4 LANDMARKS MEDICAL AND SURGICAL. 



dimensions of the great cerebral masses. The prominence 

 of the frontal and parietal ' eminences ' and of the occipital 

 region may be taken as a general indication of the develop- 

 ment of the corresponding lobes of the brain. To ascertain 

 the relative proportions of these three regions, let a thread be 

 passed from one meatus auditorius to the other, across the 

 frontal, parietal and occipital eminences respectively. 



Frontal sinuses. The ' frontal sinuses ' formed by the 

 separation of the two tables of the skull vary much in size in 

 different persons and at different periods of life. This fact 

 has an important bearing on wounds in the forehead and on 

 trephining in this situation. These ' bumps ' do not exist in 

 children, because the tables of the skull do not begin to 

 separate before puberty. From an examination of many 

 skulls in the Hunterian Museum, I find that the absence of 

 the ' bumps,' even in middle age, does not necessarily imply 

 the absence of the sinuses, since they may be formed by a 

 retrocession of the inner wall of the skull. In old persons, as 

 a rule, when the sinuses enlarge, it is by the encroachment of 

 the inner table on the brain case. The inner wall of the skull 

 here follows the shrinking brain. It is, therefore, important 

 to bear in mind that an adult, and more especially an elderly 

 person, may have a large frontal sinus without any external 

 indication of it. 



Neither does a very prominent bump necessarily imply 

 the existence of a large sinus, or indeed of even a small one. 

 The ' bump ' may be a mere heaping up of bone, a degra- 

 dation, as in some Australian skulls. 



xviastoid process. The mastoid process, which can be 

 felt behind the ear, contains air-cells, to which the above 

 observations may also be applied. 



Occipital protuberance. The occipital protuberance, 

 and the superior curved line, can be distinctly felt at the 

 back of the head. The protuberance is always the thickest 

 part of the skull-cap, and more prominent in some than in 

 others. 



The posterior inferior angle of the parietal bone, grooved 

 by the lateral sinus, is on a level with the zygoma, and a 



