234 THE HEAD AND NECK 



3. The Mandibular Nerve (p. 182) runs forwards and later- 

 ally for a short distance from the semilunar ganglion and then 

 passes downwards through the foramen ovale. At this point 

 it is joined by the whole of the small (motor) root, which lies 

 on the bone below the semilunar ganglion. 



Removal of the Semilunar (Gasserian) Ganglion. 

 The approach in this operation and in ligature of the main trunk 

 of the middle meningeal artery is similar to that described on 

 p. 230, but a somewhat wider exposure is necessary. In order 

 that this may be obtained; the temporal fascia is incised along 

 the upper border of the zygomatic arch and the bone is divided 

 at each end. When this is done the arch can be depressed 

 for half an inch ; and the temporal muscle is retracted into the 

 interval between it and the skull. A trephine can now be 

 applied a little above the level of the floor of the middle cranial 

 fossa which corresponds to the upper border of the undivided 

 arch and sufficiently far forwards to avoid the main trunk of 

 the middle meningeal artery. In this region the thickness of 

 the skull varies, being much greater below than above ; and this 

 difference must be borne in mind, as otherwise the blade of the 

 trephine may tear through the dura mater above, before it has 

 pierced the bone below. After the circular piece of bone has 

 been removed, the trephine opening is enlarged and the middle 

 meningeal artery is exposed and traced medially to the foramen 

 spinosum, where it is ligated and divided. Before the ligature 

 can be applied, the dura mater must be elevated from the floor 

 of the fossa by blunt dissection, and, after the artery has been 

 dealt with, this is continued until the lateral margin of the 

 ganglion is reached. At this point the fibrous and serous layers 

 separate to enclose the ganglion, and the elevation of the fibrous 

 layer becomes more difficult. The surgeon endeavours to incise 

 or break through the fibrous layer just beyond the splitting. If 

 this is done successfully, the maxillary nerve is exposed running 

 forwards to the foramen rotundum. If, however, the dura 

 mater is incised before the two layers separate, the subdural 

 space is opened, and the fluid which it contains floods the area 

 and greatly increases the difficulty of the operation. By 

 tracing the maxillary nerve backwards, the mandibular nerve 

 is exposed a little in front of and medial to the ligated middle 

 meningeal artery, and more posteriorly still the ganglion is 

 reached. If the serous layer of the dura is elevated in a medial 

 direction, the ophthalmic nerve will be found lying in the lower 



