MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



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states that he has found them in the brains of embryos of the third month, which 

 he had especially examined in the fresh condition. Foetal brains hardened in alco- 

 hol do, it is true, show contractions, but these are quite different in appearance from 

 these furrows, well seen in their internal and external characters in Plate XXXV, 

 fig. 4, where, through the absence of a part of the wall of the cerebral vesicle, the in- 

 ternal projection can be plainly seen in the portion marked x. Undoubtedly the 

 causes or forces acting to produce these plications are due to the fact that the hem- 

 isphere is growing much more rapidly than the surrounding skull, and the fact that 

 the cerebral sac again becomes smooth proves that later the skull accommodates 

 itself to this increased growth by a more rapid expansion, and, as a consequence, 

 these plications unfold. These foldings may, therefore, be well called the tempor- 

 ary plications or furrows. That they bear precise and definite mathematical rela- 

 tions to the relative growth of brain and skull is shown by their mode of arrange- 

 ment. On the outer surface, as can be seen by referring to Plate XXXV, figs. 2 

 and 3, they radiate toward the middle of the fossa of Sylvius which evidently is 

 the centre of pressure, and it can readily be understood from the dome shaped 

 resisting surface of the interior of the skull that the resolution of pressure 

 forces could produce just such an arrangement of plications when acting 

 upon a hollow-walled plastic body. A longer posterior radiating fissure, Plate 

 XXXV, fig. 3, Pt, has the same direction as the longitudinal axis of the Sylvian 

 fossa, indicating that the same forces are at work, at this time, in the pro- 

 duction of these two depressions. Later on, owing to the increased rapidity of 

 growth of the frontal over the temporal and occipital portions, the lines of strain 

 do not follow the same direction, the axis of the Sylvian fossa assuming a more ob- 

 lique position, as shown in Plate XXXV, fig. 3, where Pt. is the posterior temporary 

 fissure. On the mesial surface, Plate XXXV, fig. 4, the mathematical regularity is 

 even more pronounced, since the conditions are simpler, the resolution of forces tak- 

 ing place along a plane instead of upon an irregularly spheroidal surface. The ac- 

 companying diagram, fig. 2, was made directly from the brain represented in Plate 



XXXV, fig. 4, before mutilation and distortion 

 had occurred. Upon the mesial occipitofrontal 

 surface we find six temporary furrows which 

 converge to a common centre situated at about 

 the centre of the hemisphere, at the point where 

 the cerebral crus passes into the striated body 

 directly opposite or beneath the position of the 

 ~~X point of concentration of the external radiating 

 fissures, situated along the axis of the Sylvian 

 fissure, as can be seen by comparing Plate XXXV, 

 figs. 3 and 4. Moreover these axes of prolonga- 

 tion are inclined to each other at about or very 

 closely approaching our angle of 30°. One of 

 these fissures, axis of prolongation A Y, fig. 2, is perpendicular to A X, the axis of 



