DISSECTION OF THE DOG 217 



It inclines in a ventral direction and, in the dog, ends blindly about the level 

 of the tuberculum olfactorium. In many mammals the cornu is continuous 

 with the cavity of the olfactory bulb. 



The inferior cornu (cornu inferius) is a tapering curved prolongation of 

 the ventricle traceable into the pyriform lobe, where it ends on a level with 

 the point at which the optic tract makes its appearance on the surface of 

 the brain. The cornu contains the tail of the caudate nucleus (cauda nuclei 

 caudati) reduced to a narrow grey ridge, and the diminished continuations of 

 the choroid plexus, fimbria, and hippocampus. 



Septum pelltjcidum. — A thin medial partition separates the anterior 

 part of the pars centralis of the two ventricles, and fills in the triangular gap 

 between the corpus callosum and the fornix. This is the septum pellucidum, 

 in reality composed of two thin lamina? with a narrow chink-like cavity 

 intervening. 



Dissection. — The fornix must now be exposed as completely as the present 

 stage of the dissection permits. Cut across the remains of the corpus 

 callosum about the genu, and remove it from this point to the splenium. 

 Its connection with the septum pellucidum is necessarily destroyed in 

 the proceeding. 



The hemi-sected brain affords much assistance in the study of the fornix, 

 which there appears as a curved white band, ventral to the corpus 

 callosum. 



Fornix. — The fornix consists of two bands of longitudinal fibres pursuing 

 an arched course and intimately connected with each other at one place. 

 The united bands constitute the body of the fornix (corpus fornicis) and are 

 related to the corpus callosum and the septum pellucidum. At each end of 

 the body the two constituent parts of the fornix separate. At the anterior 

 end the separation is not very great and results in the production of the columnce 



fornicis, two rounded white cords which curve ventralwards in front of the 

 foramen of Monro. Continuing to the base of the brain the columns form 

 the prominent mammillary body, the double character of which is thus ex- 

 plained. From the posterior end of the body of the fornix arise two crura 



fornicis. These, widely diverging, are at first connected with the corpus 

 callosum. Soon they form the white hem — fimbria hippocampi — adherent 

 to the hippocampus ; and, as such, are continued down the inferior cornu of 

 the lateral ventricle. Some fibres of the crus fornicis are spread out as a white 

 layer (alveus) on the surface of the hippocampus. 



Dissection. — Cut across the fornix about the point where the crura are 

 leaving the body. Now, with the utmost care, remove the isolated aboral 

 part of the hemispheres ; i.e. the hippocampus and adjacent part of 

 the wall of the inferior cornu of the ventricle. 



