3b NERVOUS SYSTEM. 



An object placed at the right would thus be perceived 

 solely by means of the left optic tract, if we bear in mind the 

 points of the two retinae upon which its image would be de- 

 picted (so-called theory of coincident points ; for all the 

 points of the left half of a retina, coincident points are found 

 in the left half of the other, and inversely.) We shall see 

 when studying the retina, that this explanation, originating 

 with Wollaston, loses much of its importance in the consid- 

 eration of clear or distinct vision where the two images of 

 the object would be depicted on the macula lutea (yellow 

 spot) of each eye. 



The optic nerve conveys luminous sensations towards the 

 tubercula quadrigemina. 



Motor Nerve of the Eye (3d pair, motor oculi). 

 This nerve is solely motor; it affords movement to those 

 muscles to which it is distributed, namely, to the levator pal- 

 pebraB (elevator of the eyelid), the superior internal and 

 inferior straight muscles, and to the inferior oblique ; also by 

 means of the motor rootlet which it furnishes to the ophthal- 

 mic (or lenticular) ganglion, this nerve innervates (furnishes 

 nervous power) to the muscles of the pupil (constrictor) and 

 of the choroid (apparatus for accommodation). 



Thus when this nerve is cut, or compressed by a tumor, 

 the following symptoms may be noted, and in this way, the 

 physiology of the common motor nerve of the eye may be 

 summed up, and a priori its anatomical distribution be de- 

 duced : 1st, Exophthalmia ; 2d, Closing of the upper lid ; 

 3d, External strabismus; 4th, Inability to rotate the eye 

 when the head is inclined on the opposite side of that 

 in which the lesion is situated, or, moreover, according to 

 recent researches, when the vision is directed obliquely 

 from above outwards, or to the external side of the body 

 (Donders). There is, then, diplopia with images crossed ; 

 the image furnished to the diseased side is inclined from this 

 side, and is placed higher up than the image furnished by the 

 healthy side; 5th, Dilatation of the pupil; 6th, Inability to 

 adapt the eye for short distances. 



Patheticus (4th pair, nervus trochlearis). This nerve 

 innervates the upper oblique muscle; it presides over the 

 movements of rotation and of oblique vision. When it is cut 

 or pathologically destroyed, symptoms just the opposite of 

 those we have noted as No. 4 (see above), in the paralysis of 

 the common motor are observed. 



Motor Oculi Externus (6th nerve). This nerve inner- 



