The Cerebrum. 435 



The cortical arteries supplying the cortex of the brain, 

 ramify in the subarachnoid space and then divide into 

 smaller vessels which lie in the pia mater. Leaving the pia 

 mater, at right angles, these cortical arteries dip into the 

 cerebral tissue. There is a slight amount of anastomosis 

 permitted in these terminal branches of the cortical arter- 

 ies, and it is along the perivascular lymphatic channels of 

 these vessels that pathogenic germs spread from infected 

 foci, and thereby cause localized abscesses of the brain. 



Landmarks for the Brain. The lower level of 

 the cerebrum corresponds to a horizontal line drawn 

 across the forehead, through the glabella, to a point half an 

 inch above and behind the external angular process. From 

 this point draw a curved line to a point, one inch behind 

 and half an inch above the external auditory meatus, and 

 thence to the upper part of the external occipital protu- 

 berance. The latter portion of this line represents both 

 the transverse fissure and the attachment of the'tentorium 

 cerebelli to the skull, while the longitudinal fissure corre- 

 sponds to a line drawn over the vertex from the glabella to 

 the occipital protuberance. Reid's base line (Fig. 14) is the 

 line drawn backwards from the lower border of the orbit 

 through the middle of the external meatus. The fissure 

 of Sylvius. Draw a line from a point one and a quarter 

 inches behind the external angular process of the frontal 

 bone, to a point three-quarters of an inch below the most 

 prominent part of the parietal eminence. Three-quarters 

 of an inch behind the beginning of this line, i.e., (two 

 inches from the external angular process, draw a line, one 

 inch in tength, at right angles to the line for the fissure, 

 this represents the ascending limb. The portion of the 

 line in front of this point for the ascending limb is the 

 main fissure, while that behind it, is the horizontal limb. 

 Fissure of Rolando. Draw upwards, two parallel and 

 vertical lines from Reid's base line one immediatelv in 



