208 HEAD AND NECK 



of the base of the cranium, and perhaps carry with them the olfactory 

 bulbs. Should they remain in position, however, gently raise them with 

 the ringers, and at the same time separate the olfactory bulbs from the 

 cribriform plate of the ethmoid with the handle of the knife. As the 

 olfactory bulbs are raised the minute olfactory nerves which spring from 

 them and perforate the cribriform plate of the ethmoid bone are torn across. 

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

 come into view, as they leave the cranial cavity through the optic foramina. 

 When these are divided, the internal carotid arteries will be exposed, and 

 more posteriorly, in the median plane, the infundibulum will be seen ; it 

 is a hollow conical process which extends from the tuber cinereum at the 

 base of the brain to the hypophysis (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 anteriorly on each side of the dorsum sellse is the 

 corresponding third cerebral nerve, which must not be touched at present. 

 A little more laterally, and on a slightly lower plane, is the medial or free 

 border of the tentorium cerebelli, 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. 



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. Pass the knife posterior to the 

 dorsum sellse, immediately above the level of the third cerebral nerve, and 

 cut through the midbrain, as it ascends from the posterior fossa, from its 

 lateral surface inwards to the median plane. Repeat the operation in the 

 same way on the opposite side, and remove the cerebrum and upper part of 

 the midbrain from the cranium. 1 



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 lateral halves. On either 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. This 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 corresponding optic nerve and internal carotid artery, and 

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

 which runs anteriorly under cover of the optic nerve. Posterior to the 

 divided ends of the internal carotid arteries, and in the median plane, is the 

 infundibulum descending into the hypophyseal fossa, and more posteriorly, 

 on either side, are the projecting posterior clinoid processes. The area 

 between the four clinoid processes is covered by a fold of the inner layer 



1 For alternative method see p. 217. 



