460 



PHYSIOLOGY 



from the retina to the brain the optic tract contains a certain 

 number of efferent fibres which pass out and end in the retinae. 



It is evident from these connections that whereas section of one 

 optic 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 move- 

 ment 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 

 FIG. _204 -Diagram to show origin of the different fte semicircular canals, the 



fibres of the third and fourth nerves from the 



oculo-motor nuclei. afferent impressions from 



the eyes may serve for the 



maintenance of fairly well co-ordinated movements a compensa- 

 tion which is rendered possible by the power of the cerebral cortex 

 to learn new reactions by experience. 



The centres for the eye movements are contained in the grey 

 matter in the floor of the back part of the third ventricle and of the 

 iter of Sylvius. Here we find the nucleus of^the third or 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 different parts of the nucleus (Fig. 204). Stimula- 

 tion of the back part of the third ventricle causes contraction of the 

 ciliary muscles, and a little behind this contraction of the pupil 



