662 



THE NERVOUS SYSTEM. 



submerged cortical ridge, the gyrus cunei ; and the parieto-occipital will be found 

 to be something more than a mere sulcus. It is, in fact, a great fossa in which are 

 submerged the anterior parts of the area parastriata and area peristriata, and the 

 posterior part of the parietal area known as the prascuneus, as well as the sulci 

 which separate these territories one from the other. It is a great trough formed 

 by the splenium of the corpus callosurn as in the course of its development it 

 thrusts itself backwards and crumples up the cortex. When the corpus callosum 

 fails to develop, no parieto-occipital fossa makes its appearance. The part of the 

 sulcus that notches the supero-medial border (Figs. 589 and 593) forms a distinct 

 element, which Retzius has called the incisura parieto-occipitalis. 



Sulcus Collateralis. The collateral sulcus is a strongly marked furrow on the 

 tentorial face of the cerebral hemisphere. It begins near the occipital pole and 

 extends forwards towards the posterior end of the rhinal fissure, with which it 

 sometimes becomes confluent. In its posterior part it is placed below, and parallel 

 to, the calcarine fissure, from which it is separated by the lingual gyrus. From 

 the posterior extremity a sulcus proceeds forwards and then laterally across the 

 inferior surface of the occipital region, forming a V-shaped pattern with the 

 collateral sulcus (Fig. 585). As it is serially homologous with the latter, being, like 

 it, an inferior boundary of the area peristriata, it may be called the sulcus collateralis 

 trans versus. The lingual gyrus is sometimes subdivided by a furrow (sulcus 

 sagittalis gyri lingualis) midway between the collateral sulcus and the inferior 

 margin of the area striata. It is the line of demarcation between the parastriate 

 and peristriate areas, and when deep is often mistaken for the collateral sulcus. 



THE PAEIETAL REGION. 



We have seen that the acoustic pathway leads into the temporal region and 

 the visual pathway into the occipital region. The facts of clinical medicine show 



that large areas in these two regions 

 beyond the limits of the cortex in 

 which the acoustic and op tic radiations 

 end are concerned with the functions 

 of hearing and vision. A large part 

 of the parietal area is interposed be- 

 tween these temporal and occipital 

 territories, and its integrity and 

 normal functioning is a necessary 

 condition for the proper performance 

 of many acts, such as reading written 

 or printed documents, in the apprecia- 

 tion of which both hearing and vision 

 have played some part. But the 

 parietal region also includes the 

 cortical area in which a part, at 

 least, of the chief thalamo- cerebral 

 tract ends the bundle of fibres that 

 represents the third stage of the great 

 sensory pathway, the first stage of 

 which is formed by the spinal and 

 cerebral sensory nerves and their 



FIG. 590. LEFT CEREBRAL HEMISPHERE, from a foetus in 

 the early part of the seventh month of development. 



p.c.s. Sulcus prsecentralis superior. 



p.c.i. Sulcus praecentralis inferior. 



r 1 . Inferior part of central sulcus. 



r 2 . Superior part of central sulcus. 



p 1 . Inferior postcentral sulcus. 



p 3 . Sulcus interparietalis proprius. 



p 4 . Sulcus paroccipitalis. 



t 1 . Superior temporal sulcus. 



S. Lateral fossa. 



F.P. Fronto-parietal wall. 



F. Frontal wall. 



0. Orbital wall. 



central prolongations, and the second 

 stage by the spino-thalamic, bulbo- 

 thalamic, and ponto-thalamic fasciculi, which pass upwards in the medial lemniscus 

 and end in the ventral nucleus of the thalamus (Figs. 579 and 580). 



The sensory area in question forms part of the gyrus centralis posterior, whic 

 intervenes between two oblique furrows, the sulcus centralis and the sulcus posl 

 centralis, which extend across the whole breadth of the hemisphere above tl 

 sulcus lateralis (Fig. 581). 



Sulcus Centralis. During the sixth and seventh months of foetal life the expand- 



