THE NERVOUS SYSTEM 1501 



The superior longitudinal sinus corresponds to a line drawn from 

 the glabella to the inion, or external occipital protuberajice. For 

 the most part it occupies the median line, but, as it grooves the 

 upper portion of the tabular part of the occipital bone, it deviates 

 to one side, most commonly the right side. 



The toreular Herophili is usually situated on the right side of the 

 inion. 



The occipital sinus corresponds to a line drawn downwards from 

 the inion. 



The transverse sinus on either side is indicated by a line drawn 

 outwards from a point immediately above, and external to, the inion 

 to a point immediately above the asterion. This line is slightly 

 curved, the convexity being upwards. The sinus grooves the inner 

 surface of one-half of the tabular portion of the occipitaLbone, along 

 the line of attachment of the tentorium cerebelli ; and in the region 

 of the asterion, where the sinus, in altering its course, describes a 

 curve, it grooves the inner aspect of the parietal bone, close to the 

 postero-inferior angle, for a very short distance. 



Inasmuch as the superior longitudinal sinus usually opens into 

 the right transverse sinus, the right sinus is usually larger than that 

 of the left side. 



The sigmoid part of the transverse sinus grooves the inner surface 

 of the mastoid portion of the temporal bone, and the superior 

 surface of the jugular process of the occipital bone. It consists of 

 two limbs and a genu. The superior limb passes horizontally for- 

 wards, and the inferior limb passes downwards and inwards. The 

 genu intervenes between the two limbs, and its convexity is directed 

 forwards. The following line indicates approximately the course 

 of the sigmoid part of the transverse sinus: (i) Draw a line hori- 

 zontally forwards from the asterion for fully | inch to a point on the 

 root of the mastoid process in line with the upper part of the meatus 

 auditorius extemus; (2) the line now curves, and passes down- 

 wards and inwards on the front part of the mastoid process towards 

 its tip for I inch, lying close to the groove between the back of the 

 pinna and the mastoid process (the level to which this line descends 

 is J inch below the lower margin of the orifice of the meatus audi- 

 torius extemus) ; and (3) the line finally passes forwards for h inch to 

 meet the jugular foramen, through which the sigmoid part of the 

 transverse sinus leaves the cranial cavity to become the internal 

 jugular vein. 



Part (i) of the preceding line indicates the superior horizontal 

 limb of the sigmoid sinus; part (2) the inferior descending limb; and 

 part {3) the terminal portion of the sinus passing forwards to the 

 jugular foramen. 



The genu and descending limb of the sigmoid sinus lie directly 

 behind the mastoid or tympanic antrum, being separated from it 

 only by a very thin plate of bone. In pyogenic affections of the 

 tympanum and antrum the genu and descending Umb of the sigmoid 

 sinus are consequently liable to become affected with thrombosis. 



