140 TOPOGRAPHICAL ANATOMY OF THE 



cranial vault from the crista galli to the osseous tentorium, and contains 

 the dorsal sagittal blood-sinus. The concave border is free and ver}' 

 thin. Sometimes, indeed, it presents defects in the shape of small 

 holes. 



Dissection. — Cut across the falx cerebri as close as i)ossible to its 

 anterior end and turn it backwards. Veins from the interior of the 

 brain v ill be noted joining the system <if blood-sinuses in the region of 

 the osseous tentorium. Deep veins unite to form the straight sinus, and 

 this in turn joins the sagittal sinus at tlie confluence of the sinuses. 



Next cut through tlie tentorium cerebelli — a fold of the dura mater 

 inserted into the cleft between the cerebral hemispheres and the cere- 

 bellum — on one side of the head a little distance from the median plane. 



Now proceed to remove the brain. This is facilitated by placing the 

 head in a vertical position with the nose resting on the table. Introduce 

 the handle of a knife under the medulla oblongata and gently raise the 

 brain from the base of the cranium. Tiie nerves and vessels associated 

 with the base of the brain are thus put on the stretch, and must be 

 severed one by one. In cutting the cerebral nerves it is well to divide 

 them close to the brain on one side and clos3 to the dura mater on the 

 other. The last eight cerebral nerves are fairly close together, but may l»e 

 distinguished by differences in size and mode of exit from the cranium. 

 The twelfth nerve leaves alone by the hypoglossal foramen. Then comes 

 a group formed by the eleventh, tenth and ninth, which leave the* 

 cranium together by the jugular foramen. The eighth and seventh enter 

 the internal acoustic meatus in company. Tiie sixth, a small nerve, and 

 the fifth, the largest of all the cerebial nerves, pierce the dura mater 

 close together at the projecting edge of the temporal bone. The fourth, 

 the smadest cerebral nerve, winds round tlie lateral border of the cereljral 

 peduncle ; and the third nerve leaves the ventral surface of the peduncle. 

 The next structure that interferes with the removal of the brain is 

 the infundibulum, a hollow, median connection between the tuber 

 cinereum and the hypophysis. As it is not advisable to attempt the 

 removal of the hypophysis from the sella turcica in which it is lodged, 

 the infundibulum siiould be cut through. 



Just about the same level as the infundibulum, and at no great 

 distance from the median plane, are the two internal carotid arteries. A 

 little fartlier forwards, and also not far from the median plane, are the 

 optic or second cerebral nerves. 



The last connection of the brain with the wall of the cranium is that 

 produced by the olfactory nerves as they pierce the ethmoid bone. It is 

 generally very difficult to extract the olfactory bulbs, from which the 

 nerves arise, without injury. 



If the brain, when removed, is not sufficiently well hardened to allow 

 ' of satisfactory dissection, it should be placed in a 5 per cent, solution of 

 formaldehyde. In any case its dissection is better postponed. 



The dura mater and other structures at the base of the cranium 

 should now be examined. The dura mater where it covers the basal 

 bones of the cranium, is removed with difficulty. Not only is it 

 adherent to the bones : it also forms sheaths for the various cerebral 

 nerves, and is continuous, through the foramina of the skull, with the 

 periosteum of the exterior. The falx cerebri, one of the three folds of 

 the dura, already having been examined, there remain the other two, 

 namely, the tentorium cerebelli and the diaphragma sellse turcica^. 



