CHAP. X.] COMMISSURES OF THE BRAIN. 287 



minates in a small spherical body at the base of the brain. These 

 bodies, called corpora mamillaria, are white outside, but, when 

 cut into, exhibit a reddish-gray colour, like that of the optic tha- 

 lami. They contain nerve-tubes and vesicular matter in consider- 

 able quantity, and therefore resemble ganglia in structure. A 

 considerable fasciculus of fibres connects each mamillary body with 

 the optic thalamus. 



From each angle of the base of the fornix a broad band of 

 fibrous matter passes outwards, and spreads partly into the poste- 

 rior horn of the lateral ventricle, and partly into its descending 

 horn. These bands constitute the posterior pillars of the fornix. 

 They connect themselves with certain convolutions which project 

 into the posterior and inferior cornua of the lateral ventricles ; in 

 the latter with the hippocampus minor, and in the former with the 

 hippocampus major. 



The fornix consists of longitudinal fibres, unmixed with vesi- 

 cular matter, save in the optic thalami and corpora mamillaria. 

 The superior surface of the body of the fornix is connected to the 

 inferior surface of the corpus callosum, at its base apparently by 

 the direct adhesion of the fibres of the two planes; but towards 

 its apex by the septum lucidum, which extends vertically from the 

 middle line of the inferior surface of the corpus callosum to that of 

 the superior surface of the fornix. 



From the great extent of the fornix, and the numerous con- 

 nexions which its pillars form, it is plain that it must serve as 

 a commissure to many and distant parts. Each half of it is a 

 longitudinal or antero-posterior commissure for the hemisphere of 

 its own side. It is not improbable that some of the convolutions 

 contain antero-posterior commissures for the superficial part of the 

 hemisphere ; such is certainly the case with the longitudinal con- 

 volution above the corpus callosum. The fornix, however, con- 

 nects deep-seated parts, for it passes between the optic thalamus 

 and the deep convolutions of the posterior and middle lobes. 



Tlie septum lucidum consists of two layers of fibrous matter, 

 which enclose a space or cavity called the fifth ventricle. The 

 fibrrs of this layer radiate upwards and forwards, and connect the 

 anterior pillars of the fornix with the corpus callosum. Each 

 fibrous layer is covered on its outside by a layer of nuclear parti- 

 cles, which again is covered by the membrane of the lateral 

 ventricle. 



A band of fibrous matter, which belongs to the same system 

 of commissural fibres as the fornix, is found, on each side, in the 



