THE CEREBRAL HEMISPHERES. 239 



interior of the cerebrum, called the ventricles; these cavities are roofed 

 in, as it were, by the substance of the hemispheres, and by the corpus 

 callosum, and are also closed below by the diverging peduncles, at the 

 sides by the hemispheres, and in front by nervous substance passing 

 down from the corpus callosum to the peduncles; but posteriorly, they 

 are open, so as to communicate with the surface, and admit, into the 

 interior of the ventricles, an extension of the vascular covering of the 

 brain, or pia mater, together with many large bloodvessels. 



Fig. 58. 



Fig. 58. Horizontal section through the cerebrum, to show the mode in which the two hemispheres, a, a, 

 are joined together by the transverse band of white substance, named the corpus callosum. In front and 

 behind this, the longitudinal fissure separates the two hemispheres, b, b, is the section of the cortical pub- 

 stance; a, a, of the medullary. The section also shows the depth of the sulci, between the convolutions. 



Each cerebral hemisphere is described as consisting of certain parts 

 called lobes, which were formerly named anterior or frontal, middle or 

 parietal, and posterior or occipital. The anterior lobe was said to be 

 separated from the middle, by a deep fissure, seen on the side of the 

 brain, named the Sylvian fissure, but the limits between the middle 

 and posterior lobes were arbitrarily fixed by different anatomists, ac- 

 cording to their position in regard to the cerebellum. Certain well- 

 marked fissures, which have been observed on the outer and inner sur- 

 face of the hemispheres, serve however to divide its mass more defi- 

 nitely into such lobes. For example, one long fissure, called the fissure 

 of Rolando (see description of Fig. 57), passes obliquely forwards, from 

 a little distance behind the vertex, and separates an anterior or frontal 

 region from a middle or parietal region, the former nearly correspond- 

 ing with the frontal lobe, but including a small portion of the parietal 

 lobe. Further back, another fissure, seen on the inner surface of the 

 hemisphere (see description of Fig. 59), and called the internal perpen- 

 dicular fissure, marks off a posterior lobe or region, which is now de- 

 scribed as the occipital lobe. Below that, on the under and inner sur- 

 face of the hinder part of the hemisphere, is a horizontal fissure, called 

 the fissure of the hippocampus; this separates the occipital lobe above, 

 from the lowest portion of the middle lobe, now named the temporal 

 lobe, which is further separated, on the outer surface of the hemisphere, 

 from the frontal and parietal lobes, by the Sylvian fissure. On open- 



