260 THE NERVOUS SYSTEM 



The Cerebrum. 



The great size of the cerebral hemispheres in man obscures 

 the fact that the different parts of the brain are disposed in a 

 linear series; these, from before backward, are, the olfactory lobes, 

 cerebral hemispheres, optic thalami, corpora quadrigemina, 

 cerebellum, medulla oblongata. This arrangement exists in 

 the human fetus, and persists throughout life in some of the 

 lower animals. 



Anatomy. The substance of each hemisphere is divided by 

 fissures into five lobes (a) frontal, (b) parietal, (c) occipital, (d) 

 temporo-sphenoidal and (e) central. The main fissures are four 

 in number (i) The fissures of Sylvius running from the front 

 and under part of the brain backward, outward and upward; (2) 

 the fissures of Rolando running from the median line near the 

 center of the longitudinal fissure forward, outward and down- 

 ward; (3) the parieto-occipital fissure, little of which is evident 

 upon the surface of the brain, but which appears on longitudinal 

 section separating the occipital and parietal lobes; (4) the calloso- 

 marginal fissure, also evident only on the internal aspect of the 

 hemisphere, parallel with and above the corpus callosum. (Figs. 

 81, 82.) 



(a) The frontal lobe is bounded internally by the longitudinal 

 fissure, posteriorly by the fissure of Rolando and below by the 

 fissure of Sylvius. On its surface are seen three convolutions, 

 approximately parallel, called the superior, middle and inferior 

 frontal convolution, and occupying positions which their names 

 indicate. In addition the posterior portion of this lobe is occu- 

 pied by the ascending frontal, or the anterior central convolution, 

 lying just in front of the Rolandic fissure. 



(b) The parietal lobe is bounded anteriorly by the fissure of 

 Rolando, internally by the longitudinal fissure, posteriorly by 

 the parieto-occipital fissure and below by the fissure of Sylvius. 

 Just behind the fissure of Rolando is the ascending parietal, or 



