494 



PHYSIOLOGY 



be thought that, since the eye forms a ball-and-socket joint, the centre of rota- 

 tion would be at the geometrical centre of the eyeball. Careful measurement 

 shows that such is very nearly the case. The amount of rotation of the eyes 

 is considerable, being 88 degrees in a horizontal, and 80 degrees in a vertical 

 plane. If the sphericity of the globe of the eye is destroyed through disease, 

 myopia for example, then it is found that rotation is impaired. 



ANATOMY AND FUNCTION OF THE EXTERNAL MUSCLES OF 



THE EYEBALL 



The six external ocular muscles produce rotation of the eyeball ; four are 

 called recti and two oblique. The recti arise from a fibrous ring attached 

 to the margin of the optic foramen, and pass forward to meet the 

 eyeball at its equator, where they form tendons. These having passed 

 through Tenon's capsule are attached to the sclera about 6 mm. behind the 

 corneal margin. From the positions they occupy they are called superior, 

 inferior, external and internal. When they contract they will cause upward, 

 downward, outward and inward rotation of the eyeball respectively. 

 In the case of the first two muscles there is a turning movement 

 inwards at the same time ; this is due to the muscle attachment 

 round the optic foramen being on the inner side of the back of the orbit, 

 since the muscles can cause rotation only in the directions which their tendons 

 take. In addition to the above movements, and for the same reasons, there 

 is a very small amount of rotation of the eye about the visual (antero- posterior) 

 axis in the case of the superior and inferior recti, the directions in both cases 



FR.ONT VIEW 



TOP 



SUPOBL.'. 



NF. OBL. 



BOHt 



FIG. 2 !.". Tin- ;inMt<!uic;il position of the external muscles in rosp-ct 1<> I he cyrl>all. 



nliv'nuis from the directions of the pull of the muscles. Figures 

 245 and 246 show the above diagrammatically. The two oblique muscles, 

 the superior and inferior, are both smaller than the recti. The former arises 

 near the optic foramen, and passes forward to t he upper and inner side of the 

 orbit, forming on its way^a round tendon. It here passes through a narrow 

 fibrous ring, and then turns downwards and backwards under the superior 



