126 PRACTICAL ANATOMY. 



of the external occipital protuberance you will have trouble in removing the cere- 

 bellum. 



To Cut the Calvarium. Saw in the line as indicated in the preceding 

 paragraph. Avoid cutting too deeply in the temporal region. After you have 

 completed the incision with the saw, take a chisel and hammer and test your 

 work. If any places remain uncut, you can easily finish loosening the calvarium 

 with the chisel. The calvarium must be made loose without cutting through the 

 dura mater. Grasp the cut edge of bone above the orbits, and, while an assistant 

 presses down on the chin, pull the calvarium backward ; as it comes off you will 

 hear a noise, produced by the separation of the dura from the bone. On a 

 previous page you have learned that the dura has attachments to the interior of 

 the cranium. 



Now make the following observations : 



1. Note on the inner surface of the calvarium the meningeal grooves for the 

 meningeal arteries. These arteries are between the dura and bone. They furnish 

 blood to both dura and bone. They have nothing to do with the blood-supply 

 to the brain proper. 



2. The Pacchionian bodies and villi (Fig. 82) will be seen on the outer sur- 

 face of the dura mater, in some cases where they have perforated this structure. 

 They produce, by pressure atrophy, the Pacchionian depressions in the inner table 

 of the calvarium. They are a villous product of the arachnoid. Their function 

 is not perfectly understood. They are found in great abundance in the region 

 of the superior longitudinal sinus, and even in the sinus itself. 



3. You will note the great or middle meningeal artery. It consists of 

 several branches, which all spring from one parent stem in the middle fossa of 

 the skull. It lies in the connective tissue that held the dura to the bone. 



4. The superior longitudinal sinus (Figs. 82 and 83) lies in the mid-line. 

 Cut this open from end to end. Take note of the large amount of clotted blood 

 it contains in its posterior half. Make note also of the chordae Willisii in the 

 bottom of the sinus. Clear out the clotted blood, and these cords of Willis will 

 come into view. 



5. The Cerebral Veins. Cut through the dura, with scissors, one inch ex- 

 ternal to the sinus just opened, from before backward, parallel with the sinus. 

 Next make a lateral incision from the first incision to the ear. Do this on both 

 sides. Turn the flaps outward. Turn the. dura up over the median line, and 

 see the veins opening into the sinus from behind forward. 



Origin of the Cerebral Veins. These veins take blood from the part of the 

 brain supplied by the cortical system given off from the circle of Willis. They 

 discharge the blood into the several dural sinuses. 



6. The subdural space is exposed now. It is between the dura and arach- 

 noid. It contains a small amount of fluid for lubrication fluid derived from a 

 source explained in a previous paragraph. This space contains (i) the Pac- 

 chionian bodies ; (2) the veins to the sinuses ; (3) the cranial nerves prior to their 

 emergence. 



7. The falx cerebri (Fig. 82) you will see occupying the great longitudinal 

 tissure between the cerebral hemispheres. Cut its attachment to the crista galli in 

 front, and as you pull it gently backward, cut the remaining veins opening into 

 the sinus. In the free margin of the falx cerebri is the inferior longitudinal 

 sinus. 



8. The arachnoid membrane is the smooth, glistening, transparent structure 

 that you now see covering the brain. It is transparent, for you see the vessels 

 of the pia mater through it. It is smooth on its outer surface, for it is lubricated 

 by the subdural lymph. It is so very delicate you would scarcely suspect its 

 presence. Under it is the subarachnoid space, also containing fluid. 



