CHAPTER XIII 

 ISTHMUS 



THE strategic position of the isthmus as a transitional sector of 

 the brain stem between rhombic brain and cerebrum has been 

 commented upon (pp. 45, 118). In early neural-tube stages of the 

 developing human brain it is a rather large sector, but in the adult 

 it is hardly recognizable as a distinct entity because its tissues are 

 dispersed among other structures of relatively recent phylogenetic 

 origin. In contrast, the isthmus of adult Amblystoma retains its em- 

 bryonic separateness and, indeed, accentuates it. Here its distinctive 

 features are most clearly seen, and these will be described as fully as 

 available material permits. Its dorsal part is contracted and includes 

 only the anterior medullary velum and some adjoining structures. To 

 the intermediate zone is assigned the superior visceral-gustatory nu- 

 cleus, and for convenience of description the remainder is included in 

 the motor zone, though its functions are, in large part, of intermedi- 

 ate type. The interpeduncular nucleus is an isthmic structure, to 

 which a separate chapter is devoted. 



DEVELOPMENT 



The configuration of this sector of the brain exhibits an interesting 

 series of changes in the course of development, due to the rapid and 

 radical dislocations caused by the cerebral flexures and to other in- 

 equalities of intrinsic growth. Successive modifications of the isthmic 

 sulcus provide a useful indicator of the course of these changes. 



From the coil stage (Harrison's stage 35) to the adult, the isthmic 

 tegmentum is intimately joined with the trigeminal tegmentum, 

 with no constant external or ventricular boundary visible. "Isthmus 

 rhombencephali," therefore, is an appropriate name. But its anterior 

 boundary is sharply marked by the fovea isthmi, the deep ventricular 

 sulcus isthmi, and the external fissura isthmi. Even in early stages 

 these grooves are not exactly parallel, and as development advances 

 they deviate from this relationship more and more widely because of 

 unequal growth of the deep and superficial structures. 



Reference was made on page 177 to von Kupffer's sulcus intraen- 



179 



