CHAPTER XI 

 MEDULLA OBLONGATA 



THERE is no systematic description of the medulla oblongata of 

 adult Amblystoma comparable with my paper on Necturus 

 ('30) . Coghill's papers give many details of the early stages of develop- 

 ment, but no comprehensive description has been written. The 38- 

 mm. larva was described in 1914, and subsequent study convinces 

 me that the conditions found there and in Necturus are not funda- 

 mentally different in adult Amblystoma, though the latter is con- 

 siderably more specialized. This is evident upon comparison of the 

 figures of adult Amblystoma recently published ('446) with those of 

 the two papers just cited, to which the reader is referred for general 

 discussion. At metamorphosis the changes are far less in Amblystoma 

 than in Salamandra and some other urodeles. Descriptions of the 

 medulla oblongata and cerebellum of Cryptobranchus, Siren, Pro- 

 teus, Salamandra, and some other urodeles are found in the papers 

 cited on page 11. Some details of the schematic outline given in 

 chapters iv and v will now be filled in. 



SENSORY ZONE 



The sensory zone forms the massive dorsolateral wall of the fourth 

 ventricle from the calamus scriptorius forward, to and including the 

 thick auricle under the cerebellum (p. 44). This zone is reduced in 

 size but not suppressed in the floor of the lateral recess of the ventricle 

 spinal ward of the auricle. In the remainder of the medulla oblongata 

 the gray substance forms a low ridge bordering the taenia of the 

 fourth ventricle and projecting into the ventricle, the acousticolateral 

 area, which is smaller in the adult than in the larva (figs. 9, 89, 90). 

 Ventrally of this area is a narrower ridge, which expands posteriorly 

 — the visceral lobe (figs. 9, 88). This contains the nucleus of the 

 fasciculus solitarius. 



The white substance of the sensory zone is composed chiefly of the 

 fascicles of sensory root fibers and the associated neuropil. There are, 

 in addition, secondary arcuate fibers and two longitudinal bundles of 

 correlating fibers — tracts a and h of Kingsbury (figs. 7, 9, 87-90). 



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