The nucleus isthmi of the frog

The nucleus or ganglion isthmi is a subject of much confusion in the literature of comparative neurology. It has been described in all groups of vertebrates except cyclostomes and mammals. According to Kappers ('21, p. 789) it first appears in the Plagiostomi. There is formed in this group a new differentiation in the dorsal part of the isthmus, lateral to the anterior medullary velum, which Kappers regards as the nucleus isthmi. Franz ('12) describes it in bony fishes, and it has long been recognized in the anuran amphibia. Joustra ('18) has studied its connections in the chameleon, in which form it is very large, and in the crocodile. He concludes that in these reptiles the nucleus isthmi is homologous with the medial geniculate body of mammals. Edinger ( '11) describes a nucleus of this name in various vertebrate groups, but does not always apply the term to structures having the same connections, as Johnston ('05) has pointed out. According to Kappers ('21, p. 867) the nucleus is present in birds. Regarding its presence in the mammalian brain the statement of Kappers (p. 894) is pertinent when he remarks "doch musste es jeden vergleichenden Neurologen auffallen, dass ein so konstanter Kern wie das Ganglion isthmi, welches namentlich bei den Reptilien bereits eine so machtige Entwicklung erreicht, bei den Saugern fehlen sollte." Johnston ('05) considers the nucleus isthmi to correspond with his secondary visceral nucleus of fishes (Rindenknoten of Mayser, '82). In another place Johnston ('06) states that the nucleus isthmi of birds and of amphibia, as Edinger applied the name,