THE NERVOUS SYSTEM. 



77 



may be viewed as a whole if the dorsal half of the cere- 

 brum including the corpus callosum is cut away. The telen- 

 cephalon or end brain is composed of the two large 

 hemispheres partially surrounding the diencephalon and 

 mesencephalon. 



The brain is composed of two kinds of matter, white and 

 gray. The former is -constructed for the most part of 

 fibers, while the latter is formed largely of cells. The 

 outer or cortical portion of the 

 cerebrum and cerebellum is a 

 layer of gray matter less than 

 a half centimeter thick, and is 

 disposed in folds called gyri or 

 convolutions with intervening 

 slit-like depressions termed 

 sulci, the more important of 

 which are called fissures. 



The four different portions 

 of the cerebrum are known as 

 frontal, parietal, occipital, and 

 temporal lobes, which occupy 

 the respective regions of the 

 cranial cavity. The frontal and 

 parietal lobes are separated by 

 the crucial fissure, extending 

 transversely between them. 

 The olfactory lobe (usually 

 torn off in removing the brain 

 from the skull) projects from 

 the cranial portion of the 

 frontal lobe. The parietal lobe 

 is marked by three gyri, named 

 according to location gyrus 

 marginalis, gyrus suprasylvius, 

 and gyrus ectosylvius (Figs. 92 and 93). The caudal 



Fie. 92. DORSAL ASPECT OF 

 THE BRAIN. 



a, Gyrus marginalis; ac, white 

 matter of the cord; ad, gray 

 matter of the cord; b, gyrus 

 suprasylvius ; c, gyrus ecto- 

 sylvius ; cr, crucial fissure ; 

 ce, lateral lobe of cerebellum ; 

 d, lateral sulcus ; e, supra- 

 sylvian sulcus ; lg, great longi- 

 tudinal fissure; md, medulla 

 oblongata; n, first spinal 

 nerve; ol, olfactory lobe; p, 

 posterior pyramids ; sp, spinal 

 cord ; v, vermis of cerebellum. 



