THE FOHM-EELATIONS OF THE HUMAN BEAIN. 203 



The brain that we are considering is still smooth on the outer surface and just 

 slightly differentiated on the mesial surface by the fissura areuata. 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 callosum 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, 

 but 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 commonly disappeared by the 

 fifth month, a certain tendency to the development of similar fissures still 

 remains. The fissura parieto-oceipitalis develops very early (see Fig. 133) 

 in exactly the course of such a transitory fissure which previously had a 

 similar direction; and on the outer side of the brain there is found— in 

 apes, at least — a continuation probably passing out from it, the fissura per- 

 pendicularis ext. The fissura calcarina, which is demonstrable very early, 

 also lies in the direction of this old transitory fissure. 



Now, however, probably toward the end of the fifth month, there begins 

 the development of those fissures which we have previously become familiar 



