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The so-called Dorsotrachealis Brauch of the seyenth Cranial 

 Nerve in Amphinma. 



By H. W. NoRRis. 

 (From the Zoological Laboratory, Iowa College.) 



Fischer in his work on the Derotremes and Pereuuibranchs i) 

 describes a peculiar branch of the seventh cranial nerve in Araphiuma, 

 distributed, according to his statement, to the hyotracheahs muscle. 

 KiNGSLEY^) agrees with Fischer that the nerve is one having no 

 homologue in other Amphibians. According to him the nerve ends in 

 the dorsotrachealis muscle. My own observations are so at variance 

 with the views of these two writers that the following detailed account 

 of the course of this extraordinary nerve is hereby given. 



As KiNGSLEY says, there emerge on the posterior surface of the 

 hyomandibular trunk of the seventh cranial nerve four branches. The 

 first is Jacobson's commissure, passing posteriorly and dorsally to 

 anastomose with the glossopharyngeal nerve. The fourth branch, or 

 hyomandibular proper, arises as two branches or as one that im- 

 mediately divides into two. The second and third branches immediately 

 enter the digastric muscle. According to Kingsley the dorsal one of 

 these breaks up into smaller branches supplying this muscle, while the 

 ventral one passes postero-ventrally through the muscle. I find that 

 both branches give off fibers to the muscle and pass back, uniting into 

 one trunk near the posterior border of the muscle. In passing be- 

 tween the muscle fasciculi both branches become much flattened and 

 in some places difficult to follow. 



From the posterior border of the muscle the nerve rapidly ascends 

 nearly to the dorsal border of the thymus gland, along which organ 

 it passes posteriorly, for some distance being imbedded in the dorso- 

 lateral border of the gland. Before reaching the thymus gland the 

 nerve divides, the two divisions reuniting shortly after the gland is 

 reached. After passing back nearly to the posterior border of the 

 thymus gland the nerve enters the extreme posterior part of the dorso- 

 trachealis muscle. It possibly gives off some fibers to the muscle, but 

 the main trunk continues posteriorly into the connective tissue ventral 

 to the lateral border of the longissimus dorsi muscle and between the 

 latter and the intertransversales muscles, running approximately parallel 

 with the ramus lateralis medius of the vagus nerve. 



1) J. G. Fischer, Anatomische Abhandlungen über die Perenni- 

 branchiaten und Derotremen, Hamburg 1864. 



2) J. S. Kingsley, The Cranial Nerves of Amphiuma. Tufts College 

 Studies, No. 7, 1902. 



