790 



THE NERVOUS SYSTEM. 



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. 



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



Superior frontal fissure. 



End of calloso- 

 marginal fissure.^ 



Inferior frontal 

 lobe. 



Parieto- __ _ 



occipital 



fissure. 



Fissure of 

 Sylvius. 



FIG. 475. Drawing to illustrate cranio-cerebral topography. 

 Professor Cunningham. 



(Macalister.) Taken from a cast prepared by 



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 3$ 

 inches, its axis making an angle of 61 with the middle line. 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 and 3| inches will mark the length of the furrow. Dr. Wilson 

 has devised an instrument, called a cyrtometer, which combines the scale of measurements for 



