THE CEREBRUM 435 



the posterior horn are formed by the tapetum of the corpus 

 callosum (see p. 426). 



Upon the medial wall two elongated, curved elevations 

 may be seen. The upper of the two is termed the bulb of 

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

 as they curve, backwards, 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. If the opercula have not already been removed 

 to expose the insula, the dissector should now insinuate his 

 fingers underneath the fronto-parietal operculum of the insula 

 on the right side and tear that 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 how 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 backwards and laterally, the inferior 

 horn suddenly sinks downwards, posterior to the thalamus, into 

 the temporal lobe, in which it takes a curved course, forwards 

 and medially, to a point about 25 mm. (one 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. It 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 roof presents a slight bulging into the 

 ventricular cavity. The bulging is called the amygdaloid 

 tubercle, and it is produced by a superjacent collection of grey 

 matter, termed the amygdaloid nucleus. The stria terminal's 

 and the greatly attenuated tail of the caudate nucleus are 

 both prolonged into the inferior horn, and are carried 

 forwards, in its roof, to the amygdaloid nucleus. 



