MOVEMENTS OF THE EYEBALLS. 783 



eyeball is capable of extensive and free movement on the correspondingly excavated 

 fatty pad of the orbit, just like the head of a long bone in the corresponding socket 

 of a freely movable arthroidal joint. The movements of the eyeball, however, are 

 limited by certain conditions, by the mode in which the eye-muscles are attached 

 to it. Thus, when one muscle contracts, its antagonistic muscle acts like a bridle, 

 and so limits the movement ; the movements are also limited by the insertion of 

 the optic nerve. The soft elastic pad of the orbit on which the eyeball rests is 

 itself subject to be moved forward or backward, so that the eyeball also must 

 participate in these movements. 



Protrusion of the eyeball takes place 1. By congestion of the blood-vessels, especially of 

 the veins in the orbit, such as occurs when the overflow of the venous blood from the head is 

 interfered with, as in cases of hanging. 2. By contraction of the smooth muscular fibres in 

 Tenon's capsule, in the spheno-maxillary fissure, and in the eyelids ( 404), which are inner- 

 vated by the cervical sympathetic nerve. 3. By voluntary forced opening of the palpebral 

 fissure, whereby the pressure of the eyelids acting on the eyeball is diminished. 4. By the 

 action of the oblique muscles, which act by pulling the eyeball inwards and forwards. If the 

 superior oblique be contracted when the eyelids are forcibly opened, the eyeball may be pro- 

 truded about 1 mm. When protrusion of the eyeball occurs pathologically (as in 1 and 2), the 

 condition is called exophthalmos. 



Retraction of the eyeball is the opposite condition, and is caused 1. By closing the eye- 

 lids forcibly. 2. By an empty condition of the retrobulbar blood-vessels, diminished succulence, 

 or disappearance of the tissue of the orbit. 3. Section of the cervical sympathetic in dogs 

 causes the eyeball to sink somewhat in the orbit. The smooth muscular fibres of Tenon's 

 capsule are perhaps antagonistic in their action to the four recti when acting together, and thus 

 prevent the eyeball from being drawn too far backwards. Many animals have a special 

 retractor bulbi muscle, e.g., amphibians, reptiles, and many mammals; the ruminants have 

 four. 



The movements of the eyes are almost always accompanied by similar movements 

 of the head, chiefly on looking upwards, less so on looking laterally, and least of 

 all when looking downwards. 

 ' The difficult investigations on the movements of the eyeballs have been carried out, especially 

 by Listing, Meissner, Helmholtz, Donders, A. Fick, and E. Hering. 



Axes. All the movements of the eyeball take place round its point of rotation (fig. 566, 0), 

 which lies 177 mm. behind the centre of the visual axis, or 10*957 mm. from the vertex of the 

 cornea (Donders). In order to determine more carefully the movements of the eyeball, it is 

 necessary to have certain definite data : 1. The visual axis (S, Sj), or the antero -posterior axis 

 of the eyeball, unites the point of rotation with the fovea centralis, and is continued straight 

 forwards to the vertex of* the cornea. 2. The transverse, or horizontal axis (Q, Qj), is the 

 straight line connecting the points of rotation of both eyes and its extension outwards. Of 

 course, it is at right angles to 1. 3. The vertical axis passes vertically through the point of 

 rotation at right angles to 1 and 2. These three axes form a co-ordinate system. We must 

 imagine that in the orbit there is a fixed determinate axial system, whose point of intersection 

 corresponds with the point of rotation of the eyeball. When the eye is at rest (primary 

 position), the three axes of the eyeball completely coincide with the three axes of the co-ordinate 

 system in the orbit. When the eyeball however is moved, two or more axes are displaced from 

 this, so that they must form angles with the fixed orbital system. 



Planes of Separation. In order to be more exact, and also partly for further estimations, let 

 us suppose three planes passing through the eyeball, and that their position is secured by any 

 two axes. 1. The horizontal plane of separation divides the eyeball into an upper and lower 

 half ; it is determined by the visual transverse axis. In its course through the retina it forms 

 the horizontal line of separation of the latter ; the coats of the eyeball itself cut it in their 

 horizontal meridian. 2. The vertical plane divides the eyeball into an inner and outer half ; 

 it is determined by the visual and vertical axes. It cuts the retina in the vertical line of 

 separation of the latter and the periphery of the bulb in the vertical meridian of the eyeball. 

 3. The equatorial plane divides the eyeball into an anterior and posterior half ; its position is 

 determined by the vertical and transverse axes, and it cuts the sclerotic in the equator of the 

 eyeball. The horizontal and vertical lines of separation of the retina, which intersect in the 

 fovea centralis, divide the retina into four quadrants. 



In order to define more precisely the movements of the eyeball, v. Helmholtz has introduced 

 the following terms : He calls the straight line which connects the point of rotation of the eye 

 with the fixed point in the outer world, the visual line (" Blicklinie "), while a plane passing 

 through these lines in both eyes he called the visual plane ; the ground line of this plane is the 

 line uniting the two points of rotation, viz., the transverse axis of the eyeball. Suppose a 



