ABSCESS OF THE BEAIN. 179 



the orbital roof. These sinuses are of insignificant size until 

 the sixth or seventh year of life. A very thin plate of bone 

 separates the fully developed sinus from the frontal lobe of the 

 brain, so that caries and perforations of its inner wall may 

 cause extradural, subdural or intracerebral abscess. In the 

 latter case the brain membranes are often closely adherent to 

 the perforated bone, but in some instances there is no perfora- 

 tion and no adhesion, the infection having been carried by the 

 lymphatics or veins of the diploe. 



The anterior and posterior ethmoidal cells are separated 

 from the anterior fossa of the skull by the ethmoidal edge of 

 the frontal bone, but occasionally a posterior ethmoidal cell, 

 extending backwards, intervenes between the roof of the 

 sphenoidal sinus and the base of the brain. It is easy to see 

 that infection of the brain membranes and the brain might 

 originate from these cells. 



The roof of the sphenoidal air sinus lies in contact with the 

 olfactory peduncles, the optic commissure, the pituitary body 

 and the front part of the pons Varolii. Infection is not often 

 transmitted through it. 



An abscess in the temporo-sphenoidal lobe is so situated 

 that it may cause motor paralysis of either cortical or capsular 

 type. By pressure on the adjacent lower end of the ascending 

 frontal convolution or by the spread of inflammatory oedema to 

 that region, convulsions or paralysis may be produced, and in 

 each instance the face may be expected to suffer first, then the 

 arm, and lastly the leg, owing to the arrangement of motor 

 centres in that order from below upwards. (Fig. 14, p. 183.) 

 Any anaesthesia which is present will be of the patchy and 

 incomplete cortical type. 



By pressure exerted inwards, and by the inward spread of 

 inflammation, the internal capsule becomes involved in its 

 posterior part. The motor paralysis is then of capsular type, 

 and the leg, the fibres of which lie most posterior of the motor 

 fibres, will suffer first. (Fig. 12.) Since the posterior part of the 

 capsule is involved, hemiansesthesia and hemianopia may occur. 



122 



