THE CEAxXIAL NEliVES AND LATERAL SENSE ORGANS OF FISHES. 147 



He says (p. 58) : " The nerve of Jacobson is present in this embryo, as an outgrowth 

 from the petrous ganglion of the glossopharyngeal, but is very short. This nerve passes 

 vertically upwards, and ends a considerable distance below the level of the inferior 

 maxillary nerve." Dixon also states (p. 66) that the nerve of Jacobson grows out from 

 the petrous ganglion in the rat. 



Jacobson's anastomosis is described in ^iv/^iV by Allis as follows (6, pp. 685-686): — 

 " In the palatine canal the ramus pharyngeus lies on the median side of the ramus 

 palatinus facialis, and at the point where that nerve separates into its anterior and 

 posterior branches the pharyngeus, in one dissection, separated also into two portions, 

 one qfioJiich accompanied each of the tico branches of the palatinus [facialis]. In other 

 dissections this separation was not evident, tlie pharyngeus glossopharyngei accompanying 

 the anterior branch of the facialis and issuing on the ventral surface of the vomer to be 

 distributed to dermal tissues there." (Italics mine.) This description hardly tallies with 

 plate xxxviii., where a complete anastomosis with the palatinus facialis is tigured. Allis 

 does not homologise it with Jacobson's anastomosis, but it is obvious that this is what 

 it is, seeing that it is formed by the visceral branch of the IXth accompanying the 

 corresponding division of the facial. 



Huge (1897, 169), after referjing to anastomoses described by Stannius [Acipenser, 

 Perca, Tinea), Johannes Mliller [Tolijpterus, Lepidosteiis), and Eischer [Blenobranchus, 

 Siren), concludes with reference to the mammalian tympanic nerve : — " Es besteht aber 

 ausserdem die Moglichkeit, dass in der rortsetziing des N. tympanicus das llomologon 

 des Ramus pharyngeus (IX.) enthalten sei. 1st dies der Fall, so entspricht der N. petrosus 

 superficialis minor sowohl deni N. palatinus als audi einem der Jxv. branchiales n. vagi 

 der Fische. Die Anastomose, welche zwischen Facialis und Gl.-phar. ganz ausserhalb 

 des Schadels besteht, darf ndt der erwahnten nicht verwechselt werden. Sie ist sehr 

 wahrscheinlich dvirch die Verschmelzung der motorischen Endgebiete beider Nerven 

 (;ntstauden " (p. 213). Finally, fig. 118, p. 182, in tlie new English edition of Wiedersheim's 

 ' Comparative Anatomy ' (1897, 221), for the accuracy of which I am partly responsible, 

 should be corrected in two respects : (1) tlie dotted pharyngeal branch should be deleted, 

 and so also should the nerv e labelled Jak. (Jacobson's anastomosis). The latter, we have 

 seen, is represented by the palatinus glossophai-yugei. 



It is hence clear that thiee nerves have been described connecting the vagal and facial 

 o-roups of cranial nerves, each of which has a distinctive composition. These are : — 



(1) A somatic sensory anastomosis. Described by Strong in Amphibia. This is the 

 only instance known, I believe, of an anastomosis of this nature. It is possible that 

 Strong may have been mistaken as to its character, especially as it, too, is distinctively a 

 branch of the glossopharyngeal ; but however that may be it cannot be considered 

 comparable to a true Jacobson's anastomosis. 



(2) A lateral line anastomosis. This more usually connects the facial with the vagus, 

 but where a portion of the lateralis lateral line nerve accompanies the glossopharyngeus, 

 it may connect that nerve with the lacial. The character of this nerve cannot be mistaken, 

 since it usually exists side by side with Jacobson's anastomosis. 



(3) A visceral sensory or true Jacobson^s anastomosis. That the nerve which usually 



SECOND SEIUES. — ZOOLOGY, VOL. VII. 21 



