VENOUS SINUSES OF THE CRAXIVM 619 



and, coursing doAvnwards to the posterior inferior angle of that bone, passes either 

 through a foramen in its inner table, or through the mastoid foramen into the lateral 

 sinus. 



The occipital, or parieto-occipital, ramifies chiefly in the occipital l)one, and 

 opens into the (lecijiital vein or into the lateral sinus. 



The diploic veins freely anastomose with one another in the adult; l)Ut in the 

 foetus, before the bones have united, each svstem of veins is distinct. 



2. THE VEXOUS SINUSES OF THE CRANIUM 



The venous sinuses of the cranium are endothelially lined blood-sjoaces, situ- 

 ated between the periosteal and meningeal layers of the dura mater. They are the 

 channels by which the l)lood is conveyed from the cerel>ral veins, and from some of 

 the veins of the meninges and diploe, into the veins of the neck. The sinuses of the 

 base of the skull also carry the chief part of the blood from the orbit and eyeball to 

 the jugular veins. At certain spots the sinuses communicate with the superficial 

 veins by small vessels known as the emissary veins, which run through foramina 

 in the cranial bones. 



The venous sinuses are eighteen in number: six being disposed medianly 

 and singly; six laterally and in pairs. The median and single sinuses are: — (1; the 

 superior longitudinal; (2) the inferior longitudinal; (3) the straight; (4) the 

 occipital; (o) the circular; and (6) the transverse or basilar. The lateral and 

 ])aired sinuses are: — (7) the two lateral; (8) the two superior petrosal; (9) the two 

 inferior petrosal; (10) the two cavernous; (11) the tAvo spheno-parietal (sinus ahe 

 l)arvse), and (12) the two sigmoid — which latter, however, are usually described as 

 part of the lateral. Occasionally there are two additional sinuses (the two petro- 

 squamous), due to the persistence in the adult of what in the fcetus was the con- 

 tinuati(jn of the lateral sinus. 



(1) The superior longitudinal sinus, or superior sagittal sinus (fig. 388), 

 lies in the median groove on tlie inner surface of the calvarium along the attached 

 margin of the falx cerebri. It extends from the foramen caecum to the internal 

 occii)ital i)rotuberance. It grooves from before backwards the frontal l)one, the 

 contiguous sagittal margin of the parietal bones, and the squamous j^ortion of the 

 occipital bone. In the ftx^tus it communicates, tln-ough the foramen caecum, with 

 tlie nasal veins, and generally throughout life with the superficial temporal vein 

 througli the parietal foramen. It is triangular on section, the base of the triangle 

 corresponding to the bone. Crossing it are a number of fibrous bands known as 

 the chorda? ^\'illisii, and projecting into it in places are the Pacchionian bodies. 

 In front the sinus is quite small, but it increases greatly in calibre as it runs back- 

 wards. It receives at intervals the superior cortical cereliral veins and the veins 

 from the falx. The former, for the most part, open into it in the direction opposite 

 to that in which the blood is flowing in the sinus. They pass for some distance in 

 the walls of the sinus befcn-e opening into it. Posteriorly, at the internal nceii)ital 

 protuberance, the superior longitudinal sinus usually turns sharply to the right, 

 and ends in the right lateral sinus; the straight sinus then usually terminates in 

 the left lateral sinus, and the right and left lateral sinuses conmiunicate with each 

 other across the occipital protuberance. Occasionally, however, the superior 

 longitudinal sinus ends in the left lateral sinus, the straight then passing into the 

 right. At other times the posterior end of the superior longitudinal sinus at the 

 internal occipital ])rotuberance becomes slightly dilated, forming what is called 

 the torcular Herophili, or confluence of the sinuses. When this dilatation exists, 

 the straight sinus usually opens into it in front, the two lateral sinuses on either 

 side, the superior longitudinal above, the occijtital sinus or sinuses, when two are 

 present, ])elow. The torcular may conmiunicate with the occipital vein through 

 the occipital emissary vein, which, when present. ])asses through a minute foramen 

 in the occipital protuberance. 



(2) The inferior longitudinal or inferior sagittal sinus (fig. 388) is situated 

 at the free margin of the falx cerebri. Beginning about the junction of the anterior, 

 with the middle third of the falx, it is continued l)ackwards along the concave or 



