456 THE INTERNAL STRUCTURE 



therefore serve to bring parts of these bodies into func« 

 tional relation with one another. 



The Posterior Commissure is a small white band 

 which passes across the upper and posterior boundary of 

 the ' third ventricle ' (fig. 164, 5), and bends downwards 

 through the Thalamus on each side so as to terminate 

 in the ganglionic matter of the Tegmentum. 



The existence of these commissural connections between 

 the Thalami are specially worthy of note, when we find 

 the two Corpora Striata quite unconnected by Commissures 

 of any kind. It is, how^ever, important that the various 

 centres in relation with * ingoing ' impressions should 

 be in functional connection with one another, while no 

 similar necessity exists for such Commissures between 

 the great superior motor ganglia — since each Corpus 

 Striatum transmits and regulates those motor incitations 

 only which emanate from its own Cerebral Hemisphere. 



b. Commissures connecting clissimilar parts in the 

 same Heniisj^h ere. — Of these that which is by far the best 

 knowai is the Fornix. This is generally spoken of as a 

 longitudinal commissure, but the term is misleading, 

 though its fibres do for the most part take a longitudinal 

 direction. They serve to bring the inner aspect of the 

 Thalamus and the Hippocampus Major of the same hemi- 

 sphere into relation with one another — these being parts 

 which are almost in the same vertical transverse plane. 

 The course and functional uses of its fibres have been 

 already indicated (p. 272). 



Two accessory sets of fibres come into relation with the ' anterior 

 pillars' of the Fornix; — (1) a narrow band of fibres on each side 

 known as the tcenia semicircular is, which, after separating from the 

 ' anterior pillar ' of the same side passes backwards in the groove 

 between the Corpus Striatum and the Thalamus and disappears 

 within the substance of the latter after tnrnin"- round to the roof 



