490 THE BRAIN 



of ependyma, which excludes it from the cavity and is 

 connected on the one hand to the sharp margin of the fornix, 

 and on the other to the upper surface of the thalamus. 



Cornu Posterius Ventriculi Lateralis. The posterior 

 horn is a diverticulum which runs, from the posterior end of the 

 central part of the ventricle, into the occipital lobe. It tapers 

 to a point and describes a gentle curve, the convexity of 

 which is directed laterally. The roof and lateral wall of 

 this portion of the ventricular cavity is formed by the tapetum 

 of the corpus callosum. 



Upon the medial wall two elongated curved elevations 

 may be seen. The upper of these, termed the bulb of the 

 cornu, is produced by the fibres of the forceps major as 

 they curve posteriorly, from the lower part of the splenium 

 of the corpus callosum, into the occipital lobe. The lower 

 elevation is known as the calcar avis. It varies greatly in 

 size, in different brains, and is caused by an infolding of the 

 ventricular wall which corresponds with the anterior part of 

 the calcarine fissure. 



Dissection. The dissector should now insinuate his fingers underneath 

 the fronto-parietal operculum of the insula and tear this portion of the 

 cortex away in an upward direction. The frontal operculum (pars 

 triangularis) and the orbital operculum should be dealt with in the same 

 manner. The greater part of the temporal operculum has already been 

 removed in opening up the inferior horn of the ventricle ; therefore the 

 insula is now fully exposed to view, and its relation to the parts in the 

 interior of the ventricle can be seen. 



Cornu Inferius Ventriculi Lateralis (O.T. Descending 

 Cornu). The inferior horn must be regarded as the direct 

 continuation of the main ventricular cavity into the temporal 

 lobe. The posterior horn is merely a diverticulum from the 

 main cavity. At first directed postero-laterally, the inferior 

 horn suddenly sinks downwards, posterior to the thalamus, into 

 the temporal lobe, in which it takes a curved course, antero- 

 medially, to a point about an inch posterior to the extremity 

 of the temporal pole. In the angle between the diverging 

 inferior and posterior horns the cavity of the ventricle exhibits 

 a triangular expansion of varying capacity. This is called the 

 trigonum collaterale. 



The lateral wall of the inferior horn is formed, for the 

 most part, by the tapetum of the corpus callosum. At the 

 extremity of the horn the ra/ presents a slight bulging into the 

 ventricular cavity. This is the amygdaloid tubercle, and it is 



