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PHYSIOLOGY 



THE CHIEF TRACTS OF THE CEREBRAL HEMISPHERES 

 We may divide the tracts of the upper brain or cerebral hemispheres into 

 three classes : 



I. Tracts connecting the brain with lower levels of the central nervous 

 system. 



II. Tracts connecting different parts of the cortex of one hemisphere 

 and serving as a means of association between these different parts. 



III. Tracts (commissural) connecting the two cerebral hemispheres 

 together. 



I. THE PROJECTION FIBRES 



These are the fibres which connect the cerebral cortex with the different 

 lower levels of the central nervous system. They form a great part of the 



FIG. 213. Schema of course of olfactory impulses. (RAM6N Y CAJAL.) 



A, olfactory mucous membrane ; B, olfactory glomeruli ; c, mitral cells ; 

 E, granule cells ; D, olfactory tract ; L, centrifugal fibres. 



fibres of the corona radiata and are condensed at the base of the brain into 

 the broad band of fibres known as the internal capsule. A few of the fibres 

 of the projection system may gain the cortex through the lenticular nucleus 

 and by the external capsule. The projection fibres may be divided into 

 two groups according as they conduct impulses to or away from the cerebral 

 cortex : the afferent or corticipetal, and the efferent or corticifugal. 



A. AFFERENT TRACTS OF THE CEREBRUM. 



(1) THE THALAMO-CORTICAL. From all parts of the optic thalamus fibres 

 arise as axons of the cells of its grey matter and streaming out from its outer 

 and under surfaces pass to every part of the cortex. Although there is no 

 division of them into distinct groups as they leave the thalamus, they are 

 often described as constituting a frontal, a parietal, an occipital, and a ventral 

 stalk. The front fibres pass through the anterior limb of the internal capsule 

 to reach the cortex of the frontal lobe, many of the fibres, however, termina- 

 ting in the caudate and lenticular nuclei. The parietal fibres issuing from 

 the lateral surface of the thalamus pass through the internal capsule to be 

 distributed chiefly to the parietal lobe. The occipital fibres issue from 



