the retina upon which the rays of light directly impinge, but 

 that the sensitive elements immediately contiguous are always more or less involved. 

 In looking at powerfully-illuminated objects, the irradiation is considerable, as compared 

 with objects which send fewer luminous rays to the eye. 



In experiments analogous to those just described, made with strongly colored objects, 

 it has been observed that the border of irradiation takes a color complementary to that 

 of the object itself. This is particularly well marked when the objects are steadily looked 

 at for some time. Illustrations of this point also are very simple. If we looked fixedly 

 at a red spot or figure on a white ground, we soon see surrounding the red object a faint 

 areola of a pale green ; or, if the image be yellow, the areola will appear pale blue. 

 These appearances have been called accidental areolae. 



Movements of the Eyeball. 



The eyeball nearly fills the cavity of the orbit, resting, by its posterior portion, 

 upon a bed of adipose tissue, which is never absent, even in extreme emaciation. Out- 

 side of the sclerotic, is a fibrous membrane, the tunica vaginalis oculi, or capsule of 

 Tenon, which is useful in maintaining the equilibrium of the globe. This fibrous mem- 

 brane surrounds the posterior two-thirds of the globe and is loosely attached to the 

 sclerotic. It is perforated by the optic nerve posteriorly, and by the tendons of the 

 recti and oblique muscles of the eyeball in front, being reflected over these muscles. It 

 is also continuous with the palpebral ligaments and is attached by two tendinous bands 

 to the border of the orbit at the internal and the external angles of the lids. 



The muscles which move the globe are six in number for each eye. These are, the 

 external and internal recti, the superior and inferior recti, and the two oblique muscles. 

 The four recti muscles and the superior oblique arise posteriorly from the apex of the 

 orbit. The recti pass directly forward by the sides of the globe and are inserted by 

 short, tendinous bands into the sclerotic, at a distance of from one-fourth to one-third of 

 an inch from the margin of the cornea. The superior oblique, or trochlearis muscle 

 passes along the upper and inner wall of the orbit to a point near the inner angle. It 

 here presents a rounded tendon, which passes through a ring, or pulley of fibro-cartilage ; 

 and it is from this point that its action is exerted upon the globe. From the pulley, or 

 trochlea, the tendon becomes flattened, passes outward and backward beneath the supe- 

 rior rectus, and is inserted into the sclerotic, about midway between the superior and the 

 external rectus and just behind the equator of the globe. The inferior oblique muscle 

 arises just within the anterior margin of the orbit, near the inner angle of the eye, and 

 passes around the anterior portion of the globe, beneath the inferior m-tus and between 

 the external rectus and the eyeball, taking a direction outward and slightly backward. 

 Its tendon is inserted into the sclerotic, a little below the insertion of the superior 

 oblique. The general arrangement of these muscles is shown in Fi.ir. 'J.VI. 



The various movements of the eyeball are easily understood by a study of the asso- 

 ciated movements of the muscles just enumerated, at least, as far as is necessary to tlu> 

 comprehension of the mechanism by which the eyes are directed toward any ptrtkulai 

 object. We have already seen that the centre of exact vision is in the fovra : and r 

 evident that, in order to see any object distinctly, it is necessary to bring it within the 

 axes of vision of both eyes. As the globe is so balanced in the orbit as to be capable of 

 rotation, within certain limits, ki every direction, we have only to note the exact mode 



