286 INNERVATION. [CHAP. X. 



vesicular matter with nerve-tubes -which, stretches from one optic 

 thalamus to the other, having no other connexion, and being free on 

 its upper as well as its under surface. This layer, thus extended 

 horizontally between the thalami, divides the third ventricle into a 

 superior and an inferior portion. As it comprises vesicular matter, 

 it is not a commissure in the same sense as the others, which 

 contain none. 



The superior longitudinal commissure is enclosed in the internal 

 convolution overhanging the corpus callosum. Posteriorly it passes 

 over the posterior border of the corpus callosum, to the under part 

 of the middle lobe, where it is chiefly connected with the hippocam- 

 pus major. Anteriorly it winds over the front border of the corpus 

 callosum to join the lower convolutions of the anterior lobe in front 

 of the fissure of Sylvius. Thus it takes a course similar to that of 

 the fornix, though more extensive and superficial. 



Thefornix or vault is the most extensive, and in every way the 

 most remarkable of the cerebral commissures. It is placed imme- 

 diately beneath the corpus callosum, with the posterior half of which 

 it is intimately connected, and from which it is with difficulty sepa- 

 rated. A principal portion of the fornix consists of a horizontal 

 lamella of fibrous matter, parallel to the corpus callosum, of a trian- 

 gular shape, with the apex forwards (corpus for nicis). The base is 

 enclosed by the posterior reflection of the corpus callosum, the ter- 

 minal transverse fibres of which are seen on its interior surface, 

 forming the appearance which has been designated lyra. 



The fornix may be divided along the middle line into two equal 

 and symmetrical portions, one belonging to each hemisphere. Suf- 

 ficient indication of its double form is evinced by the prolongation 

 from its apex of two cylindrical cords, which curve forwards and 

 downwards, then backwards, with their convexities touching the 

 posterior border of the anterior commissure. These are the ante- 

 rior pillars of the fornix. In their descent they diverge slightly 

 from each other, leaving an interval between them, through which 

 the anterior commissure appears. These pillars form the anterior 

 boundary of the foramen commune anterius, through which the 

 lateral ventricles communicate with the third, and with each other. 



Each anterior pillar of the fornix in its descent penetrates the 

 anterior and inner part of the optic thalamus. Here it is sur- 

 rounded by vesicular matter, which may be readily scraped away 

 from it. Numerous striae of fibrous matter join the pillar as it 

 passes through the vesicular matter ; their constituent fibres, doubt- 

 less, being derived from the thalamus. Finally, each pillar ter- 



