8 THE NERVOUS SYSTEM 



elevation immediately above the root of the nose) to the 

 external occipital protuberance ; its lower extremity lies 2 inches 

 vertically above the pre-auricular point, which is situated on 

 the zygomatic process of the temporal bone immediately in 

 front of the tragus of the external ear. The line joining these 

 two points indicates, on the surface of the head, the -position 

 and direction of the central sulcus, and the area which extends 

 for three-quarters of an inch anterior to it overlies the anterior 

 central gyrus. Firm pressure or percussion over this area may 

 produce pain in organic lesions of the motor cortex. 



The Middle Frontal Gyrus lies anterior to the middle third 

 of the anterior central gyrus, from which it is separated by 

 the precentral sulci (Fig. 3). It is said to contain the motor 

 centres for the muscles of the eye. Turner and Ferrier 

 removed this portion of the cortex in monkeys, but, although 

 the operation produced temporary conjugate deviation of the 

 head and eyes towards the side of the lesion, the condition 

 was rapidly recovered from, and the animal regained free 

 control over all the muscles of the eye and the head and neck. 

 Irritative lesions in this region may give rise to deviation of 

 the head and eyes to the opposite side, but the great majority 

 of such lesions give rise to no localising motor symptoms 

 unless they extend backwards and involve the anterior central 

 gyrus. 



Lesions of the frontal lobe, anterior to the precentral sulcus, 

 may give rise to mental symptoms, but these vary so much 

 that they are not of great help in topical diagnosis. Failure 

 of memory, alterations in personal disposition, loss of concen- 

 trative powers, are features which have been noted in some 

 cases. 



The posterior part of the middle frontal gyrus is said to 

 contain the higher centres for written speech. In cortical 

 lesions of this area the patient is unable to write intelligible 

 sentences or words, although he can read and speak quite 

 intelligently and understands what is said to him. 



The Lateral Fissure (of Sylvius) begins on the basal 



