34 APPLIED ANATOMY. 



THE MEDIAL SURFACE OF THE HEMISPHERES. 



If now the medial surface of the hemisphere, which forms one side of the longi- 

 tudinal fissure, be examined, there is seen a large convolution running just above and 

 parallel with the corpus callosum. It is called thegyrus cinguli {fornicatus} . Below 

 and separating it from the corpus callosum is the callosal fissure; above it is the calloso- 

 marginal fissure. The convolution above the latter, forming the margin of the hem- 

 isphere, is the marginal convolution. The callosomarginal fissure at its posterior 

 portion turns upward and ends on the margin of the hemisphere, just posterior to the 

 fissure of Rolando, or central fissure, and serves to identify it. This marks the pos- 

 terior limit of the frontal lobe. The posterior end of the frontal lobe surrounds the 

 upper end of the central fissure and on that account is called the paracentral lobule. 

 Its anterior boundary is marked by the paracentral fissure, or sulcus. Between the 

 callosomarginal fissure in front and the parieto-occipital fissure behind is the parietal 

 lobe, called, from its square shape on the medial surface, the quadrate lobule, or from 

 being anterior to the cuneus lobule, the precuneus. Running downward and back- 



Fissura centralis . ^_ / Sulcus cinguli (callosomarginalis) 



(Rolandi) 



Fissura parieto- 

 occipitalis 



Fissura calcarina 

 posterior 



Fissura calcarina 

 anterior 



Fissura collateralis 



Sulcus temporalis inferior 



FIG. 42. Gyri, sulci, and fissures of the medial surface of the cerebral hemisphere. 



ward from the deeper portion of the parieto-occipital fissure is a very distinct depres- 

 sion called the calcarine fissiire. These two fissures include a wedge-shaped piece 

 of the occipital lobe called, from its shape, the cuneus lobule. It is of interest in 

 reference to the sense of sight. 



FUNCTIONS OF THE CORTEX OF THE BRAIN: CEREBRAL 



LOCALIZATION. 



A knowledge of the functions of the various portions of the brain is necessary 

 in order to localize a diseased area. The diseases and injuries to which the brain 

 is exposed oftentimes do not involve the whole brain, but only certain distinct and 

 isolated parts. The brain is not a single, homogeneous organ that acts only as a 

 whole; it is complex. It is composed of a number of separate parts or areas, which 

 may act either singly or in conjunction with other areas. These separate areas have 

 different functions, so that if the disease or injury is limited to one of them, we have 

 its functions abolished, and the symptoms produced indicate the area affected. 



These areas are situated on the surface or cortex of the brain in the gray matter. 

 They receive impressions from, and transmit impulses to, all parts of the body 

 through the white matter or fibres of the brain. An injury to the cortex or gray 

 matter destroys the originating and receptive centres. An injury to the white matter 

 destroys the paths to and from these centres and therefore prevents them from 

 receiving impressions or sending out impulses. Thus, we may have a paralysis of 

 the leg and arm caused by an injury to the leg and arm centres in the cortex of 

 the brain, as by a hemorrhage from a fracture, or we can have the same paralysis 



