OCCIPITAL ASD TEMPORAL LOBES 687 



by the superior border of the petrous portion of the temporal Ijone, bus been 

 sonu'tinies described as the pra^occipital notch. 



It is generally stated that the occipital lobe passes into the temporal lobe on the tentorial 

 surface without any line of" demarcation. A number of" methods of marking off" the occipital lobe 

 have been jiroposed by different authors. These are all largely artificial. The occipital lobe is 

 due to a backward growth common to the parietal and temporal lobes. That growth carries with 

 it a jtrolongation of the lateral ventricle, forming the posterior cornu. The occipital lobe is there- 

 fore that jiart of the hemisphere which contains the posterior cornu. It maj^ be maiked off on 

 the teutoiial surface by a line drawn from the praeoccipital notch to the extremity of the jjarieto- 

 occij)ital fissure at the isthmus of the gyrus fornicatus (fig. 4f 1). The remaining boundaries will 

 be the parieto-oecipital fissure for its entire length and (as before) a line drawn from the extremity 

 of the external parieto-oecipital fissure to the prseoccipital notch. 



The fissures on the convex surface of the occipital lobe are the superior, middle, 

 and inferior occipital fissures; to these may be added the ramus occipitahs of the 

 intraparietal fissure with its appendage the transverse occipital fissure. 



The superior occipital fissure may be represented by a direct continuation of 

 the intraparietal fissure to the occipital pole of the cerebrum. More commonly, 

 however, it is limited to the posterior part of the occipital lobe (fig. 409, right 

 side), and is not continuous with the intraparietal fissure. The ramus occipitalis 

 of the intraparietal fissure may be directly continued into the occipital lobe from 

 the horizontal limb of the main fissure, or it may commence in the superior parietal 

 lobule. In either case it passes backwards into the occipital lobe between the first 

 and second annectant gyri, and usually terminates a short distance behind the 

 external parieto-oecipital fissure by dividing into two branches which extend 

 upwards and downwards at right angles to the main fissure, and together constitute 

 the transverse occipital fissure. 



The middle occipital fissure is often feebly developed. It is directed from 

 l)efore IjackAvards and meets the superior occipital fissure at the occipital pole. 



The inferior occipital fissure is placed along the margin which separates the 

 convex surface from the tentorial surface of the hemisphere. It may extend in a 

 tortuous manner from the praeoccipital notch to the occipital pole of the hemi- 

 sphere but is often broken up by one or more convolutions crossing it. 



The convolutions on the convex surface are three in number, and are imper- 

 fectly marked off from one another by the fissures above described. They pass 

 from before backwards and become confluent at the occipital pole. 



The superior occipital convolution is placed between the superior occipital 

 fissure and the margin of the great longitudinal fissure. It is continuous with the 

 cuneus on the inner surface of the hemisphere, and is joined in front by the first 

 annectant gyrus to the superior parietal lobule. 



The middle occipital convolution is situated between the superior and middle 

 occipital fissures, and is united in front to the i)Ost-parietal by the second and third 

 annectant gyri. 



The inferior occipital convolution occupies the interval between the middle 

 and inferior occipital fissures, and may be connected to the middle temporal convo- 

 lution by a fourth aimectant gyrus. 



The annectant gyri are four small sagittally-directed convolutions which cross the boundary 

 line which separates the occipital from the parietal and temporal lobes. They are numbered from 

 above d(jwnwards. The first and second of these are the most constant and are placed one on each 

 side of the ramus occii)italis of the intraparietal fissure. The third and fourth are not infre- 

 quently absent as they may be cut through by a fissure, the sulcus occipitalis anterior. 



The sulcus occipitalis anterior is an inconstant fissure which is parallel to, and placed a little 

 in front of, the anterior boundary of the occipital lobe on the convex surface of the hemis]ihere. 



The TEMPORAL LOBE (temporo-sphenoidal lobe) is sharply marked ofif in front 

 by the Sylvian fissure, but, as we have already seen, it jiasses behind Avithout any 

 surface line of demarcation into the parietal and occipital lobes. Its anterior 

 extremity forms a bold prominence directed downwards and forwards (the temporal 

 pole of Broca). It has four siu'faces: an external, forming part of the general con- 

 vexity of the hcmis])here; an inferior, Avhieh rests on the great wing of the sphe- 

 noid and the adjacent part of the petrous jjortion of the temporal bone; a tentorial. 



