128 PROFESSOR DAVID HEPBURN ON 



the fornix, which, although attached to the under side of the corpus callosum in the 

 mesial plane, nevertheless was spread outwards as far as the tsenia semicircularis, thus 

 forming a complete layer above the velum interpositum and the optic thalamus, of 

 which, indeed, no part was visible until the fornix was removed. 



At the hinder and outer end of the optic thalamus, the fornix was raised from 

 below by a subjacent object so that it appeared as if the fornix itself contained a 

 rounded mass of material in the position just stated. However, this underlying rounded 

 projection was the corpus geniculatum externum of the optic thalamus, which would 

 have been visible in the floor of the lateral ventricle but for its concealment by the 

 expanded overlying base of the triangular fornix. The foramen of Monro was clearly 

 denned and occupied its customary position. 



The fornix was remarkably well developed and of large size as compared with that 

 of man ; but, as in man, its body or central portion was triangular in shape and flattened 

 from above downwards. By its upper or callosal surface it was attached to the under 

 surface of the corpus callosum along a narrow mesial line which extended from Verga's 

 ventricle posteriorly to the septum lucidum in front. Elsewhere the cavity of the 

 lateral ventricle on each side extended between the corpus callosum and the fornix. 

 The lateral margins of the fornix were sharply defined and free. The deep surface of 

 the fornix rested upon the velum interpositum, but no vessels could be seen passing 

 between the two structures. The two anterior pillars of the fornix followed their 

 usual course towards the base of the brain, curving round in front of the foramen of 

 Monro. The two posterior pillars were wide like the body from which they started. 

 Each entered the descending horn of a lateral ventricle having its anterior margin 

 closely adapted to the concave margin of the hippocampus major, so as to form the 

 fimbria hippocampi in the manner already described. A closer examination of its 

 disposition now revealed a somewhat remarkable fact which had so far escaped observa- 

 tion viz. the surface of the hippocampus major, although rounded and solid in appear- 

 ance, was now found to consist for the most part of the fibres of the posterior pillar of 

 the fornix arranged somewhat like an incomplete hollow tube, within which there lay 

 concealed a much smaller ridge of grey substance representing the grey matter of the 

 hippocampus major, which became continuous with the isthmus of the limbic lobe at a 

 point below the splenium of the corpus callosum. 



It is probable, therefore, that the longitudinal fibres of the posterior pillars of the 

 fornix are distributed to the hippocampus major ; to the hippocampal gyms with its 

 uncus, as well as to the gyrus dentatus. Further, such an increase of the amount 

 of grey matter in the hippocampus major as would deepen the sulcus dentatus would 

 also probably lead to the obliteration of the fissure on the floor of the descending horn 

 and to a thinning out of the fibres of the posterior pillar of the fornix, and thus produce 

 appearances which are characteristic of the brain of man without materially increasing 

 the total size of such a hippocampus major as is presented by the brain of the seal. 



The velum interpositum was chiefly notable on account of its large choroid fringes 



(ROY. soc. EDIN. TRANS., VOL. XLVIII., 840.) 



