324 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



in highly convoluted brains, areas of growth rising from the depths of the original 

 and primitive fissures. In this way we may have developed in certain regions gyri 

 that become characteristic of a genus or a species, or that may even present them- 

 selves as individual peculiarities, such as the bridging of the central fissure in the 

 brain of Dr. Fuchs figured by Wagner. 



Now the fissura paroccipitalis of Wilder is of this nature. It is simply the 

 remains of a gap made between two separated extremities of a fissure produced by 

 the breaking of the continuity of the original fissure, due to the development of a 

 localized swelling springing from its depths. By examining figs. 18 and 19 from 

 this standpoint, I think the difficulties of the subject entirely disappear. In fig. 

 18, we have the condition as found in the majority of the Simiadge. The inter- 

 parietal, i p, parieto-occipital, p o, and Ecker's fissura occipitalis transversa meet 

 together at the margin of the mesial aspect of the hemisphere. Certainly in these 

 forms there can be no doubt of the typical nature of these fissures. In fig. 19, we 

 find coming to the surface a small gyrus, 2, which arises from the depths of the 

 occipital cleft, 0\ and which can be seen in its concealed condition in Cebus appella, 

 Plate XXXVIII, fig. 12. Evidently, as this concealed gyrus gradually reaches the 

 surface, it will present the appearance as shown in the Chimpanzee, Plate XXXVIII, 

 fig. 13, where Wilder's paroccipital gyrus, the superior external pit de passage, is 

 seen in a condition which is intermediate between the stage as found in the lower 

 Simiadse and Man, and which is almost identical with the arrangement as found in 

 the human foetus toward the end of the ninth month (compare figs. 3 and 4 with 

 7 and 8, Plate XXXVI). Instead, therefore, of the " transverse occipital" of Ecker 

 being merely the caudal stipe and ramus of the zygon of a new fissural integer the 

 paroccipital, it appears to me that this fissure is merely a communication across a gap 

 in the fissura perpendicularis externa caused by the development of the convolution, 

 2. Viewed in this light it is not a fissural integer at all, but merely a modification 

 produced in the manner of connection of the originally confluent interparietal and 

 fissura perpendicularis externa, and as such I regard it. It is for these reasons 

 also, that this so-called paroccipital is deepest at its middle point and gradually 

 becomes shallower as it joins the interparietal and backwardly displaced fissura 

 perpendicularis externa, which Ecker, in Man, terms the " transverse occipital." 

 From its mode of development it cannot be considered as a distinct and separate 

 fissural integer comparable with the more important fissures of the cerebral hemi- 

 sphere. There is no objection, however, to calling the convolution (2) by the name 

 paroccipital, as it represents a distinct morphological development, the nature of 

 which we shall more fully discuss under the heading of the plis de passage. 



It should ever be borne in mind, in comparing the occipital lobe of Man with 

 that of the Simiadse, that whilst the posterior portion of the hemisphere is increas- 

 ing largely in development, this is due in the human brain principally to growth 

 taking place in the occipito-parietal region. The occipital lobe as it is sharply de- 

 fined by the perpendicular or so-called "Ape fissure" is becoming relatively smaller, 

 and the primitive simplicity of the primary occipital arch (as we have termed it 



