106 HEAD AND NECK 



the handle of the knife. As the olfactory bulbs are raised the 

 minute olfactory nerves, which perforate the cribriform plate of 

 the ethmoid bone and pass to the bulbs, will be torn across. The 

 large, round and white optic nerves (second pair of cerebral nerves) 

 then come into view, as they pass towards the optic foramina. 

 Divide the optic nerves and the internal carotid arteries will be 

 exposed. More posteriorly, in the median plane, the infundi- 

 bulum will be seen ; it is a hollow conical process which extends 

 from the tuber cinereum, at the base of the brain, to the hypo- 

 physis (O.T. pituitary body), which lies in the fossa hypophyseos 

 (O.T. pituitary fossa). Divide the carotid arteries and the 

 infundibulum. Posterior to the infundibulum is the upper 

 border of the dorsum sellae, terminating on each side in the 

 rounded posterior clinoid process. Passing forwards, on each 

 side of the dorsum sellae, is the corresponding oculomotor nerve, 

 which must not be touched at present. A little more laterally, 

 and on a slightly lower plane, is the free border of the tentorium 

 cerebelli. The tentorium cerebelli is a fold of the inner layer of the 

 dura mater which lies above the cerebellum and forms the roof 

 of the posterior fossa of the cranium (Figs. 34, 35). 



Carefully displace the temporal pole of the brain from under 

 cover of the posterior border of the small wing of the sphenoid, 

 which lies to the lateral side of the optic nerve and the cut end 

 of the internal carotid artery ; then raise the temporal lobe 

 from the floor of the middle fossa, and from the upper surface 

 of the tentorium cerebelli, and note a thick stalk the midbrain 

 ascending from the posterior fossa. Push the knife back- 

 wards, along the side of the midbrain, immediately above the 

 level of the oculomotor nerve, and cut through the midbrain, 

 from its lateral surface inwards to the median plane, slanting the 

 knife so that it is in the same plane as the surface of the tentorium 

 cerebelli. Repeat the operation in the same way on the opposite 

 side ; then turn the hemispheres backwards, divide the great 

 cerebral vein, immediately behind the cut midbrain, and remove 

 the cerebrum and upper part of the midbrain from the cranium. 



Place the removed cerebrum in the vault of the cranium and 

 lay it aside. Then note the relative positions of the parts exposed. 

 Anteriorly lies the floor of the anterior fossa of the cranium ; 

 behind it, on a more depressed plane, the middle fossa, and still 

 more posteriorly the sloping tentorium cerebelli. 



In the median plane anteriorly is the projecting crista galli, 

 partially dividing the anterior fossa into halves. On each side 

 of the crista galli is the depression from which the olfactory 

 bulb was dislodged, and still more laterally are the portions of 

 the floor of the anterior fossa which form the roofs of the orbits ; 

 they bulge upwards as well-marked convexities. Each lateral 

 part of the floor of the anterior fossa terminates posteriorly in 

 a sharp margin formed by the posterior border of the small wing 

 of the sphenoid. That margin overhangs the anterior part of the 

 middle fossa. It is covered with a thickening of dura mater 

 in which runs the spheno -parietal blood sinus, and it terminates 

 medially in a projecting process, the anterior clinoid process. 

 On the medial side of each anterior clinoid process lie the corre- 

 sponding optic nerve and internal carotid artery, and springing 

 from the upper surface of the artery is its ophthalmic branch, 

 which runs forward under cover of the" optic nerve. Posterior 



