176 THE HUMAN BODY 



At the base of the cerebrum, where it rests upon the midbrain, 

 all the projection fibers, both afferent and efferent, are crowded 

 together into a restricted space between two of the basal nuclei. 

 This region is known as the internal capsule. As the fibers emerge 

 thence into the roomy cerebrum they spread apart on their way 

 to the different parts of the cortex forming the corona radiala. 



The second group of cerebral axons are the association fibers. 

 These pass between one part of the cortex and another within 

 the same hemisphere, enabling impulses to travel freely among 

 the cortical cells. The third group of cerebral axons are the 

 commissural fibers which pass between cortical areas in opposite 

 hemispheres; these serve to unify the anatomically double cere- 

 brum into a single physiological organ; the corpus callosum (Cc, 

 Fig. 60) is made up of commissural fibers. 



Lobes of the Cerebrum. The convolutions of the cerebrum are 

 sufficiently constant in number and position to serve as land- 

 marks in locating particular regions. The individual convolutions, 

 or gyri, have been given specific names, as have also the fissures, 

 or sulci, which separate them. For our purposes it is necessary 

 to mention by name only those fissures which mark off the grand 

 divisions, or lobes, of the cerebrum. The division of the cerebrum 

 into lobes is purely arbitrary, and is made for greater ease in 

 describing it. In general the lobes correspond in position to the 

 overlying skull bones for which they are named. The fissures 

 which mark the boundaries of the lobes are indicated in Fig. 59. 

 They are the fissure of Sylvius, the fissure of Rolando, and the 

 Parieto-occipital fissure. The frontal lobe is that part of the cere- 

 brum above the fissure of Sylvius and in front of the fissure of 

 Rolando; the parietal lobe is between the fissure of Rolando and 

 the parieto-occipital fissure; the occipital lobe is the wedge-shaped 

 portion behind the parieto-occipital fissure; the temporal lobe is 

 below the fissure of Sylvius; it is the only one of the lobes which 

 is sharply set off as a distinct region. 



Cortical Localization. A problem of much interest in connec- 

 tion with the study of cerebral functions is whether there is di- 

 vision of labor among the various parts of the cortex. Do certain 

 groups of cells perform certain special functions, or are all cortical 

 activities shared in by all the cells? This is not the place for a 

 history of the solution of this problem. Suffice it to say that we 



