MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 279 



anterior and middle or descending horn of the lateral ventricle. This stage of 

 development is well represented in the figure of a three months human foetal 

 brain, Plate XXXV, fig. 1, where cp. is the cerebral peduncle, F the frontal. T 

 the temporal and P the posterior portion of the crescentic cerebral vesicle, whilst 

 S shows the position of the commencing fossa of Sylvius. It will also be noticed 

 that the mechanical action due to the surrounding cranial bones is exerting its 

 influence in moulding or pressing the hemispherical mass into shape ; for whilst 

 the outer portion of the cerebral sacs retain their primitive rotundity, owing to 

 the external resistance, they have been forced together in the median line, thus 

 producing two opposed flat surfaces, the future mesial portions of the hemispheres. 

 In this way is produced the great longitudinal fissure, separating the two hemi- 

 sphei'es in the median plane. This fissure in all animals below the mammalia 

 extends downward and forward as far as the lamina terminalis, Lt., fig. 1, which is 

 simply the anterior wall of the first primitive cerebral vesicle. In the mammals, 

 however, owing to the evolution of the corpus callosum, it extends anteriorly and 

 above merely to this great commissure. Plate XXXV, fig. 2, is a lateral view of 

 the same foetal brain shown in Plate XXXV, fig. 1. The crescentic shape of the 

 hemispherical sac is very apparent and in OF and OT can be seen those portions 

 which develop into the frontal and temporal lobes. The occipital lobe has not 

 as yet made its appearance but develops shortly afterward, as can be seen in 

 Plate XXXV, figs. 3 and 4, which represent the human foetal brain towards the 

 end of the fourth month. The posterior portion of the hemispherical sac has 

 developed backward, producing a distinct occipital lobe, 0, the lateral ventricle 

 following to form the posterior horn. 



In this way we have developed from the original ovoidal shaped cerebral vesicle 

 a hollow nervous bud connected with the thalamencephalon and posterior portions 

 of the central nervous system by the cerebral crura. This bud is arranged after 

 a tri-radiate plan and marks out what I believe to be the future lobes of the hemis- 

 phere. I distinguish, therefore, only three lobes to each hemisphere : an anterior, 

 an inferior and a posterior ; thus returning to the old sub-divisions of Burdach. 

 For these three lobes I propose the following names : 



1. Occipito-Frontal= Anterior of Burdach et al. 



2. Occipito-Temporal=Inferior of " " 



3. Occipital=Posterior lobe of " " 



As regards the so-called parietal lobe of recent writers I can by no means ad- 

 mit that it represents a lobe of the hemisphere in any morphological sense compara- 

 ble or homologous to the three lobes described above. The consideration of this 

 region however, we will leave for the present. 



These three lobes, as above described, are related to the fundamental structure 

 of the cerebral vesicle, its cavity and the three horns of the lateral ventricles. The 

 occipito-frontal lobe arches forward and descends around the position of the cere- 

 bral crus above. The occipitotemporal arches downward and forward beneath the 



35 JOUEN. A. N. S. PHILA.. VOL. X. 



