THE FROG 23 



cranium. These are known both by name and by number. In 

 some cases the roots of two or more of these nerves may be 

 so closely associated that only microscopical analysis can dis- 

 tinguish them as they emerge from the brain or as they pass 

 through the skull. 



Carefully pick away the muscles lying between the brain and 

 the tympanum until at about the level of the side of the brain 

 the nerve-trunks are reached. These will be recognized by their 

 white color and, in this region, the transverse direction of the 

 larger trunks. 



Most posterior of the series is the complex of the ninth and 

 tenth nerves. This emerges from the brain at about the level 

 of the middle of the fossa rhomboidalis, passes outwards behind 

 the otic capsule and soon presents a small enlargement, the 

 jugular ganglion, beyond which the trunk separates into several 

 branches. 



The two anterior of these form the glossopharyngeal (IX) 

 nerve. At their common base these have a small glossopharyn- 

 geal ganglion, lateral to which the two branches separate. The 

 posterior is the glossopharyngeal proper, which passes outwards 

 and then downwards. The other is Jacobson's commissure, 

 which should be traced laterally to the medio-posterior side of 

 the tympanum, where it unites with a branch (hyomandibular) 

 of the seventh nerve, the common trunk now passing straight 

 outwards behind the tympanum. 



The other branches, which extend laterally, form the vagus 

 nerve (X), while from the jugular ganglion a commissure of the 

 sympathetic system extends backward to the second spinal 

 nerve (so-called hypoglossus, XI), which leaves the spinal cord 

 between the first and second vertebrae. 



The roots of the fifth and seventh nerves are also closely 

 associated with each other. They arise from the antero-lateral 

 side of the medulla and unite almost immediately to form a 

 large common prootic ganglion* inside the cranium. Outside 

 the skull four main trunks are immediately visible. Of these the 

 hindermost is the hyomandibularis branch of the facial nerve (VII) 



can be given in the ordinary course, and hence only the roots and main 

 trunks are noticed here. Those wishing to follow it farther should use 

 Gaupp's description. 



* A fusion of Gasserian (5th) and geniculate (7th) ganglia. 



