MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 343 



2. The posterior portion of the hemisphere becomes marked off into a dis- 

 tinct lobe by the symmetrical arching around the ends of the transverse calcarine, 

 of the superior or first occipital fissure, O 1 , above, and the inferior or second 

 occipital, O 2 , below. 



3. The fissures of this occipital lobe, when they are present, follow the 

 lines of the primary arch and are related in the same symmetrical scroll-like 

 manner to the two extremities of the transverse calcarine, as are the posterior 

 ends of that arch. 



4. The occipital lobe as a whole is formed by a regular scroll-like infolding 

 around the fissure of the posterior horn of the lateral ventricle, the calcarine, Plate 

 XXXVII, fig. 5. 



5. From this single symmetrically developed occipital lobe eight convolu- 

 tions separated by six fissures pass forward to the two anterior extremities of 

 the divided cerebral hemisphere. These are split by the Sylvian fissure into two 

 groups, equal in the number of fissures and convolutions composing them and 

 similarly related, — an occipitofrontal and an occipito-temporal lobe. 



6. Of these two lobes the occipito-temporal always retains its primitive 

 simplicity, but the occipito-frontal group owing to its greater antero-posterior 

 extension is exposed to pressure forces that tend to produce a vertical fissuration, 

 as a result of which we have the production of the fissura centralis with its 

 vegetative repetitions, the post-central and precentral fissures in the parietal 

 region, whilst anteriorly the type remains unchanged. 



7. The so-called plis de passage of Gratiolet, the bridging, annectant con- 

 volutions, etc., of other writers, are merely developments of parts that were at 

 first suppressed and altered by the evolution of the occipital lobe. They repre- 

 sent the occipital origins and connections of the occipito-frontal and occipito- 

 temporal convolutions and correspond in number and relations, as we have seen, 

 with their posterior modified extremities. 



In other words in the Primates each hemisphere, with respect to its convo- 

 lutions, is a symmetrical bud, arranged around the point of entrance, the cerebral crus; 

 its posterior portion involuted - in a regular and symmetrical manner around the 

 fissure of the posterior horn of the lateral ventricle (fissura calcarina), whilst the 

 anterior portion is split by the fissure of Sylvius into two symmetrical halves, 

 consisting of the same number of similar fissures and convolutions in each divi- 

 sion. Of these the occipito-frontal division or lobe is related to the upper branch 

 of the fundamental mesial arched fissure (fissura callosalis), and the occipito- 

 temporal in a corresponding manner to the inferior branch (fissura hippocampi), 

 the relations of which to the ventricle have already been pointed out. 



We thus come to the conclusion that the occipito-frontal and occipito-temporal 

 lobes, together with their corresponding fissures and convolutions are morphologi- 

 cally homologous to each other. 



The mesial surface of the occipito-frontal and occipito-temporal retain their 

 primitive and exact similarity throughout all the Primates and the lateral surface 



43 JOUEN. A. N. S. PHILA., VOL. X. 



