362 THE BRAIN 



sulci, called the lateral occipital sulcus and paramedial occipital 

 sulcuS) into three gyri the superior, middle, and inferior. 



The lateral surface of the temporal lobe is divided by two 

 sulci, which run antero-posteriorly, into superior, middle, and 

 inferior temporal gyri. 



When the survey of the supero-lateral surface is completed, 

 a specimen should be examined in which the lips of the 

 lateral fissure have been separated or removed. In such a 

 specimen it will be obvious that at the bottom of the fissure 

 there is a sunken area of the brain cortex (Fig. 157). It is 

 called the insula, and it is separated from the adjacent parts 

 by a sulcus called the circular sulcus. 



After the general relations of the supero-lateral surface 

 have been noted the medial surface of the hemisphere should 

 be examined. Upon it, nearer its anterior than its posterior 

 end, and nearer its lower than its upper border, will be seen 

 the surface of section of the severed corpus callosum (Fig. 136). 



The corpus callosum consists of a trunk, which terminates 

 posteriorly in a free, thick, rounded posterior end, called the 

 splenium, and anteriorly in a bent anterior extremity called 

 the genu. From the genu a tapering portion of the corpus 

 callosum, termed the rostrum^ passes downwards and back- 

 wards. It ends below in a thin lamina, called the lamina 

 terminalis, which descends till it reaches the ovoid transverse 

 section of the optic chiasma, which connects together the two 

 optic nerves. The lamina terminalis passes behind the optic 

 chiasma, and joins the tuber cinereum in the floor of the 

 third ventricle (Figs. 136, 159). The transverse, small and 

 round, white bundle which passes through the lamina termin- 

 alis, above the optic chiasma, is the anterior commissure. 



In the median plane, in the angle between the body, genu, 

 and rostrum of the corpus callosum, there is a thin vertical 

 lamina called the septum pellucidum. It is bounded below 

 and behind by a flat band of white matter, called the fornix 

 (Figs. 136, 156). 



The boundaries of the medial surface of the hemisphere 

 are (i) The supero-medial border, which extends from the 

 frontal pole to the occipital pole, and separates the medial 

 from the supero-lateral surface. (2) The medial occipital 

 border, which extends from the occipital pole to the splenium 

 of the corpus callosum ; it separates the medial surface from 

 the posterior part of the inferior surface. (3) The medial 



