386 



PHYSIOLOGY 



of the fifth nerve, and is known as the trigemino-thalamic tract. All 

 these fibres pass up in the tegmentum of the mid-brain and finally end, partly 

 in the grey matter of the subthalamic region and partly in the grey matter of 

 the thalamus itself. To the thalamus are also continued a few fibres from the 

 lateral fillet. By this means the head ganglion of the fore-brain is in a posi- 

 tion to receive, so to speak, samples of the afferent impressions derived from 

 every sense-organ of the body. 



THE VISUAL PATHS. Two classes of afferent impressions which arrive at 



the optic thalamus are probably of 

 equal importance to all the other 

 afferent impressions taken together. 

 These are impulses derived from 

 the organs of vision and of smell. 

 The greater part of the fibres com- 

 posing the optic nerves arise as 

 axons of the ganglion-cells of the 

 retinaB. Passing backwards, the 

 nerves of the two sides join in the 

 optic chiasma, which is clost-ly 

 attached to the floor of the third 

 ventricle. After joining in the 

 chiasma the optic nerves are ap- 

 parently continued round the crura 

 cerebri as the optic tracts. These 

 pass round on each side and can be 

 seen to make connection with the 

 back part of the thalamus, the 

 external geniculate body, and the 

 superior corpus quadrigeminum. 

 Part of the tract, which is some- 

 times called the mesial root, passes 

 into the internal geniculate body. 

 This part of the tract has probably 

 nothing to do with vision and 

 forms a commissure running in 



the optic chiasma < -oimecting the internal geniculate bodies of the two 

 aides. The course of the optic fibres is shown in the diagram (Fig. 198). In 

 man and in some other mammals, e.g. dog, monkey, the nerve fibres decn>-a t e 

 incompletely in the chiasma. The uncrossed bundle is derived from the 

 outer half of the retina of the same side, whereas the crossed bundle is derived 

 from the mesial half of the retina on the other side. The right optic nerve 

 thu> carries all the impulses originating in the ri^ht eye. The right optic 

 tract carries all the impulses originating from stimuli occurring in the left 

 Held of vision. It HUM be remembered that vision in man is binocular, both 

 ivt hue being concerned in the perception of each Held of vision. The external 

 and internal geniculate bodic> may be regarded as extensions of the optic 



FlG. 198. Diagram- 

 matic representa- 

 tion of the optic 

 tracts and their 

 connections. 

 (CUNNINGHAM.) 



