THE ANATOMY OF GUSTATION 129 



course, entering the tympanum, crossing the auditory ossicles, to 

 make its way out at the front wall of the tympanum, then uniting 

 ^^ith the gustatory division of the trigeminal, or Vth nerve, and, 

 passing along the inner side of the ramus of the mandible with it, 

 until eventually it leaves it to become connected with the sub- 

 maxillary ganglion. The principal portion of the portia dura, on 

 the other hand, makes its way out by the stylo-mastoid foramen 

 and is distributed to the facial muscles, some comparatively insignifi- 

 cant branches only being furnished to the levators of the hyoid and 

 depressors of the lower jaw. 



We can now refer to the studies of Gushing and others which 

 prove that the Vth nerve takes no part in the innervation of taste- 

 buds in man. According to the diagram after Gushing, the taste- 

 buds of the tongue are innervated by the IX or glossopharyngeal 

 nerve and fibres pass along the lingual nerve which are continuous 

 with the chorda tympani and end in the geniculate ganglion. It is 

 established, therefore, that the true courses of the nerves of the 

 taste-buds of the tongue are through the VII and IX nerves. 



An interesting and difficult problem arises. How are we to 

 explain the fact that a taste-bud seems to be associated with a 

 particular function according to its situation, and that there is no 

 anatomical difference in its structure wherever it may be situated. 

 It would seem that taste-buds have varying functions, or rather 

 that the afferent impulses from a taste-bud have a different effect 

 according to the part of the brain, in fish a lobe, which receives the 

 stimulus. If we assume that the facial lobe receives impressions 

 recognised as of a true gustatory type and the vagal lobe receives 

 impressions recognised as of a warning type we explain the varying 

 functions of the taste-buds as due to their central nervous repre- 

 sentation. 



The facts remain clear, however, that the facial nerve in fish is 

 primarily a sensory nerve carrying gustatory stimuli and has only 

 a small motor element. The facial in man is an important motor 

 nerve supphing the face muscles and also has a stylohyoid and 

 digastric branch ; its small sensory element appears as the chorda 

 t\nnpani and this is the nerve that carries true gustatory stimuli 

 from the anterior part of the tongue and the circumvallate papillae. 

 The warning and discriminatory impulses in fish are carried by the 

 glossopharjTigeal and vagal nerves from the branchial region and 

 palatal organ to the vagal and glossopharyngeal lobes and the vagal 

 receives also sentinel impulses from the taste-buds surrounding the 

 orifice of the pneumatic duct. 



In man the taste-buds of the palate and pillars of the fauces are 

 I 



