Ilimites of Proceedings. 197 



I cannot dwell. The transitory fissures result from deep infoldings of 

 the thin cerebral wall. The influence at work in their production 

 appears to be purely mechanical — a restraint placed on the longitudinal 

 growth of the hemispheres. Most anatomists have regarded the 

 occipital lobe as a secondary formation — a local out- growth from the 

 hinder part of the hemisphere, the early condition of the brain present- 

 ing only the frontal, temporal, and parietal lobes. From this view Dr. 

 Cunningham dissents. He holds the occipital lobe to be an original 

 formation which arises from the general growth of the brain, and 

 whose shape is determined by the restricted space it has to occupy 

 above the cerebellum. Now, the appearance and the obliteration of 

 the transitory fissures seem to be closely related to the mapping-out of 

 the occipital lobe. Those fissures are due to a difference between the 

 rate of growth of the hemisphere wall and the skull capsule contain- 

 ing it. The development is initiated in the brain, and the changes 

 involved in it are resisted by the enclosing cranium. "When the 

 primate brain is in the quadruped stage, there is an e:ffort towards the 

 formation of a distinct occipital lobe ; but there is a pause in the 

 growth of the cranium, and hence a temporary pressure which gives 

 rise to infoldings of the cerebral wall. (It would seem to follow that, 

 as Benedikt of Vienna has asserted, not only in the primates, but 

 in the mammalia generally, there is an occipital lobe ; this, however, 

 cannot yet be stated with certainty.) As the lobe is formed, the 

 foldings disappear, part of the agency in their obliteration being, 

 however, most probably, in addition to the extension of the surface, 

 the absorption of the folds. The transitory fissures have, with two 

 exceptions, disappeared when the coi^pus callosum is fully formed, and 

 its development may possibly have something to do with their 

 disappearance. 



Dr. Cunningham next proceeds to the study of the fissure of Sylvius, 

 that important sulcus which separates the frontal and parietal lobes 

 above from the temporo-sphenoidal lobe below, and which, with the 

 exception of the great longitudinal, is the most conspicuous sulcus in the 

 brain. It is sometimes, but wrongly, described as a complete fissure ; the 

 projection into the cavity of the hemisphere corresponding to it, is not the 

 result of a folding in of the mantle wall, but is an elevation of the floor 

 of the prosencephalon. The thi'ee limbs usually recognised as com- 

 posing the fissure, namely, the anterior ascending, the anterior horizon- 



