THE HEAD. 5 



trifle more than one inch behind the front border of the 

 mastoid process. 



Lines of cerebral sinuses. A line drawn over the 

 head from the root of the nose to the occipital protuberance 

 corresponds with the superior longitudinal sinus. Another 

 line drawn from the occipital protuberance to the front border 

 of the mastoid process corresponds with a part of the lateral 

 sinus. 



Middle meningeal artery. The trunk of the middle 

 meningeal artery runs along the front lower corner of the 

 parietal bone, about one inch and. a half behind, and half an 

 inch above, the external angular process of the frontal. 



A straight line drawn from the front of one mastoid 

 process to the other would pass through the middle of the 

 condyles of the occiput, showing how nearly the skull is 

 balanced on the top of the spine in the erect posture. 



6. Thickness of skull-cap. The average thickness of 

 the cap of an adult skull is about 1 of an inch. The thickest 

 part is at the occipital protuberance, where it is often of an 

 inch or more, even in an otherwise thin skull. The thinnest 

 part is at the temple, where it may be almost as thin as 

 parchment. Everyone in the habit of making post-mortem 

 examinations knows how much the skull-cap differs in thick- 

 ness in different persons and in different parts of the same 

 skull. In old persons it is often in some parts not thicker 

 than a shilling, owing to absorption of the diploe. Another 

 point of interest is that the inner plane of the cap is not 

 always parallel with the outer. Hence, in applying the 

 trephine this is not a bad rule i Think that you are operating 

 on the thinnest skull ever seen, and thinner in one half of 

 the circle than the other.' 



7. Levels of the brain. The level of the anterior lobes 

 in front corresponds with a straight line drawn across the 

 forehead, just above the eyebrows. The lower level of the 

 anterior and middle lobes of the cerebrum corresponds with 

 a line drawn from the external angular process of the frontal 

 bone to the upper part of the meatus auditorius. Another 

 line drawn from the meatus to the occipital protuberance 



