THE CEREBRUM. 121 



Their action is crossed ; that is, lesions of the left side produce 

 right blindness. From these bodies also is derived the power of 

 co-ordination of the movements of the eyes and the control of the 

 the reflex of the pupil. These centres are closely related, and 

 the disturbance of the one by a lesion in this part usually involves 

 the other. 



THE CEREBRUM. 

 Describe briefly the cerebrum. 



It is composed of two parts, or hemispheres, connected by a com- 

 missure of white fibres, the corpus callosum. The two hemispheres 

 are separated by a deep fissure extending fore and aft, and in the 

 interior of each is found a cavity known as the lateral ventricle. 

 The hemispheres are connected directly with the spinal system by 

 the crura cerebri and medulla, and with each other by the corpus 

 callosum. They are composed of white and gray nerve-substance, 

 and the latter is arranged largely at the periphery of the hemi- 

 spheres ; the former being made up of communicating nerve-fibros 

 which connect the various portions of the hemispheres, and the 

 hemispheres with other parts of the cerebro-spinal system, thus 

 allowing a free control of the impulses arising from one cell or set 

 of cells by other cells in the gray matter. 



How is the surface of the cerebrum marked ? 



It is divided into regions by fissures, which separate one part 

 from another. These fissures are always present, and upon them 

 depends the determination of the division of the cerebrum into 

 lobes. The fissures which are of most use in locating the lobes of 

 cerebral matter are the fissure of Rolando, the fissure of Sylvius, 

 and the parieto-occipital fissure. 



What are the convolutions of the cerebrum ? 



The surface of the brain is further cut up by a number of other 

 clefts, known as sulci; and these separate the surface into a num- 

 ber of distinct masses or convolutions. The depth of the sulci 

 and their number determine the quality of the brain in respect to 

 its degree of development ; thus, the convolutions in man are much 

 deeper and more numerous than in the lower animals. The sulci 

 are not invariable in position or number in different brains. 



Into what regions is the cerebrum divided by the fissures ? 

 (1) Frontal Lobe. This lobe is bounded by the fissure of Rolando, 



