414 HISTORY OF THE HUMAN BODY 



In other cases localized thin areas push out instead of in and 

 form evaginations of more or less importance in the formation 

 of various organs, which are supplementary to the actual brain. 

 In this way are formed the retina of the eyes, a portion of the 

 hypophysis, and one or two problematic structures arising dor- 

 sally from the diencephalon. 



3. Folding or creasing of a certain part of the wall. This 

 mechanical device means here as elsewhere an increase of sur- 

 face, and hence of physiological efficiency, without a corre- 

 sponding increase in bulk. Its best manifestation is, perhaps, 

 that of the cerebellum, morphologically formed from roof and 

 sides of the metencephalon. In some forms, as in the adult 

 dog-fish, these folds, three or four in number, are seen with 

 the clearness of a diagram; in others, as in adult birds and 

 mammals, the original creases become shallow by a coalescence 

 of the applied surfaces of adjacent folds, but the structure is 

 still marked by the characteristic dendritic arrangement of the 

 white matter, which marks the core of each fold. In some 

 cases a secondary or even tertiary folding is thus marked. 



4. Flexure, or the bending upon itself of the entire longi- 

 tudinal axis of the neural tube (Fig. 116). The possible 

 flexures are three in number, apical flexure, flexure of the pons, 

 and cervical, the two first appearing in birds and quadrupedal 

 mammals, the last found only in Man, caused directly by the 

 erect position and the consequent bending of the skull over an 

 angle of nearly 90. The reason for the other flexures is un- 

 doubtedly the same as in the case of the foldings of the surface, 

 since by folding the parts on themselves a larger brain may be 

 accommodated within the length limits of a given skull. The 

 gradual formation of these flexures may be well seen during 

 the embryonic development of a bird or mammal, preferably, 

 however, in Man, in which alone the third or cervical flexure 

 is involved. They occur in the order of their position, begin- 

 ning anteriorly, the first being dorsal, the second ventral, and 

 the third dorsal again, in accordance with the natural law of 

 folded objects. 



With the above principal in mind, the further history 



