THE CEREBRUM, AND ITS FUNCTIONS. 



687 



Sylvian Fissure outwards upon the external aspect of the hemisphere it is seen to 

 bifurcate, one branch running upwards, the other almost horizontally backwards. 

 Anterior to the Sylvian Fissure we have the frontal lobe, between its two branches 

 the parietal lobe, below the posterior branch ihetemporo-ftphenoidal lobe, and forming 

 the posterior extremity of the hemisphere the occipital lobe, while concealed within 

 the Fissure of Sylvius is the central lobe, more commonly known as the Island 



FIG. 242. 



Brain of Man. 



1, 2, 3, Frontal convolutions. 4, 5, G, A & B, Parietal convolutions. 



7, 8, 9, Temporo-sphpnoklal convolutions. 10, 11, 12, Occipital convolutions. 



A is the supramarginal lobule; B is the postero-parietal lobule; and is on the bridging or an- 

 nectent convolutions ; E, Fissure of Rolando ; E, parieto-occipital fissure. 



of Reil. This last presents a few short and almost straight gyri radiating from the 

 inner end of the tissue called the Gyri operti. The four other lobes are each made 

 up of three more or less tortuous gyri. The frontal convolutions have a horizontal 

 direction, and form three tiers, named respectively the superior, middle, and inferior 

 frontal convolutions, the superior close to and parallel with the great longitudinal 

 fissure, the inferior resting upon the orbital plate of the frontal bone. It is the 

 posterior part of this inferior frontal convolution which has been said to be the seat 

 of the faculty of language, and disease here to be one of the causes of aphasia. The 

 convolutions of the parietal lobe are nearly vertical, and run up to the margin of the 

 longitudinal fissure; between the first or anterior and the second is a well-marked 

 sulcus, which appears early in the development of the brain, and is called the Fissure 

 of Rolando. The tcmporo-sphenoidal gyri are again almost horizontal, lying parallel 

 to the horizontal branch of the Sylvian Fissure. The occipital lobe is small and not 

 well defined, a series of short irregular convolutions connecting it with the parietal 

 lobe. These have received the name of annectent or bridging gyri; the convolutions 

 of the occipital lobe itself are irregularly horizontal, and its anterior limit is a sulcus 

 which issues from the longitudinal fissure, and is called the parieto-occipital fissure. 

 On the opposed flat surfaces of the hemispheres the principal convolutions seen are 

 the marginal convolutions, which form the margin of the great longitudinal fissure 

 and theffyrusfornicatus, which encircles the corpus callosum, commencing anteriorly 

 near the anterior perforated space, and terminating at the point of the temporo- 

 sphenoidal lobe. By some anatomists these two great convolutions have been sub- 

 divided. 



559. The Cortical substance or " gray matter " of the Hemispheres essen- 

 tially consists of that vesicular nerve-substance, which, in the Spinal Cord, 

 as in gangliouic masses generally, is found to occupy the interior. Its usual 

 thickness is about one-fifth of an inch ; but considerable variations present 

 themselves in this respect, as also in the depth of the convolutions. Thus the 

 plications are deepest, and the layer of "gray matter" the thickest, during 

 the period of greatest nervous energy, that is, in middle life ; in infancy and 



