870 HUMAN ANATOMY. 



auditory meatus. Here the sinus turns inward and forward to reach the jugular 

 foramen and has no further close relation to the lateral cranial wall. A curved line 

 drawn 1 2 mm. ( % in. ) below the horizontal and behind the vertical portions of the 

 curved line last described represents approximately the inferior and posterior 

 boundary of the sinus. The width thus indicated — a half inch — varies ; it is usually 

 greater in the descending part of the sinus. So, too, the space intervening 

 between the genu and the posterior wall of the external auditory meatus may vary 

 from 2-12 mm. 



The direction of the sinuses is also indicated (Macewen) by a line from the 

 upper edge of the external meatus to the asterion, and by one from the tip of the 

 mastoid to the parieto-squamo-mastoid junction, the latter corresponding to the 

 midportion of the sinus, or that most often involved in middle-ear disease. The 

 region of danger in trephining is enclosed (Birmingham) by two lines, one from a 

 point 3.3 cm. (1}^ in.) above and 3.8 cm. {i}i in.) behind the centre of the 

 external auditory meatus to a point 12 mm. (}4 in.) above the inion ; the other 

 from a point 3. 8 cm. (i}4 in. ) behind the meatus and on the same level to a point 

 12 mm. {}4 in.) below the inion. The sinus almost never overpasses these limits 

 in either a downward or an upward direction, and hence the trephine or chisel may 

 be safely applied either below or above these lines. 



Fracture of the base of the skull may extend into the lateral sinus, in which 

 case the blood may pass outward into the tympanum and thence by way of the Eusta- 

 chian tube to the pharynx, or — if the tympanic membrane is torn — may find exit, 

 mingled with cerebro-spinal fluid, at the external auditory meatus (page 1505). 



For further remarks on the practical relations of this important sinus, see 

 page 1508. 



2. The Superior Longitudinal Sinus. — The superior longitudinal sinus 

 (sinus sagittalis superior) (Fig. 756) is an unpaired sinus which lies along the line of 

 attachment of the falx cerebri to the cranial vault. It begins blindly anteriorly by a 

 small vein-like portion which lies in the foramen caecum between the frontal and 

 ethmoidal bones, but soon becomes a true sinus which passes upward and backward in 

 the median line of the frontal bone, beneath the sagittal suture of the parietals, and down 

 the median line of the squamous portion of the occipital to terminate at the internal 

 occipital protuberance by opening into the torcular Herophili, or, usually, more or 

 less directly into the right lateral sinus. 



The sinus is triangular in section and increases gradually in size from before back- 

 ward, measuring about 1.5 mm. in diameter at the level of the apex of the crista 

 galli and 1 1 mm. at its termination. Its lumen is usually traversed by numerous 

 irregular bands of connective tissue known as chordce Willisii, and frequently, espe- 

 cially in aged persons. Pacchionian bodies, which are numerous along its course, 

 project into it (Fig. 1039), 



Tributaries. — In the foetus and in early childhood the superior longitudinal sinus communi- 

 cates with the veins of the nasal cavity through the foramen caecum, but this connection is dissolved 

 in the adult. In addition, it communicates with the neighboring blood-lakes and through these 

 with the meningeal veins, and receives {a) branches from the adjacent portions of the dura 

 mater \ [b) the superior cerebral veins, from ten to fifteen in number (page 877) ; and (r) diploic 

 veins, some of which traverse the parietal bone and constitute emissary veins, the most noticeable 

 of these being one which traverses the parietal foramen (page 876). 



Variations.— The superior longitudinal sinus varies considerably in size and is occasionally 

 exceedingly small, the tributaries which normally open into it passing downward in the falx 

 to open into the inferior longitudinal sinus. It has been observed to diyide into two trunks 

 throughout a portion of its course, and also to divide at the apex of the occipital bone into two 

 trunks which followed the lines of the lambdoid suture to open into the lateral sinuses. Usually, 

 as .stated, the sinus communicates more or less directly with the right lateral sinus, but occasion- 

 ally it may bend to the left of the internal occipital protuberance and open into the left lateral. 



Practical Considerations. — The superior longitudinal sinus may become 

 infected {a) from the scalp through the diploic veins ; {b') from foci of cerebral or 

 meningeal disease through the contiguous blood-lakes or through the cerebral veins ; 

 (^) in childhood from the nose through the veins traversing the foramen caecum. 



