. The anatomy of the central nervous system of man and of vertebrates in general. Neuroanatomy; Central Nervous System. THE FOKM-EELATIONS OF THE HUMAN BKAIN. 203 The brain that we ai'e considering is still smooth on the outer surface and just :slightly differentiated on the mesial surface by the fissura arcuata. Now, however, about the beginning of the third month, there is presented a beautiful confirmation of the proposition, previously stated, namely: that the fissures of the brain result from the difference in growth between the roof of the skull and the developing fundaments within the b
. The anatomy of the central nervous system of man and of vertebrates in general. Neuroanatomy; Central Nervous System. THE FOKM-EELATIONS OF THE HUMAN BKAIN. 203 The brain that we ai'e considering is still smooth on the outer surface and just :slightly differentiated on the mesial surface by the fissura arcuata. Now, however, about the beginning of the third month, there is presented a beautiful confirmation of the proposition, previously stated, namely: that the fissures of the brain result from the difference in growth between the roof of the skull and the developing fundaments within the brain. Fissures appear only in Primates, the brains of which, as is known, attain the greatest expansion, which fissures are arranged in the form of a fan on the inner and outer side of the brain. They converge toward the base of the skull and, varying in number and formed essentially on the mesial wall, have exactly the direction that would be expected and required if the brain met with compression or pressure during the expansion of its mantle. Some time during the course of the fourth month, simultaneously with the de- velopment of the fibers of the corpus eallosum between the hemispheres, these pri- mary fissures disappear, and at the beginning of the fifth month the entire hemisphere is again smooth. These transitory fissures have never been found in other mammals, Tjut I may communicate to you the interesting fact that, under certain circumstances, in cases with an abnormal rate of development, fissures having a similar direction. Fig. 136.—Transitory fissures of the brain. Brains of fetuses of the eleventh and thirteenth weeks. (After Cunningham.) ?cover the surface of the adult brain. Purely mechanical disturbances lead to such radial Assuring, as the example illustrated by Fig. 137 at once shows. In several of the lower mammals also, as in the Marsupials, such radially placed fissures are here and there found on the brain. Although the transitory fissures have commonl
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