406 



PHYSIOLOGY 



nerve, say the right, will only cause loss of vision in the right eye, section 

 of the right optic tract will divide the fibres coming from the right halves of 

 both retinae. This portion of the retina in each eye is stimulated in the 

 normal position of the eyes by rays of light coming from the objects lying to 

 the left of the field of vision. Section of the right optic tract therefore causes 

 blindness to all objects to the left of the median line, left hemianopia. 

 Section of both optic tracts of course causes complete blindness. 



Every movement of the head involves compensatory movements of the 

 eyes, and conversely, in any change in the environment of the animal which 



demands its attention, there is 

 a movement of the eyes so as to 

 turn the gaze on to the origin of 

 the disturbance as an antecedent 

 to any body movement. In the 

 absence of normal regulative im- 

 pulses from the skin or from the 

 semicircular canals, the afferent 

 impressions from the eyes may 

 serve for the maintenance of 

 fairly well co-ordinated move- 

 ments a compensation which is 

 rendered possible by the power 

 of the cerebral cortex to learn 

 new reactions by experience. 



The centres for the eye move- 

 ments are contained in the grey 

 matter in the floor of the back 

 part of the third ventricle and of 

 Fio. 204. Diagram to show origin of the different the iter of Sylvius. Here we find 

 fibres of the third and fourth nerves from the tne nuc l eus o f the third or 

 oculo-motor nuclei. 



oculo-motor nerve. The oculo- 

 motor nucleus consists of several divisions, viz. a lateral part containing 

 large motor cells, a superficial median nucleus with small cells, and a 

 deeper median nucleus with large cells. By localised stimulation it has been 

 found possible to differentiate the functions of the various parts of the 

 nucleus (Fig. 204). Stimulation of the back part of the third ventricle causes 

 contraction of the ciliary muscles, and of the part immediately behind this 

 contraction of the pupil. On stimulating the floor of the iter from before 

 backwards, we obtain contractions in order of the rectus internus, the rectus 

 superior, the levator palpebrae superioris, the rectus inferior, and the inferior 

 oblique muscle. On stimulating more laterally, or exciting the corpora 

 quadrigemina, dilatation of the pupil is obtained. 



It seems probable that the optic thalamus and the closely related external 

 geniculate body are mainly concerned with the reception of visual impulses 

 and their forwarding to the cerebral cortex. On the other hand, the anterior 

 or superior corpora quadrig ^mina are mainly concerned with the co-ordination 



