MOVEMENTS OF THE EYEBALL. 719 



the sides of the globe and are inserted by short, tendinous bands into the scle- 

 rotic, at a distance of one-fourth to one-third of an inch (6-4 to 8*5 mm.) 

 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. Fro*m the pulley, or trochlea, the tendon becomes flattened, 

 passes outward and backward beneath the superior 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 rec- 

 tus and between the external rectus and the eyeball, taking a direction out- 

 ward 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 Fig. 259. 



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

 the associated movements of the muscles just enumerated, at least as far as 

 is necessary to the comprehension of the mechanism by which the eyes are 

 directed toward any particular object. The centre of exact vision is in the 

 fovea ; and it is evident that in order to see any object distinctly, it is neces- 

 sary 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, in 

 every direction, it is necessary only to note the exact mode of action of each 

 of the muscles, in order to comprehend how the different movements are 

 accomplished ; and it is sufficient for practical purposes to admit that ap- 

 proximately there is a common axis of rotation for each pair of muscles. 



Under ordinary conditions, in the human subject, the action of the six 

 ocular muscles is confined to the movements of rotation and torsion of the 

 globe. It is said that in the human subject, there is no such thing as pro- 

 trusion of the eye from general relaxation of these muscles, and that it is 

 impossible, by a combined action of the four recti muscles, to retract the 

 globe in the orbit ; but those who have operated upon the eyes assert posi- 

 tively that this statement is erroneous, and that the globe is almost always 

 suddenly and powerfully drawn within the orbit, when a painful impression 

 is made upon the cornea. This is stated as a matter of common observation 

 by ophthalmic surgeons. 



The extent to which the line of vision may be turned by a voluntary effort 

 varies in different individuals, even when the eyes are perfectly normal. In 

 myopic eyes, the centre of rotation is deeper in the orbit than normal, and 

 the extent of the possible deviation of the visual line is correspondingly di- 

 minished. Helmholtz stated that, in his own person, with the greatest effort 

 that he was capable of making, he could move the line of vision in the hori- 

 zontal plane to the extent of about fifty degrees, and in the vertical plane, about 

 forty- five degrees; but he added that these extreme rotations were very 

 forced, and that they could not be sustained for any considerable length of 



