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A TEXTBOOK OF PHYSIOLOGY 





production make no thalamic appeal. Most sensations in patient* 

 with a thalamic lesion are of painful quality. One patient, for instance, 

 could not bear the singing of hymns. It produced unpleasant sensa- 

 tions on his affected side, " and during the singing he rubbed his 

 affected hand." Another patient, during the scraping of his palm 

 on the affected side, said: "It is a horrid sensation. It feels as if 

 my hand were covered with spikes, and you were running them in. 

 It is not painful, but very unpleasant." Another nighty educated 

 patient said: " I crave to place my right hand [the affected one] on 

 the soft skin of a woman. It's my right hand that wants the con- 

 solation. I seem to crave for sympathy on my right side. My right 

 hand seems to be more artistic." 



Thalamic stimulations have a high threshold value. Stimuli of 

 low intensity arouse the sensory cortex, which is quick in reaction 

 and controls the thalamic centre. The aim of evolution is the domina- 

 tion of feeling and instinct by discriminating mental activities. The- 



forebrain 



optic uesicle 

 epiblast 

 neuroblast 

 mesoblast 

 mid-brain 



hind-brain 



FIG. 435. DIAGRAM OF THE ELEMENTS WHICH FORM THE EYEBALL. (Keith.) 



sensory cortex cerebri is an organ by which attention can be concen- 

 trated on any part of the body that is stimulated. The focus of 

 attention being arrested, the stimuli from this part are sorted out 

 in the cortex, and brought into relation with their sensory processes, 

 past or present. The thalamus is aroused by impulses of affective 

 activity. It refuses to react to those which underlie the purely dis- 

 criminative aspects of sensation. Long latency, persistent character, 

 and freedom from control mark the thalamus sensations when acting 

 by themselves. 



The Paths concerned in Vision The Visual Tract. The optic nerve is 

 an outgrowth of the brain (Fig. 436) . It consists in the main of afferent 

 fibres coming from the retina. The study of the degenerations which 

 follow its section shows that there are also some fibres running in the 

 nerve from the brain to the eye. Degenerative changes are found in 

 the other optic nerve, indicating that fibres pass from one retina to- 

 the other through the optic chiasma, where the two optic nerve* 



