2^2 THE BRAIN OF THE TIGER SALAMANDER 



isthmus. In the human brain there are no definite structures com- 

 parable to the amphibian dorsal and isthmic tegmentum. 



The isthmus is much more clearly defined than in adult higher 

 brains, it is relatively larger, and its physiological importance is cor- 

 respondingly greater, as will appear later. It is bounded anteriorly 

 by the sharp isthmic sulcus and posteriorly by the cerebellum, 

 auricle, and trigeminal tegmentum. The so-called "pons" sector of 

 the human brain stem is named from its most conspicuous compo- 

 nent, but this name is meaningless in comparative anatomy. In man 

 it is the pons and the sector of the stem embraced by it; but in no 

 two species of mammals is the part embraced by the pons equivalent; 

 and below the mammals the pons disappears entirely. The medulla 

 oblongata, on the other hand, is a stable structure, extending from 

 the isthmus to the spinal cord, and for it the shorter name "bulb" is 

 sometimes used, especially in compounds. 



I outlined the development and morphological significance of the 

 urodele cerebellum ('14, '24), and this was followed by detailed de- 

 scriptions of the development and adult structure of this region of 

 Amblystoma by Larsell ('20, '32), whose observations I have sub- 

 sequently confirmed, including his fundamental distinction between 

 its general and its vestibular components. 



Some features of the larval medulla oblongata and related nerves 

 have been described ('14a, '396) and, more recently ('446), addi- 

 tional details of the adult, particularly the structures at the bulbo- 

 spinal junction. Much remains to be done to clarify the organization 

 of the medulla oblongata and spinal cord. 



The cranial nerves and their analysis into functional components 

 (chap, v) were described by Coghill ('02). The embryological devel- 

 opment of these components also has been extensively studied (chap. 

 x). The arrangement and composition of these nerves are fundamen- 

 tally similar to those of man, with a few notable exceptions. The 

 internal ear lacks the cochlea, which is represented by a very primi- 

 tive rudiment; a cochlear nerve, accordingly, is not separately dif- 

 ferentiated. There is an elaborate system of cutaneous organs of the 

 lateral lines, whose functions are not as yet adequately known. These 

 are supplied by very large nerves commonly assigned to the VII and 

 X pairs, though it would be more appropriate to regard them as 

 accessory VIII nerves, for all these nerve roots enter a wide zone at 

 the dorsolateral margin of the medulla oblongata known as the "area 

 acusticolateralis." There is no separate XI cranial nerve, this being 



