110 THE CEREBRUM. 



the thalami, the third ventricle, is separated from the broader, 

 superior part of the fore-brain cavity by the body of the fornix 

 together with the chorioid tela and a layer of epithelium. The 

 body of the fornix is produced by the approximation of two 

 bundles of white fibers, one belonging to each hemisphere. These 

 bundles are the crura of the fornix. 



The cms fornicis (Figs. 29, 38 and 39) may be traced from 

 the uncus and the hippocampus, its chief origin, upward through 

 the inferior horn and into the floor of the body of the lateral ven- 

 tricle, where it unites with its fellow of the opposite side in forming 

 the body of the fornix. At the apex of the body of the fornix, 

 which is the anterior end, the bundles again separate and become 

 the columnae of the fornix. The crura are united at the back 

 part of the body of the fornix by a few transverse and obh'que 

 fibers which form the lyre, or commissura hippocampi (Fig. 36). 

 The commissure is best seen when the corpus callosum and fornix 

 are viewed from below; its fibers connect each crus of the fornix 

 with the hippocampus and uncus of the opposite side. 



The columnae fornicis (Figs. 29 and 40), one on either side 

 pass down in front of the thalami, bounding the foramina inter- 

 ventricularia (Monroi) ; and then descend to the corpora mammil- 

 laria, at the base of the brain. On the way down, each columna 

 (pars libera) passes behind the anterior commissure, beyond 

 which (pars tecta) it pierces the inner part of the thalamus of the 

 same side. The fibers of the columna fornicis for the most part 

 terminate in the medial nucleus of the corpus mammillare, from 

 which other fibers take their origin, forming the fasciculus mam- 

 millaris princeps. This bundle divides Y-like; the anterior 

 branch is the fasciculus thalamo-mammillaris (Vicq d'Azyri) 

 and ascends to the anterior nucleus of the thalamus ; the posterior 

 bundle is the tegmental part of the fasciculus pedunculo-mam- 

 millaris and probably ends in the stratum griseum centrale of 

 the mid-brain. 



At the lower border of the interventricular foramen a small 

 bundle of fibers leaves the columna of the fornix and, bending 

 backward, runs as medullary stria along the thalamus to the 

 nucleus of the habenula; some of the fibers decussate through 



