420 SPECIAL PHYSIOLOGY. 



are conveyed by the optic nerve to the sensorium, and excite the sen- 

 sation of light. 



A straight line passing directly backwards, through the centre of 

 the cornea, or transparent part of the eyeball, is named its antero-pos- 

 terior, visual, or optic axis. This does not correspond with the axis of 

 the orbit, which passes obliquely backwards and inwards. The antero- 

 posterior axes of the two eyeballs are parallel when the eyes are at 

 rest, and also in certain motions. The optic tracts, on each side, arise 



Fig. 80. 



Fig. 80. Left eyeball, seen from above, with a portion of the bone at the bottom of the orbit, the left optic 

 nerve, and the optic commissure, showing some of the ocular muscles. 1, superior rectus muscle. 2, ex- 

 ternal rectus mu-cle. 3, internal rectus muscle. 4, 4. superior oblique muscle, passing through the trochlea 

 or pulley, by which the direction of its tendon is changed, before it is inserted into the eyeball, t. common 

 tendinous origin of the ocular muscles, surrounding the optic foramen, at the bottom of the orbit, g, the 

 lachrymal gland, c. the transparent coat of the eyeball or cornea. The rest of the eyeball is covered by 

 the sclerotic, oc, the optic commissure, n, the left optic nerve passing obliquely forwards, in the axis of the 

 orbit, to reach the eyeball. The an tero posterior axis of the eyeball, when at rest, is not oblique, but is di- 

 rected forwards, the axes of the two eyeballs being then parallel. 



from the optic thalami and corpora quadrigemina, and may be regarded 

 as prolongations of the cerebrum, rather than as nerves; they converge, 

 and join in the middle line, to form the optic commissure, o c, from 

 which, in front, the optic nerves are given off. These nerves diverge 

 to enter the optic foramina, t, of the orbits, where they receive a pro- 

 tecting sheath from the dura mater, processes from which pass between 

 the nervous funiculi. Each nerve, after entering the orbit, pierces the 

 sclerotic and choroid coats of the eyeball, about y^th of an inch to the 

 nasal side of, and a little below, its antero-posterior axis, and then 

 expands into the retina. 



The muscles which move the eyeball are six in number. Of these, 

 four are called straight, and two oblique. The straight, or recti, mus- 

 cles are named, respectively, the superior, 1, inferior, external, 2, and 

 internal, 3, rectus. They arise from the borders of the optic foramen, 

 where they surround the optic nerve, and pass forward, to be inserted 

 respectively into the upper, lower, outer, and inner sides of the eye- 



