MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 307 



of this region as compared with the occipitotemporal lobe produces a change in the 

 con volution al configuration, producing lines of depression or fissuration having the 

 same general relations to the fossa of Sylvius as regards inclination as the primary 

 radiating furrows of the foetal brain possessed, and due no doubt to a somewhat 

 similar composition of pressure forces as was found at that early period. These 

 facts seem to show an alternation in the lines of direction of the resultant pressure 

 forces in different regions of the brain, from a radiating arrangement to a curvili- 

 near one and vice versa. These reversals take place, not only during the develop- 

 ment of the individual but also during the evolution of the phylum, and are due to 

 the varying rates of growth of different portions of the brain and skull at corres- 

 ponding periods of development. Besides the fissure of Rolando we find several 

 other vertical depressions originating in this region, such as precentral, post-central, 

 etc. ; but we leave the consideration of these sulci and their relations until we 

 have discussed the general structure of the fronto-parietal region. 



As already remarked, I cannot accept the parietal portion of the hemisphere 

 as a distinct and separate cerebral lobe comparable to the three lobes previously 

 pointed out, but for purposes of description divide the lateral surface of the occipito- 

 frontal lobe into two regions, a frontal and a parietal, separated from each other by 

 the fissura centralis. 



I. ANTERIOR OR FRONTAL DIVISION OF THE OCCIPITO-FRONTAL LOBE. 



On the lateral surface of this region in Man and most of the Primates, three 

 convolutions, separated by two fissures, are found. These convolutions are known 

 respectively as the first, second and third, or superior, middle and inferior frontal 

 convolutions, F 1 , F 2 and F 3 , whilst the fissures are designated as the first and second 

 or superior and inferior frontal fissures, f 1 , f 2 , PI. XLIII, fig. 7. The inferior 

 frontal fissure at its posterior part joins a perpendicular fissure, f 3 , the sulcus prse- 

 centralis of Ecker, the antero-parietal fissure of Huxley. Much confusion has 

 arisen in regard to this fissure; many writers entirely disagreeing as to its relations. 

 The names given by Ecker and Huxley are inappropriate and liable to create 

 confusion. This is not a distinct and separate fissure morphologically as these 

 names would seem to indicate, but is merely the posterior portion of the inferior 

 frontal fissure which has assumed a more pr less perpendicular direction, following 

 the line of inclination of the fissura centralis. Pansch recognized this and he 

 called it ramus descendens sulci frontalis medii. He called it a branch of the middle 

 frontal because in some of the monkeys another fissure runs beneath it. I should 

 prefer to term it the vertical or branch sulcus of the inferior frontal fissure. 



Turner has identified this fissure with the ascending branch of the Sylvian, 

 and he considers its separation from the latter by a part of the lower frontal 

 convolution as exceptional. This identification is incorrect. It is true that it 

 sometimes is continuous with the ascending ramus of the Sylvian, as in fig. 1, Plate 

 XLI, but it is this condition which is exceptional. Occasionally it extends as far 

 as the Sylvian fissure and joins it superficially, but this is always back of the 



