STRUCTURAL ARRANGEMENTS OF CEREBRUM 477 



olfactory tubercle. The primitive rhinencephalon includes in the 

 adult human brain the olfactory bulb and tract, together with the 

 anterior perforated space, the anterior part of the uncinate gyrus, 

 the subcallosal gyrus, the septum lucidum, and the hippocampal 

 convolution. The two sides of the rhinencephalon are united by fibres 

 passing through the anterior commissure. Other tracts subserving 

 this apparatus include the habenula passing from the fornix to the 

 ganglion of the habenula, the fasciculus retroflexus passing from this 

 to the interpeduncular ganglion, and the corpus mammillare which 

 is connected with the column of the fornix on the one hand and through 

 the bundle of Vicq d'Azyr with the thalamus on the other. 



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 hemi- 

 sphere and serving as a means of association between these different 

 parts. 



III. Tracts (commissural) connecting the two cerebral hemi- 

 spheres 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 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 corticipital, 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, terminating in the 

 caudate and lenticular nuclei. The parietal fibres issuing from the 

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



