866 OCULAR MOVEMENTS AND OCULAR MUSCLES. 



OCULAR MOVEMENTS AND OCULAR MUSCLES. 



The spherical eyeball is capable of extensive and free movement in the 

 correspondingly excavated cushion of fat in the orbit, like the head of a 

 bone in the corresponding socket of a freely movable arthrodial joint. 

 The motion is limited, in the first place, by the attachment of the muscles 

 and in such a manner that in the action of one muscle its antagonist, 

 acting like a rein, serves to limit the movement; and secondly by the 

 insertion of the optic nerve. The soft elastic orbital pad on which the 

 eyeball rests may itself be moved backward and forward, so that the 

 eyeball must follow these movements. 



Protrusion of the eyeball takes place: (i) As a result of marked distention 

 of the blood-vessels, especially of the orbital veins, when there is an obstruction 

 to the outflow of venous blood (for example in the head after execution by hang- 

 ing). (2) As a result of contraction of the unstriated muscle-fibers in Tenon's 

 capsule, in the sphenomaxillary fissure, and in the eyelids, which are innervated 

 by the cervical sympathetic. (3) As a result of voluntary, forcible opening of 

 the palpebral fissure, because the pressure of the lids from before backward is 

 diminished. (4) As a result of the action of the oblique muscles, which pull the 

 eye inward and forward. If the superior oblique is made to contract, while the 

 palpebral fissure is forcibly widened the eyeball may protrude about i mm. Patho- 

 logical protrusion of the eyeball (especially caused by 2 and i) is called exophthal- 

 mos. 



Conversely, retraction of the eyeball is caused: (i) By forcible closure of the 

 palpebral fissure. (2) By an empty condition of the retrobulbar blood-vessels, 

 diminished succulence or disappearance of the orbital tissue. (3) In dogs, section 

 of the cervical sympathetic causes recession of the eyeball. The smooth muscula- 

 ture of Tenon's capsule probably prevents the four rectus muscles from pulling 

 the eye backward unduly. Many animals possess a special retractor muscle 

 of the eyeball, for example amphibians, reptiles, and many mammals; the rumi- 

 nants have, in fact, four of them. 



The ocular movements are almost always accompanied by similar 

 movements of the head, especially in looking upward, less in looking 

 laterally and least in looking downward. 



Difficult investigations into the ocular movements have been carried out 

 especially by Listing, Meissner, v. Helmholtz, Donders, A. Pick, E. Hering and 

 others. 



Orchansky placed a closely fitting hemisphere against the eyeball, inside 

 of the conjunctival sac (with an opening cut for the pupil), and in this way 

 could observe the simple and combined movements, and also register them graphic- 

 ally by means of a writing lever. 



All movements of the eyeball take place about its center of rotation (Fig 

 302, O), which lies 1.77 mm. behind the center of the visual axis, or 10.957 mm - 

 from the vertex of the cornea. In order to study the movements more exactly, 

 certain fixed data must be determined. Three axes, intersecting at right angles 

 in . th e center of rotation, are conceived to be erected, namely: (i) The visual 

 axis (S Si) or sagittal axis of the eyeball, which connects the center of rotation 

 with the fovea centralis, and is prolonged forward in a straight line to the vertex 

 of the cornea. (2) The transverse or horizontal axis (Q QJ, being the direct pro- 

 longation outward of the line connecting the centers of rotation of the two eyes 

 (naturally at a right angle with i). (3) The vertical axis, erected perpendicularly 

 to i and 2 at the center of rotation. These three axes form a physical system of 

 coordinates. Further, a similar, but always fixed system of axes may be con- 

 ceived to be erected in the orbital cavity, the center of which coincides with the 

 center of rotation of the eyeball. In the position of rest (primary position) of 

 i eye, the three axes of the eye coincide exactly with the axes of the orbital 

 system. 11 however, the eyeball is moved, two or three of the axes cease to 

 coincide and must form angles with the fixed orbital system of axes. 



For further study, partly also for further determinations, three planes may 

 be imagined as passing through the eye, each of whose positions is determined 



