EYE-MUSCLES. 191 



In a recent paper, Engelinann has shown that the cones lengthen under the 

 influence of light and shorten in darkness. This contraction of the cones and 

 pigment- epithelium is directly connected with the nervous system, as is proved 

 Ly the fact that it may occiir in eyes which are entirely shut off from the light, 

 as well as in those of decapitated animals when the brain is intact. The optic 

 nerve must thus be looked upon as being made up of both centripetal sensory 

 and centrifugal motor fibres. A reflex excitation of the cones may also be 

 produced in Frogs by allowing light to fall on any one part of the body only, 

 and the same occurs in strychnine-tetanus, quite independently of light. 



Accessory Organs in Connection with the Eye. 



(a) EYE-MUSCLES. 



The movement of the eyeball is always (except in Myxinoids) 

 effected by six muscles, which may be divided, according to the 

 direction they take, into four straight (rectus superior, in- 

 ferior, externus, and internus) and two oblique muscles 

 (obliquus superior and inferior). The former, which arise 

 from the inner portion of the orbit, usually from the dural sheath 

 of the optic nerve, together circumscribe a pyramidal cavity, the 

 apex of which lies against the inner portion of the orbit, while the 

 base surrounds the equator of the eyeball, that is, the region in 

 which the muscles are inserted into the sclerotic. 



Both the oblique muscles usually take their origin, in close 

 proximity to one another, from a point on the anterior or nasal 

 side of the orbit, and as they respectively pass from this point 

 dorsally and ventrally in an equatorial direction round the eyeball, 

 they constitute a sort of incomplete muscular ring. 



A deviation from this arrangement is seen in Mammals, in which the 

 superior oblique arises far down in the inner part of the orbit, and then 

 passes forwards in the long axis of the latter towards its anterior (internal) 

 angle, where it becomes tendinous, and passes through a nbro-cartilaginous 

 pulley (trochlea) attached to the upper border of the orbit, on the frontal 

 bone. Hence it is sometimes called the trochlear muscle. From this point it 

 changes its direction, and becomes reflected obliquely outwards and backwards 

 to the globe of the eye. 



Besides these six muscles, others are usually present which are 

 known as the retractor bulbi (which is most developed in 

 Ungulates), the quadratus (bursalis), and the pyramidalis. 

 The last two are in connection with the nictitating membrane, and 

 are present in Reptiles and Birds. All three are supplied by the 

 abducent nerve. For an account of the innervation of the straight 

 and oblique muscles, the reader is referred to the chapter on the 

 cranial nerves (p. 154). 



EYELIDS 



In Fishes the upper and lower eyelids are very rudimentary, 

 having simply the form of stiff folds of the skin ; and in all other 

 Vertebrates below the Mammalia they never reach a very high stage 



