THE MEMBRANES OF THE BRAIN. 31 



right side. The fact that it is frequently separated from the mastoid cells by only a thin plate 

 of bone (see page 59) is of great practical importance, since the sinus not infrequently becomes 

 involved (sinus thrombosis) from suppuration extending into the mastoid cells from the middle 

 ear. The external guide for the sinus is a point on the mastoid process, two centimeters behind 

 the posterior margin of the external auditory meatus. The posterior border of the mastoid 

 process is frequently employed as a landmark, but it cannot be utilized in all cases, as the varying 

 size of the bony prominence and its digastric groove of inconstant depth may give rise to error. 

 It may cause the operator to go too far forward and consequently to strike the petrous portion of 

 the temporal bone. [The term "lateral sinus" should be restricted to that portion of the sinus 

 between the external occipital protuberance and the posterior inferior angle of the parietal bone ; 

 the remainder of the sinus, to the jugular foramen, is known from its tortuous course as the 

 sigmoid sinus. According to Wolsey, the genu of the sigmoid sinus the part most frequently 

 involved in middle-ear inflammations is convex forward, reaches to a point | to ^ of an inch 

 behind a coronal plane through the posterior border of the external osseous meatus, and is on 

 the level of the upper part of the meatus. An opening may be made into the genu at a point 

 half an inch behind the posterior wall of the bony meatus between the levels of its roof and 

 floor, where the more superficial part of the sinus is within \ or even \ of an inch of the surface 

 and is therefore placed much less deeply than the antrum. ED.] 



3. The third of the most important sinuses is the cavernous sinus, situated to either 

 side of the body of the sphenoid bone [continuous with the ophthalmic veins in front, dividing 

 into superior and inferior petrosal sinuses behind, and communicating by means of the 

 circular sinus. ED.]. It gains additional interest since it surrounds the internal carotid artery. 

 As it is separated by only a very thin plate of bone from the sphenoidal air-cells communicating 

 with the nasal cavity (see Fig. u and page 53), we can understand why fatal hemorrhage from 

 the nose has been observed as the result of rupture of the sinus and carotid artery after fractures 

 of the base of the skull. An injury to the carotid artery in the sinus, which is also possible from 

 a penetrating wound through the sphenoidal fissure, may lead to the formation of an arterio- 

 venous aneurysm and to a subsequent stasis in the superior ophthalmic vein of the orbit (see 

 page 46) which empties into the sinus through the sphenoidal fissure. This symptom, known as 

 pulsating exophthalmos, is due to the fact that the pulsations of the carotid artery are trans- 

 mitted to the engorged venous spaces and displace the eyeball anteriorly. 



[Cavernous sinus thrombosis most frequently follows inflammatory affections of face or 

 upper lip, the extension taking place through facial, nasal, and ophthalmic veins. Thrombosis 

 also occurs but less frequently through the supraorbital or frontal veins, through the spheno- 

 parietal sinus, and from the pterygomaxillary region through the vein of Vesalius. This 

 sinus is placed between the two layers of dura mater, is intimately related to the Gasserian 

 ganglion, and may, during operations on the latter, become a source of some danger and great 

 annoyance from bleeding, sometimes preventing the completion of the operation. ED.] 



The other sinuses are : The inferior longitudinal sinus, in the concave lower margin of the 

 falx cerebri; the straight sinus, at the attachment of the falx cerebri to the tentorium; the superior 

 and inferior petrosal sinuses, along the superior and posterior borders of the petrous portion of 

 the temporal bone respectively; the circular sinus, composed of the two transverse venous con- 



