686 THE NERVOUS SYSTEM 



is limited below Ijy the posterior limb of the Sylvian fissure in the horizontal part 

 of its course, and behind this its lower limits are artificially maj^ped out by 

 l)roducing the horizontal part of the Sylvian fissure backwards to meet the posterior 

 boundary. The ])arietal lobe contains one fissure of importance — the intraparietal. 

 The intraparietal fissure is a system of three fissures, viz. a superior and an 

 inferior vertical, })hu-cd jiarallel to the fissure of Rolando; and a horizontal placed at 

 right angles to the other two. These parts may in rare cases remain sejjarate, but 

 most connnonly are united with one another, forming a T-shaped figure. Moreover, 

 the horizontal part may unite with a fourth part, the ramus occipitalis, which 

 extends into the occipital lobe. 



Tlie iiitniparietal fissure has been hitherto regarded as consisting of an ascending and a liuri- 

 zontal part ; the upper vertical limb (postcentral sulcus) being detached. This is a coiuukjh 

 variety of the fissure, but it has recently been shown that the T-shaped form is the commonest 

 or normal arrangement. (Cunningham.) 



The convolutions of the parietal lobe are: an ascending parietal, bounding the 

 fissure of Rolando posteriorly and placed between that fissure and the intraparietal 

 sulcus; and two parietal lol^ules, a superior and an inferior, placed above and below 

 the horizontal limb of that fissure. The inferior parietal lobule is further sub- 

 divided into an anterior part called the supramarginal convolution, a middle, termed 

 the angular gyrus, and a posterior part, the post-parietal convolution. 



The ascending parietal convolution extends from the posterior liml) of the 

 Sylvian to the great longitudinal fissure, and is bounded in front by the fissure of 

 Rolando, and behind by the intraparietal sulcus. It is continuous Avith the supra- 

 marginal convolution below the intraparietal fissure, and at its upper end is con- 

 tinued into the paracentral lobule on the inner surface of the hemisphere. 



The superior parietal lobule is a scpiarish mass indented on the surface by 

 tertiary fissures. It is limited internally by the margin of the great longitudinal 

 fissure where it becomes continuous with the prsecuneus. The anterior end of the 

 ramus occipitalis of the intraparietal fissure terminates in this lobule Avhen it is 

 not continuous with the remainder of the intraparietal fissure, and, in such cases, 

 the superior and inferior parietal lobules are continuous posteriorly. The superior 

 ])arietal loljule is joined in front, by a convolution of variable breadth, with the 

 ascending parietal, and is connected behind to the suj^erior occipital convolution by 

 the first annectant gyrus. 



The supramarginal convolution is the anterior part of the inferior parietal 

 lobule. It arches around the upturned extremity of the posterior limb of the 

 fissure of Sylvius. It is continuous in front with the ascending parietal convolu- 

 tion, behind Avith the superior temporal gyrus and also Avith the angular gyrus. It 

 is often very obscurely marked off from the latter. 



The angular gyrus is placed behind the preceding and embraces the extremity 

 of the parallel fissure. It is continuous behind Avith the middle temporal convolu- 

 tion. It is also usually connected to the middle occipital convolution by the third 

 annectant gyrus. 



The post-parietal convolution forms the posterior ])art of the inferior parietal 

 lobule. It curves round the second temi^oral sulcus and joins the third temporal 

 convolution. 



The OCCIPITAL LOBE occupies, approximately, the portion of the cranial cavity 

 Avhich is interjwsed betAveen the superior fossae of the occipital bone and the tent- 

 orium cerebelli. It presents three very distinct surfaces: a convex su])ero-externa], 

 a mesial or internal, and a tentorial. The latter looks doAVUAvards and slightly 

 iuAvards. These three surfaces meet at the occii)ital ]>ole. It is shar])ly marked 

 off on the mesial surface by the internal parieto-occipital fissure. On the convex 

 surface the line of demarcation (as already described in treating of the parietal 

 lobe) is made l\v draAving a line from the extremity of the external i)arieto-occipital 

 fissure to the pra;'occi))ital notch (fig. 410). This line separates the occipital from 

 the parietal and temjjoral lobes. 



The praeoccipital notch (fig. 410) is produced by the impression of the A'eins 

 AA'hich enter the lateral sinus. Another impression (fig. 410, n), Avhich is produced 



