126 



HUMAN PHYSIOLOGY 



quadrigemina, while the centre for the sixth is in the pons 

 Varolii (Fig. 86, p. 159, and Fig. 85, p. 158). The various 

 centres are joined by bands of nerve fibres which pass 

 between the sixth and fourth and third centres, and in part 

 at least cross the middle line. 



A combined mechanism, each 

 part of which acts harmoni- 

 ously with the other parts, 

 thus presides over the ocular 

 movements, and this mechanism 

 is controlled by impulses con- 

 stantly received from the two 

 retinae, from the ear and from 

 the brain. 



2. Connections of the Eyes 

 with the Central Nervous 

 System. 



From each eye the optic nerve 

 extends backwards and inwards 

 to join the other optic nerve 

 at the chiasma. From the 

 chiasma the two optic tracts 

 pass upwards round the crura 

 cerebri to end in two divisions 

 1. A posterior division passing 



internal rectus of the opposite to the anterior corpora quadri- 

 side through the nucleus of the gemma on the same side (Fig. 66 

 *"'*-" A.C.Q.). 



2. An anterior running to the geniculate body on the 

 posterior aspect of the thalamus opticus (Fig. 66, Op. Th.). 



A partial crossing of the fibres takes place in the chiasma 

 fibres from the middle and internal part of the retina 

 decussating, those from the outer part remaining on the same 

 side. For this refs%n, when the right optic tract is cut, it 

 leads to partial blindness of both retinaa on the outer part of 

 the right eye and on the inner and middle part of the left 

 eye. Thus objects on the left side of the field of vision are 

 not seen. 



The fibres of the posterior termination of the optic tract 



FIG. 65. The Nervous Mechanism 

 presiding over the combined 

 movements of the two Eyes. 

 IR. , Internal rectus ; ER. , ex- 

 ternal rectus; CO., convergent 

 centre acting on the internal 

 recti through the nuclei of the 

 third nerve; S.O., superior olive 

 (centre for lateral divergence) 

 acting on the external rectus 

 of the same side through the 

 nucleus of the sixth, and on the 



