706 



THE NERVOUS SYSTEM. 



devised by which one may localize these parts from an examination 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 sulci to the surface are found to be very 

 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 sulci 

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

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

 out with 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. 



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 (see page 222), 

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



Longitudinal Fissure. This corresponds to a line drawn from the glabella at the root of 

 the nose to the external occipital protuberance. 



The Fissure of Sylvius. The position of the fissure of Sylvius and its horizontal limb is 

 marked by a line starting from a point one inch and a quarter horizontally behind the external 

 angular process of the frontal bone to a point three-quarters of an inch below the most promi- 

 nent point of the parietal eminence. The first three-quarters of an inch will represent the main 

 fissure, the remainder the horizontal limb. The bifurcation of the fissure is, therefore, two 

 inches behind and about a quarter of an inch above the level of the external angular process. 

 The ascending limb of the fissure passes upward from this point parallel to, and immediately 

 behind, the coronal suture. 



Fissure of Rolando. To find the upper end of the fissure of Rolando, a measurement 

 should be taken from the glabella to the external occipital protuberance. The position of the 

 top of the sulcus will be, measuring from in front. 55.6 per cent, of the whole distance from the 

 glabella to the external occipital protuberance. Professor Thane adopts a somewhat simpler 

 method. He divides the distance from the glabella to the external occipital protuberance over 

 the top of the head into two equal parts, and, having thus defined the middle point of the ver- 

 tex, he takes half an inch behind it as the top of the sulcus. This is not quite so accurate as 

 the former method, but it is sufficiently so for all practical purposes, and on account of its sim- 

 plicity is very generally adopted. From this point the fissure runs downward and forward for 3J 



FIG. 376. Relations of the principal fissures and convolutions of the cerebrum to the outer surface of the 

 scalp. (Reid.) 



inches, its axis making an angle of 67 with the middle line. Cunningham states that this angle 

 more nearly averages 71.5. In order to mark this groove, two strips of metal may be employed 

 one, the shorter, being fixed to the middle of the other at the angle mentioned. If the longer 

 strip is now placed along the sagittal suture so that the junction of the two strips is over the 

 point corresponding to the top of the furrow, the shorter, oblique strip will indicate the direction 



