LEPUS CUNICULUS 347 



ventrally and the bend is termed the genu, and then passes ventro- 

 posteriorly for a short distance as the somewhat pointed rostrum. 

 The hinder end also bends round, but only for a short distance, 

 enlarging to form a blunt mass, the spleniuni. On the lower mesial 

 wall of the ventricle, separated from the corpus callosum by the 

 septum lucidum, is the fornix. This is an elongated triangular 

 band of fibres lying in the middle line. Anteriorly it passes down- 

 wards and bifurcates to give rise to two small but distinct rounded 

 cords of white tissue which pass downwards and backwards in the 

 walls of the third ventricle to terminate in the corpus mammilare. 

 These are known as the eolumnse fornicis or anterior pillars of the 

 fornix. At the hinder end it reaches back to the splenium of the 

 corpus callosum, and there bifurcates into two coids, the posterior 

 pillars of the fornix or cruree fornicis posterior, which pass along the 

 front or concave margins of the hippocampi as the fimbriae hippo- 

 campi. The main mass of the fornix is composed of fibres running 

 in a longitudinal direction, but at the hinder end, connected with 

 the posterior pillars, there is a band of transversely running fibres 

 which serve to form a means of communication between the two 

 hippocampi. These constitute the hippocampal commissure or 

 psalterium. 



The hippocampus itself is a large crescent-shaped mass of grey 

 matter which starts as a narrow band on each, side just behind the 

 psalterium. It increases rapidly in size and passes ventrally, 

 first posteriorly, then laterally and finally anteriorly, reaching its 

 maximum size in its transverse or lateral portion and narrowing 

 again as it runs forwards. Thus it takes part in the formation of 

 the mesial, posterior and latero- ventral wall of the descending cornu. 

 As already noted, the fibres of the posterior horn of the fornix are 

 closely attached to its anterior concave border as the fimbriae. 



Before leaving the cerebral hemispheres we must consider 

 how they are connected together. The nerve cells of the cortex of 

 the two sides are put in communication with one another by three 

 different transversely running bands of fibres which are called the 

 cerebral commissures. Two of these we have already noted, viz. 

 the corpus callosum and the posterior region of the fornix. The 

 third is a small compact band of fibres known as the anterior com- 

 missure, situated at the front end of the third ventricle. In saggittal 

 section it appears as a circular area just below the anterior end of 

 the fornix, whose anterior pillars pass one on each side of it. If this 

 cord be traced by scraping away the ventrally lying grey matter it 

 will be seen to be horse-shoe shaped, its transverse portion lying just 

 above and partly in front of the optic chiasma. and its two ends pass- 

 ing outwards and forwards to terminate in the olfactory bulbs. A 



