THE CEREBKOSPINAL NERVOUS SYSTEM. 529 



Sensory Centres in the Cerebral Cortex. 



Experimental lesions of various portions of the cerebral cortex and 

 stimulation of such parts appears to show that the special senses are in 

 some way represented at definite spots in the convolutions. 



Thus (a) the visual or optic centre is localized in the occipital lobe on 

 either side on the outer convex part (Fig. 358). This has been demon- 

 strated in the dog's brain by Munk. In the human brain there seems 

 to be a very complex mechanism about this centre. The optic nerve- 

 fibres having partially decussated in the chiasma pass in the optic tract 

 to the optic thalami, and thence to the cortical substance of the occipi- 

 tal lobe. Hemianopia, restriction of the field of vision of opposite sides 

 of the two eyes, may be produced, either by a lesion of one optic tract, 

 in which are (chiefly) the crossed fibres from the nasal portion of the 

 retina of the opposite eye and the uncrossed fibres of the external por- 

 tion of the retina of the corresponding eye; or of the occipital centre. 

 Part of the fibres of the optic tract pass to the corpora geniculata and 

 to the corpora quadrigemina. Each of these so-called half-vision cen- 

 tres of opposite sides, situated in the occipital lobes, appears to be in 

 connection with a higher centre in which the retinae of both eyes are 

 represented, but especially that of the opposite eye. If both occipital 

 lobes be extensively diseased total blindness results. 



(b) The Olfactory centre, is said to be localized in the anterior ex- 

 tremity of the uncinate gyrus. The fibres, however, appear to be con- 

 nected with a centre on the same side; others cross over to a centre on 

 the opposite side. 



(c) The Auditory centre, is situated (according to Ferrier and Munk) 

 in the monkey's brain in the first temporo-sphenoidal convolution. The 

 auditory fibres pass up the pons in which they cross, and then in the 

 superior portion of the tegmentum through the hinder portion of the 

 internal capsule to this centre. Destruction of the entire region causes 

 deafness of the opposite ear. 



(d) The centre for Taste has not yet been localized. According to 

 Gowers, it is quite probable that the whole of the taste-fibres belong to 

 the fifth nerve. Those which are distributed to the anterior parts of the 

 tongue in the chorda tympani, coming from that nerve through the Vid- 

 ian, which passes from the spheno -palatine ganglion to the facial, and 

 those which are distributed to the back of the tongue through the glosso- 

 pharyngeal, being derived from the otic ganglion of the fifth nerve 

 through the small petrosal nerve and the tympanic plexus. 



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