442 THE BRAIN 



ependyma. Each lateral margin is a sharp edge, from 

 under which the chorioid plexus projects into the cavity of 

 the corresponding lateral ventricle. The lower surface of 

 the body of the fornix rests upon the tela chorioidea of the 

 third ventricle (O.T. velum interpositum), a fold of pia mater 

 which separates it from the third ventricle and the two 

 thalami (Figs. 168, 187). 



Columns Fornitis (O.T. Anterior Pillars}. The two 

 columns of the fornix are two rounded strands which emerge 

 from the anterior end of the body of the fornix, and then, 

 diverging slightly, pass downwards, anterior to the inter- 

 ventricular foramina. They then sink into the grey matter 

 on the side walls of the third ventricle, and end at the 

 base of the brain in the corpora mamillaria (Figs. 160, 173). 



Each mamillary body has the appearance of being a twisted loop of 

 the corresponding column of the fornix, in which the fibres turn upon 

 themselves, and are then continued upwards and backwards into the 

 anterior tubercle of the thalamus. The appearance, however, is decep- 

 tive. In the interior of the corpus mamillare there is a nucleus of grey 

 matter. In that nucleus the fibres of the column end ; while the other 

 fibres, which seem to be continuous with the fornix fibres, take origin 

 within the nucleus. The strand, thus formed, is called the fasciculus 

 mamillo-thalamicus (O.T. bundle of Vicq d'Azyr) (Fig. 173). 



The connections which have just been described cannot be made out at 

 present, but at a later period the dissector will experience little difficulty 

 in tracing a column of the fornix to the corresponding corpus mamillare, 

 and in displaying the connection of corpus mamillare with the fasciculus 

 mamillo-thalamicus. 



Crura Fornicis (O.T. Posterior Pillars). The crura of the 

 fornix are flattened bands which diverge from the posterior 

 part of the body of the fornix. At first they are adherent 

 to the under surface of the corpus callosum, but soon they 

 sweep downwards, round the posterior ends of the thalami, 

 and enter the inferior horns of the lateral ventricles. There 

 each cms comes into relation with the corresponding hippo- 

 campus, and some of its fibres spread out on the surface 

 of that prominence, where they form the alveus, whilst the 

 remainder constitute the fimbria, which has been described 

 already (p. 436, Fig. 171). 



The transverse fibres of the fornix cross the lower surface 

 of the body and the anterior part of the interval between the 

 diverging crura. In the latter place they may be adherent 

 to the lower surface of the corpus callosum. On each side 

 they are continuous with the longitudinal fibres of the crura 



