THE OCCIPITAL LOBE 863 



lobule about the end of this fissure presents the appearance described as the 

 parieto-occipital arch. 



The inferior parietal lobule is hmited in front by the inferior postcentral 

 sulcus, and above bj^ the horizontal ramus of the interparietal sulcus. It is con- 

 tinuous with the cortex of the temporal lobe below, and with that of the occipital 

 lobe behind, and is therefore invaded by the ends of the sulci belonging to these 

 lobes. Its anterior portion is separated from the temporal lobe by the horizontal 

 portion of the posterior ramus of the lateral fissure. The upturned end of this 

 ramus invades the anterior portion of the lobule and the broad fold, arched around 

 this end and continuous behind it into the superior temporal gyrus, is known as 

 the supramarginal gryus — the area to which auditory word- and tone-images are 

 attributed. The angular gyrus is the portion which embraces the posterior end 

 of the superior temporal sulcus, and is continuous behind this into the middle 

 temporal gyrus and in front with the superior temporal gyrus. It is the area for 

 visual word images. Its shape is usually such as to suggest its name. The most 

 posterior part of the inferior parietal lobule, when arching in a similar way about 

 the end of the middle temporal sulcus and continuous with the temporal gyri 

 on its either side, is known as the post-parietal gyrus. This is a smaller area than 

 either of the other two, and, owing to the variability of the end of the middle tem- 

 poral sulcus, is not always evident. 



The mesial surface of the parietal lobe is divided into two parts by the marginal 

 portion of the sulcus cinguli. The anterior and smaller part is the mesial con- 

 tinuation of the posterior central gyrus, and comprises the posterior portion of 

 the paracentral lobule. It is limited from the part of this lobule belonging to the 

 frontal lobe by a vertical line drawn from the marginal extremity of the central 

 sulcus. The praecuneus (quadrate lobule) is the posterior and larger part of the 

 mesial surface of the parietal lobe. It is separatd from the cuneus of the occipital 

 lobe by the parieto-occipital fissure, and is imperfectly separated from the gyrus 

 cinguli (limbic lobe) below by the subparietal sulcus (postlimbic fissure), branches 

 of which invade it extensive^. 



The occipital lobe. — This is a relatively small, trifacial, pyramidal segment, 

 comprising the posterior extremity of the hemisphere, its apex being the occipital 

 pole. Though one of the natural divisions of the cerebral hemisphere, it is very 

 indefinitely marked off from the lobes anterior to it. Though it contains the 

 cortical area of the visual apparatus, only in the brains of man and the apes does 

 it occur as a well-defined posterior projection. In most of the mammalia it is 

 not differentiated at all. Its three surfaces comprise a convex, a mesial, and a 

 tentorial surface. 



Its convex surface is separated from that of the parietal and temporal lobes by 

 the superior and external extremity of the parieto-occipital fissure, and by an 

 arbitrary line drawn transversely from this extremity to the infero-lateral border 

 of the hemisphere, or so draw^n as to bisect the pre-occipital notch when this is 

 evident. The sulci which occur on the convex surface may be described as two, 

 though both of these are very variable in their extent and shape, and their 

 branches are inconstant both as to number and length. (1) the transverse oc- 

 cipital sulcus is the most constant in shape. It extends a variable distance 

 transversely across the superior portion of the lobe, and, as noted above, it is 

 frequently continuous with the interparietal sulcus through its occipital ramus, 

 and when so, it appears as the posterior terminal bifurcation of this sulcus 

 (fig. 674). When detached, it often occurs merely as a definite furrow with few 

 rami, and sometimes the ramus by which it otherwise would join the inter- 

 parietal sulcus is entirel}^ absent. (2) The lateral occipital sulcus is always 

 short, and has its deepest portion below the transverse sulcus. It usually has a 

 somewhat oblique course toward the supero-mesial border. Sometimes it occurs 

 in several detached pieces, then known collectively as the lateral occipital sulci. 



Therefore, the gyri of the convex surface of the lobe are also variable. The}'- are not 

 sufficiently constant to merit individual names. The lateral occipital sulcus or sulci roughly 

 divide them into an inferior and lateral area, kno-mi as the lateral occipital gyri, and into a 

 uperior larea, the superior occipital gyri. The lateral area is continuous into the g\Ti of the 

 temporal lobe, while the superior area is continuous into the gyri of the parietal lobe. 



■ The mesial surface of the occipital lobe is separated from that of the parietal 

 lobe (precuneus) and from the gyrus cinguli of the limbic lobe by the well- 



