NERVOUS CENTRES. (!!UMAN ANATOMY. THE MENINGES.) 



631 



The sinuses of the dura mater. At certain 

 situations, processes of the inner membrane of 

 the venous system are included in canals formed 

 by the separation of the laminae of the dura 

 mater. The channels that are thus formed for 

 the passage of the venous blood do not admit 

 of being dilated beyond a certain size, and in 

 this consists an important peculiarity in the 

 venous system within the cranium. These 

 channels empty themselves into the internal ju- 

 gular vein, which thus forms almost the sole 

 channel by which the venous blood is returned 

 from the brain and its membranes as well as in 

 a great measure from the bones of the skull. 

 And thus is explained the rapid influence which 

 is produced upon the brain by any impediment 

 to the passage of the blood through the superior 

 vena cava. 



It is important to notice that the sinuses 

 communicate with and receive blood from cer- 

 tain external veins which carry blood derived 

 from parts exterior to the cranium. Among 

 these may be enumerated the ophthalmic vein, 

 and several small veins in the neighbourhood of 

 the mastoid and condyloid processes, and in 

 the parietal bones. 



The following sinuses may be described. 



The superior longitudinal sinus. This 

 sinus corresponds to the superior margin of the 

 falx cerebri. It commences very narrow by one 

 or two small veins from the dura mater in the 

 vicinity of the crista galli and cribriform plate 

 of the ethmoid bone. Thence it proceeds back- 

 wards, gradually increasing in calibre, and it 

 terminates a little above the internal occipital 

 protuberance by communicating with a small 

 cavity or reservoir, situated between the layers 

 of the dura mater there, which is called Tor- 

 cular Herophili. If a vertical section of this 

 sinus be made in the transverse direction, it will 

 be seen to be triangular in shape, the apex cor- 

 responding to the falx, the base slightly curvi- 

 linear and lodged in the groove which passes 

 along the median line of the cranial vault. 

 When the sinus is laid open in its length by 

 slitting up its superior wall, we find that its 

 sides are perforated by a great number of mi- 

 nute orifices, which are the openings of veins 

 passing into it from the dura mater and from 

 the brain itself. These veins pass into the 

 sinus chiefly at right angles to it, or in the 

 direction from behind forwards; a few, situate 

 in front, enter the sinus from before backwards. 

 In the interior of the sinus we observe little 

 bands (trabecula of Haller, chords Willisii), 

 stretching across from right to left, connected 

 only with the lateral walls and leaving a free 

 space above and below them. These bands 

 are numerous, and various as regards breadth. 

 Haller has seen them so numerous that they 

 appeared like a septum dividing the sinus into 

 two portions, of which the superior was the 

 larger. 



The walls of the sinus, towards its inferior 

 angle, have frequently a cribriform appearance, 

 which puts on somewhat the aspect of erectile 

 tissue. There is no appearance of valves in 

 the interior of the sinus ; frequently, however, 

 the oblique entrance of a small vein into the 



sinus produces a fold near the venous aperture, 

 which, under the retrograde pressure of the co- 

 lumn of blood, might close the orifice, and 

 probably, when the veins open into the sinus 

 from behind forwards, they may be protected 

 fiom the regurgitation of the blood by this 

 mechanism. 



Several of the small bodies, previously al- 

 luded to by the name of Pacchionian glands, 

 project into the cavity of the sinus through 

 apertures in its wall. They appear as if they 

 had worn their way by pressure and friction 

 through the walls of the sinus, and it is here 

 that the appearance of an erectile structure is 

 most manifest. We cannot suppose that these 

 bodies are bathed in the blood of the sinus, but 

 rather that they push the lining membrane of 

 the sinus befoie them. It has been supposed 

 that these bodies are natural structures destined 

 to perform a mechanical office somewhat on 

 the principle of the ball-valve, but they are 

 frequently absent altogether, and when present 

 they have no constant relation to the venous 

 orifices. 



The inferior longitudinal sinus (sinus infe- 

 rior J'ulcis) is a small vein lodged in the inferior 

 part of the falx cerebri, running parallel to and 

 a little above its inferior margin for about the 

 two posterior thirds of its length. It gradually 

 increases in calibre from before backwards, 

 being formed by tributary veins from the falx ; 

 it opens into the strait sinus. 



The strait sinus corresponds to the middle 

 line, at the place where the falx cerebri unites 

 with the tentorium cerebelli. It seems to be 

 enclosed between the layers of the former. Like 

 the other large sinuses, it presents in its section 

 the form of a triangle, whose base is inferior. 

 Its direction is from before backwards and a 

 little downwards, with a slight degree of curva- 

 ture corresponding to that of the tentorium. It 

 corresponds at its commencement to the space 

 between the posterior reflected portion of the 

 corpus callosum and the quadrigeminal bodies, 

 and here it receives two large veins (vena magn<e 

 Galeni), which carry the blood from the inte- 

 rior of the ventricles, and a third vein, the in- 

 ferior longitudinal sinus. It opens into the 

 conflux of the sinuses or torcular by a round 

 opening or sometimes by two openings, sepa- 

 rated by a slip of fibrous membrane. This 

 sinus likewise receives veins from the inferior 

 surface of the posterior and middle lobes of 

 the brain, and from the superior surface of the 

 cerebellum. 



At the posterior extremity of the straight sinus 

 we find a reservoir somewhat polygonal in 

 shape, which corresponds to the occipital pro- 

 tuberance; this is called the Torcular Hero- 

 phili,* (the press of Herophilus,) the conflux 

 of the principal sinuses of the dura mater; it 

 has six openings, one for the superior longitu- 



* This absurd name might with great advantage 

 be discarded, for it seems quite uncertain what 

 precise part Herophilus intended to apply it to. 

 The term proposed by Cruveilhier is much better, 

 the occipital conflux of the sinuses. Various other 

 names were applied to it formerly, such as Laemia, 

 platea, pelvis, lagwtcitla. 



