REMOVAL OF THE BRAIN 103 



sinus, and four triangular flaps. The flaps must be turned down- 

 wards over the cut margin of the skull, as on the left side in Fig. 

 32. In that position they cover the sharp edge of the bone 

 and prevent laceration of the brain during its removal. 



Cavum Subdurale. The term subdural space is applied 

 to the interval between the dura mater and the arachnoid 

 Figs. 29 and 31. It contains a very small quantity of serous 

 fluid, which moistens the opposed surfaces of the membranes. 

 A striking contrast between the two surfaces of the dura 

 mater will be noted. The external surface is rough and 

 flocculent ; the internal surface is smooth and glistening. 



Venae Cerebri. After the dura mater is reflected, the 

 cerebral veins which return the blood from the surface of 

 the cerebral hemispheres can be seen shining through the 

 arachnoid. They are lodged, for the most part, in the sulci 

 between the gyri of the brain, and those at present visible 

 run upwards to the median plane. When they reach the 

 superior sagittal sinus they turn forwards, and lie against the 

 wall of the sinus, for some distance, before they open into it. 



Dissection. Open into the superior sagittal sinus by running 

 a knife through its upper wall, from behind forwards, Figs. 31 

 and 32. 



Sinus Sagittalis Superior (O.T. Superior Longitudinal). 



The superior sagittal sinus begins, anteriorly, at the crista 

 galli of the ethmoid bone, where it not infrequently com- 

 municates with the veins in the nasal cavity through the 

 foramen caecum. It extends backwards, grooving the cranial 

 vault in the median plane, to the internal" occipital protuber- 

 ance, on the right aspect of which it becomes continuous 

 with the right transverse sinus (O.T. lateral). Its lumen, 

 which is triangular in cross-section, is very small anteriorly, 

 but expands greatly posteriorly. On each side of the sinus, 

 and opening into it, are a number of clefts between the two 

 layers of the dura mater ; they are the lateral lacuna. The 

 inferior angle of the sinus is crossed by a number of minute 

 bands, named chordcz Willisii\ and arachnoideal granulations 

 bulge into it. The mouths of the superior cerebral veins 

 open into the sinus, or into the lateral lacunae, pouring their 

 blood into the sinus in a direction contrary to that in which 

 the blood flows within the channel that is, the terminal 

 portions of the veins are directed forwards, whilst the blood 

 in the sinus flows backwards, 

 in 7 c 



