MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 323 



"On page 38 it is admitted that the fissure is less distinct (manchmal weniger 

 deutlich), because often interrupted, and this more frequently on the right side. 

 Nevertheless on both sides of the diagram, fig. 2, the fissure is made continuous, 

 and his view seems to have been generally accepted. 



"After a careful study of all the specimens and figures obtainable, I am led to 

 conclude that this view is erroneous ; that the true parietal and the " superior 

 occipital' do not form parts of one fissure, and that the latter is the principal and 

 primar}' constituent — zygon — of a paroccipital fissure. 



" The evidence is threefold : (1) as admitted by Ecker, the zygon always 

 appears independently in the foetus; (2) as also admitted, it often remains separate 

 in the adult ; (3) when the union does occur, in all cases examined in reference to 

 this point, excepting one, the combined fissure is shallower at the presumed place 

 of junction, and deeper at or near the middle of the two constituents — the true 

 parietal and the paroccipital zygon. 



" So far then as reliable evidence is attainable by me at the present time, it 

 appears that Ecker' s interparietal is interrupted as often as it is continuous.. On 

 page 38 Ecker endeavors to diminish the force of what would be commonly regarded 

 as evidence adverse to his view, by affirming that the interruption of his 'long' 

 parietal occurs no more frequently than, for example, the temporal fissures. Even 

 if this be true, it is by no means certain that the temporal fissures are the integers 

 they are ordinarily admitted to be ; they certainly, like all other fissures, need 

 monographic treatment." (Wilder.) 



The various opinions expressed above depend, I believe, on the non-recognition 

 of the fact that the fissures of the hemispheres are produced in two entirely dif- 

 ferent ways. In discussing the evolution of the calcarine fissure and the bridging 

 gyri that often appear in its course, I pointed out the fact that whilst the majority 

 of the cerebral fissures are produced by the sinking or depression of the cerebral sur- 

 face, due to the general growth of the hemisphere as a whole, other fissures are 

 produced by an exactly opposite process ; namely, by the development upward 

 from the floor of previously existing fissures, of local swellings, the so-called bridg- 

 ing or annectant gyri. Evidently the fissures produced in this way will have some- 

 what different relations to the surrounding parts from those produced originally as 

 depressions of a smooth surface. We have already given sufficient evidence, to 

 show that the typical fissures represent the lines of least resistance to the differ- 

 ential action of the pressure forces ; and as long as they remain represented as 

 deep and constant fissures, the same revolution of forces must be taking place. As 

 development proceeds and the skull begins to assume a more fixed and rigid shape, 

 new conditions and relations of the growth-forces take place, as a result of which 

 those portions of the cerebral surfaces lying in the depths of some of the fissures, 

 originally produced by depression, are placed under new dynamical conditions. 

 Instead of being situated, as formerly, along lines of resolution of greatest pressure- 

 force and least resistance ; they become centres of relatively greatest growth-force 

 as compared with the resolution of pressure-forces. Hence we may expect to find 



