II58 



HUMAN ANATOMY. 



pole of the hemisphere, whilst those constituting the greater part of the splenium 

 are consolidated into a robust strand, the forceps posterior, which sweeps 

 abruptly backward into the occipital lobe and in its course produces a curved ridge 

 on the fore-part of the inner wall of the posterior horn of the lateral ventricle. 



The Fornix. — The fornix is an arched structure, white in color, and composed, 

 for the most part, of two crescentic tracts of longitudinally coursing nerve-fibres. 

 The two ends of these narrow crescents are free for some distance, but along their 

 medial borders the intervening parts are connected with the under surface of the cor- 

 pus callosum and with each other (Fig. 998), thus producing a triangular field, the 

 body (corpus fornicis), whose apex is directed forward and is prolonged into two 

 slender diverging stalks, the anterior pillars, and whose lateral angles are con- 

 tinued into the downwardly arching posterior pillars. The upper surface of the 

 body is subdivided into an attached and an unattached area. The former is a small 



Fig. 99S. 



Body of fornix 



Mainmillary bodie: 



Splenium of 

 corpus callosum 



Lyra 



Free margin of 

 fornix 



Under surface of 

 corpus callosum 



Cut surfaces of 

 hemisphere 



Septum lucidum 



'Anterior pillar of fornix 

 ITnder surface of genu of corpus callosum 



Dissection of brain, showing under surface of fornix and corpus callosum 



narrow triangle, the posterior and broader part of which corresponds-with the attach- 

 ment of the fornix to the under surface of the corpus callosum ; whilst the anterior 

 part is a mere mesial strip denoting the line along which the arching fornix is blended 

 with the septum lucidum, the sickle-shaped partition that fills the interval between 

 the corpus callosum and the fornix and separates the anterior horns of the lateral ven- 

 tricles. On either side of the attached field, the fornix presents a smooth and some- 

 what thicker marginal zone, which forms part of the floor of the lateral ventricle and, 

 depending upon the size and distention of the ventricular space, either extends later- 

 ally as a horizontally directed wing that overlies a part of thb thalamus, or descends 

 obliquely towards the thalamus upon whose upper surface the margin of the fornix 

 indirectly rests. The triangular central sheet of the fornix, bounded by its unattached 

 margins laterally and the splenium behind, exhibits transverse striation due to the 

 presence of bundles of commissural fibres connecting the hippocampi of the two sides. 

 This part of the fornix constitutes the commissura hippocampi, also known as the 

 psalter ium or lyra. A narrow horizontal cleft, the so-called ventricle of Verga (cavum 



