THE SUPERIOR CAVAL SYSTEM. 869 



Tributaries. — The lateral sinuses, in addition to the sinuses which communicate with them 

 at the torcular Herophili, receive the following tributaries, most of which will be described in 

 greater detail later: {a) the posterior inferior cerebral veins, which pass backward from the 

 temporo-sphenoidal regions of the cerebral hemispheres ; {d) some of the inferior cerebellar veins ; 

 {c) the superior petrosal sinus, this latter communicating with it just where it leaves the line of 

 attachment of the tentorium cerebelli. Into the sigmoid portion there open [d) the internal au- 

 ditory veins (vv. auditivaeinternae ), which issue from the internal auditory meatus ; [e) the mastoid 

 emissary vein (page 876) ; and (/) some of the veins of the medulla oblongata and pons. 



Variations. — Considerable variation exists in the relative sizes of the right and left lateral 

 sinuses, in accordance as the superior longitudinal sinus opens more or less directly into one 

 or the other. As stated, the tendency is for the superior longitudinal to open into the right 

 lateral ; quite often, however, it opens into the left, and occasionally it may communicate equally 

 with both. In 100 crania, Riidinger found that the right lateral sinus was the larger in 70 cases, 

 the left in 27, and the two were equal in size in only 3 cases. 



The horizontal portion of the left sinus has been observed to be lacking or reduced to an 

 exceedingly fine channel, and one or both of the sinuses have been observed to pass through a 

 greatly enlarged mastoid foramen to open into the posterior auricular vein, the sigmoid sinus 

 being represented only by a very small channel. 



In a considerable number of cases a small sinus, known as the pch-o-se/uamosa/ shins, opens 

 into the lateral just as it bends downward and inward upon the mastoid portion of the temporal 

 This sinus passes downward over the anterior surface of the petrous portion of the temporal, 

 along the line of its junction with the squamous portion, and occasionally passes through a 

 foramen — the foramen jugulare spurium — which opens to the exterior just behind the articular 

 eminence of the zygomatic process. The sinus represents the original terminal portion of the 

 lateral sinus, the sigmoid portion of that sinus being a secondary formation, and opened after 

 its exit from the foramen jugulare spurium into the internal jugular, although its connection in the 

 adult is with the temporal vein. 



Practical Considerations. — By reason of its proximity to the middle ear, 

 mastoid antrum and cells, the sigmoid por'tion of the lateral sinus is more often the 

 subject of throinbosis than any other sinus (page 1509). This may arise in the 

 following six ways, mentioned in the order of frequency, the first outnumbering all the 

 others : (i) Extension from chronic purulent inflammation of the middle ear; (2) 

 extension of acute inflammatory disease from the mouth, pharynx, and tonsils into 

 the middle ear, antrum, and cells ; (3) extension of thrombosis from other sinuses, 

 especially the so closely associated superior petrosal ; (4) trauma, such as fracture of 

 the base extending through the middle ear to the sinus ; (5) pressure of tumors or 

 discharge associated with them ; (6) infection from septic wounds of the head, neck, 

 or mastoid region (Macewen). 



The anatomical symptoms of thrombosis of this sinus may be due to (a) obstruc- 

 tive distension of the superficial veins communicating with the sinus, chiefly the 

 mastoid vein {^q.v.') ; (b) mastoid inflammation (osteitis) resulting from contiguity 

 and from the venous connection ; (e) phlebitis of the veins communicating with the 

 sinus, especially the internal jugular (page 863), condyloid (page 876), and, occasion- 

 ally, the mastoid. 



The subject of sigmoid sinus thrombosis is further considered in relation to the 

 mastoid (page 1508). 



The k?iee (genu) of the sigmoid portion of the lateral sinus extends further 

 inward and forward on the right side than on the left, and this fact, together w ith the 

 larger size of the right lateral sinus as compared with the left, aids in explaining the 

 greater frequency of sinus thrombosis, septic meningitis, and cerebral abscess as 

 sequelae of otitis media on the right side (page 1509). The infection is carried 

 by the veins which connect the mastoid cells and antrum with the genu of the 

 sigmoid sinus. 



On the surface the top of the curve represented by the horizontal and descending 

 (sigmoid) portion of the lateral sinus should correspond to a point (asteric) 2.5 cm. 

 above and 3.8 cm. {1% in.) behind the centre of the auditory meatus. This is 

 about the infero-posterior parietal angle. The superior limit of the horizontal portion 

 of the sinus is represented by a line from this asteric point to 3.8 cm. ( i ^ in. ) above 

 the inion. The superior and anterior boundary of the sigmoid portion is indicated 

 by a line from the same point curving downward and forward along the skin 

 groove at the auriculo-mastoid junction to a little below the level of the external 



