THE FORNIX. 129 



Those from the body and the upper part of the splenium which curve over the 

 lateral ventricle form the tapetum, whilst a large mass of fibres from the 

 splenium proper curves round into each occipital lobe and is known as the forceps 

 major. 



The septum lucidum (fig. 91, sept. Inc.}, a thin double partition, separates the 

 anterior part of the two lateral ventricles. It is composed of two distinct laminae, 

 having an interval between them (ventricle of the septum) which contains fluid 

 probably of the nature of lymph. It occupies the interval between tho corpus 

 callosum above and in front, and the anterior part of the fornix behind (fig. 91) ; 

 and gradually tapers backwards to end over the middle of the third ventricle, from 

 which it is separated by the fornix and velum interpositum. Antero-inferiorly it 

 extends downwards in an angle which reaches the anterior commissure. 



The laminae of the septum are formed of a part of the mesial wall of the hemi- 

 spheres which has remained free, while the surrounding parts have become united 

 by the development of the corpus callosum above and in front and the fornix 

 below and behind. The ventricle of the septum was therefore originally a part of 

 the great longitudinal fissure, and has no connection with the internal ventricular 

 cavity of the brain. Accordingly we find that it is not lined by ciliated epithelium, 

 but each lamina consists of a thin layer of grey matter next to the cavity, and 

 homologous with the grey matter of the cerebral cortex, and a thicker layer of 

 white matter continuous below on either side with the general white matter of 

 the hemisphere. Externally, next to the lateral ventricle, is a layer of ependyma, 

 and covering this the epithelium which lines the lateral ventricle ; internally is a 

 layer of connective tissue, homologous with the pia mater. 



The fornix (fig. 91, /) is an arched longitudinal white tract consisting of two 

 lateral halves, which are separated from each other in front and behind, but in the 

 intermediate part are joined together in the median plane. The two parts in front 

 form the anterior pillars (columnce fornicis} ; the middle conjoined part is named the 

 body , and the hind parts, which are again separated from each other, form the 

 posterior pillars (crurafornicis). 



The body of the fornix is triangular in shape, being broad and flattened behind, 

 where it is adherent to the under surface of the callosum, and narrower in front, 

 where it is attached to the septum lucidum. Its lateral edges are free and are seen 

 projecting into the lateral ventricles just above the choroid plexuses. Its under 

 surface rests upon the velum interpositum, which separates it from the optic thalami 

 and the third ventricle (fig. 94). 



Beevor distinguishes (in the monkey) two sets of fibres in the body of the fomix. Of 

 these, the mesial set, when traced backwards, appear to turn dorsally and pierce the corpus 

 callosum just in front of the splenium ; above the corpus callosum they may perhaps join the 

 posterior fibres of the cingulum (see p. 158), but their actual destination has not been certainly 

 followed out. 



The anterior pillars (fig. 82, Cf}, cylindrical in form, can be traced downwards, 

 slightly separated from each other, in front of the foramina of Monro, of which 

 they form the anterior boundaries, and through the grey matter on the sides of the 

 third ventricle, curving backwards to the corpora albicantia. There each column 

 turns upon itself, making a twisted loop which forms the white portion of the 

 corpus albicans, and from this it appears to be continued, as the bundle of Vicq- 

 ffAzyr, upwards into the anterior nucleus of the optic thalamus (fig. 93). Each 

 pillar is connected near the foramen of Monro with the stria pinealis (taenia 

 fornicis), and with the taenia semicircular is, and it gives fibres to the septum 

 lucidum (for other connections of the anterior pillar, see p. 158). 



According to the experiments of Gudden and Fore! the fibres of the anterior 



