964 



THE NER VE SYSTEM 



But this development of mature nerve fibres does not occur simultaneously throughout the brain, 

 but step by step in a definite order of succession; equally important bundles are myelinated 

 simultaneously, but those of dissimilar importance develop one after another in accordance 

 with Flechsig's law. The successive myelinization of fibre bundles to and from the cerebral 

 cortex corresponds to the successive awakenings of mental activities and faculties in the growing 

 child. Flechsig's method of investigation has been of great service in the elucidation of the 

 problems of cerebral localization. 



Craniocerebral Topography. The position of the principal fissures ana convolutions of the 

 cerebrum and their relation to the outer surface of the scalp (Fig. 673) have been the subject of 

 much investigation, and many systems have been devised by which one may localize these parts 

 from an exploration of the external surface of the head. 



These plans can only be regarded as approximately correct for several reasons; in the first 

 place, because the relations of the convolutions and fissures to the surface are found to be quite 



FIG. 715. Drawing to illustrate craniocerebral topography. (Taken from a cast in the Museum of the Royal 

 College of Surgeons of England, prepared by Professor Cunningham.) 



variable in different individuals; secondly, because the surface area of the scalp is greater than 

 the surface area of the brain, so that lines drawn on the one cannot correspond exactly to fissures 

 or convolutions on the other; thirdly, because the fissures and convolutions in two individuals 

 are never precisely alike. Nevertheless, the principal fissures and convolutions can be mapped 

 out w T ith sufficient accuracy for all practical purposes, so that any particular convolution can be 

 generally exposed by removing with the trephine a certain portion of the skull's area. An 

 excellent method is given by Chipault in his Chirurgie operatoire du systeme nerveux, 1894, vol. i. 

 The following systems have been the longest in vogue: 



The various landmarks on the outside of the skull, which can be easily felt, and which serve 

 as indications of the position of the parts beneath, have been already referred to, and the rela- 

 tion of the fissures and convolutions to these landmarks is as follows: 



Intercerebral or Longitudinal Fissure. This corresponds to a line drawn from the 

 glabella at the root of the nose to the external occipital protuberance. 



