496 



PHYSIOLOGY 



associated accommodation of the lens for near objects, and at the same time 

 some contraction of the pupil. By experiments in which prisms are placed in 

 front of the eyes, thus calling for convergence or divergence without accommo- 

 dation, and by others in which lenses are placed there instead, thus requiring 

 accommodation without change in the angle between the axes, it can readily 

 be shown that the association between the functions of accommodation and 

 convergence is not very rigid. The co-ordinated deviations of the eyes appear 

 to be much more closely connected. Thus Bonders found co-ordinated 

 deviations both in the newly born and in congenital blindness. This is 

 probably due to the close anatomical relationship which exists between the 

 nerve centres of the muscles on the two sides. This relationship may be 

 explained with the help of Figure 247, which shows roughly the relative 

 positions of the various nerve centres in the central part of the Sylvian 

 grey matter at the level of the quadrigeminal bodies. 



PUPIL 

 ACCOMMODATION 



LZVATOR. PALP 

 SUP. RECT. 



EXT. RCT. 



Fi. 247. Diagram to shew relationships of different parts of oculo-motor nuclei, 

 and the principle connections between them. 



It will be seen that the third or oculo-motor nerve supplies all the external 

 eye muscles except three, viz. : Tenon's capsule, which is supplied by the sym- 

 pathetic ; the superior oblique, which is supplied by the fourth or trochlear 

 nerve ; and the external rectus, which is supplied by the sixth or abducent 

 iH'ivc. Further, while most of its nuclei supply muscles on the same side, 

 two are found to go to muscles on the opposite side, namely the internal and 

 interior recti. Another eye muscle also has a crossed connection, namely 

 the superior oblique (4th nerve). Of the many bundles of association fibres 

 which connect these different nuclei, the following may be mentioned as 

 being of special importance: (1) From the external rectus of one side 

 through the posterior longitudinal fasciculus to the internal rectus of the 

 other ; thus allowing conjugate deviation of the eyes. (2) Between the 

 nuclei of the pupil sphincter, of the mechanism for accommodation, and of the 



