THE NERVOUS SYSTEM 



211 



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 most important of which are called fissures. 



The four different portions of 

 the cerebrum are known SiS 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, supra- 

 sylvius, and gyrus ectosylvius 

 (Figs. 103 and 104). The caudal 

 portions of the gyri marginalis 

 and suprasylvius constitute a 

 portion of the occipital lobe. 

 These two gyri are separated by 

 the lateral sulcus. The supra- 

 sylvian sulcus extends between 

 the gyrus suprasylvius and the 

 gyrus ectosylvius. The splenial 



sulcus (Fig. 106) separates the gyrus marginalis from the gyrus 

 fornicatus on the medial aspect of the parietal and occipital 

 lobes. There are no definite sulci or fissures separating 

 the parietal, occipital, and temporal lobes. The postrhinal fis- 

 sure, extending caudad from the fissure of Sylvius, divides the 

 temporal lobe into two portions on the ventral aspect. 



Fig. 103. — Dorsal Aspect of 

 THE Brain. 

 a. Gyrus marginalis; ac, white 

 matter of the cord; ad, gray mat- 

 ter of the cord; b, gyrus supra- 

 sylvius; c, gyrus ectosylvius; cr, 

 crucial fissure; ce, lateral lobe of 

 cerebellum; d, lateral sulcus; e, 

 suprasylvian sulcus; Ig, great 

 longitudinal fissure; md, medulla 

 oblongata; n, first spinal nerve; ol, 

 olfactory lobe; p, posterior pyra- 

 mids; sp, spinal cord; v, vermis 

 of cerebellum. 



