vin OCULAK MOVEMENTS 397 



parallel (short distance vision of points situated to the right or 

 left of the median plane). 



The latter are accompanied by a sensation of effort. They 

 rarely occur, and are difficult to perform at will. When we 

 attempt to fixate a near object situated laterally (above, below, 

 or in the horizontal plane), we instinctively prefer to turn the head 

 to the right or left rather than the eyes, so as to avoid the sense 

 of effort associated with convergent and lateral movements, 

 particularly with those that bring the eyes into tertiary positions. 



It follows that the movements which we actually carry out 

 with our eyes are far less numerous than appears from the theo- 

 retical consideration of the action of their muscles. This restric- 

 tion of the eye-movements depends on the co-ordination of the 

 nerve-centres which innervate the muscles of the eye. It is, as we 

 shall see, of great importance in visual perception, because it 

 produces a closer and more constant relation between the retinal 

 images and the positions of the eyes. 



III. We have already seen, in speaking of the central origin 

 and the peripheral distribution of the cranial nerves (Vol. III. vii. 

 p. 411), that the motor nerves of the eye are the third, fourth, and 

 sixth cranial. The first-named (oculomotor), besides innervating 

 the levator palpebrae, the pupillary sphincter, and the ciliary 

 muscle, also supplies all the external muscles of the eye, except 

 the superior oblique which is innervated by the fourth (or 

 trochlear) nerve, and the external rectus innervated by the sixth 

 nerve (or abducens). 



These three motor nerves are exceptionally large in comparison 

 with the cross-section of the ocular muscles. The oculomotor has, 

 on an average, a cross-section of 3 sq. mm. and contains 15,000 

 nerve-fibres (Krause) ; the abducens has a cross-section of 2 sq. 

 mm. and 3600 fibres (Tergust) ; the trochlear of 04 sq. mm. and 

 2150 fibres (Merkel). 



The third and fourth cranial nerves arise from a common 

 nucleus of grey matter 5-6 mm. in length, which lies below the 

 aqueduct of Sylvius at the level of the anterior, and of the most 

 anterior part of the posterior corpora quadrigemina ; the sixth 

 nerve springs from a small nucleus, situated about the middle of 

 the sinus rhomboidalis, a little above the striae acusticae, within 

 the genu of the facial nerve (Fig.- 204, Vol. III.). 



Although anatomically undivided, the nucleus of the third and 

 fourth nerves falls, according to the clinical studies of many 

 authors, particularly the interesting experiments of Bernheimer 

 (1899-1902) upon the monkey, into a number of small nuclei, 

 which overlap more or less, and are connected with separate eye- 

 muscles. From above caudalwards these are the nuclei for the 

 levator palpebrae, superior rectus, internal rectus, inferior oblique, 

 inferior rectus, and finally the superior oblique (nucleus of the 



