CHAPTER XIV 

 INTERPEDUNCULAR NUCLEUS 



THE interpeduncular nucleus, like the habenula with which it 

 is connected, is one of the most conservative structures in the 

 brains of vertebrates, but in urodeles it has some characteristics 

 which have not been reported elsewhere. The general features are 

 outlined on page 46, and the details are described here, together 

 with some speculations about the probable functions. The chief con- 

 nections are shown very diagrammatically in figures 19 and 20, the 

 spiral endings of the tractus habenulo-interpeduncularis in figure 50, 

 the composition of the glomeruli in figure 84, and some typical con- 

 nections of its neurons in figure 83. 



COMPARATIVE ANATOMY 



This nucleus was discovered by Forel in 1872 and described in the 

 rabbit in 1877, with confirmation by von Gudden in 1881 and Ganser 

 in 1882. It is a constant feature in all vertebrate brains, much larger 

 in some lower groups than in the higher. In urodeles it is a well-de- 

 fined column of cells, embracing the ventral angle of the ventricle 

 and extending from the fovea isthmi backward to the level of the V 

 nerve roots. It is a ventromedian structure, differentiated in situ in 

 the floor plate and the adjoining borders of the basal plates'. The 

 fovea isthmi marks the anterior end of the embryonic floor plate as 

 defined by Wilhelm His, and this plate is generally regarded as a non- 

 nervous structure characterized by specially differentiated ependyma 

 (Kingsbury, '30, p. 182) ; but Coghill ('24, Paper III) has shown that 

 neuroblasts are differentiated here intrinsically. Some of these per- 

 sist in the adult medulla oblongata as nucleus raphis, and between 

 the fovea isthmi and the V nerve roofes this differentiation goes much 

 further, producing the interpeduncular nucleus, as pointed out in my 

 description of this nucleus of Necturus ('34c). 



The differentiation is primarily isthmic, as was recognized by van 

 Gehuchten ('00, 2, 199). In Amblystoma it extends spinalward into 

 trigeminal territory, and below the level of the V nerve roots it is 

 continuous with the nucleus raphis of the medulla oblongata. In cy- 

 clostomes the fasciculus retroflexus extends spinalward as far as the 



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