8 THE BRAIN OF THE TIGER SALAMANDER 



Part II is written for specialists in comparative neurology. It 

 covers the same ground as the first part, reviewing each of the con- 

 ventional subdivisions of the brain, giving details of the evidence 

 upon which conclusions are based, with references to sources and 

 much new material. This involves some repetition, which is unavoid- 

 able because all these structures are interconnected and in action 

 they co-operate in various ways. Many structures must be described 

 in several contexts and, accordingly, the Index has been prepared 

 with care so as to enable the reader to assemble all references to every 

 topic. 



The most important new observations reported in Part II relate to 

 the structure and connections of the isthmus (chap, xiii), inter- 

 peduncular nucleus (chap, xiv), and habenula, including analysis of 

 the stria medullaris thalami and fasciculus retroflexus (chap, xviii). 

 In chapter xx a few of the more important systems of fibers are de- 

 scribed, including further analysis of the tegmental fascicles as enu- 

 merated in the paper of 1936 and references to other lists in the litera- 

 ture. The composition of the commissures of the brain is summarized 

 in chapter xxi. The lemniscus systems are assembled in chapter xi, 

 and other tracts are described in connection with the structures with 

 which they are related. 



Since most neurologists are not expert in the comparative field, 

 where the nomenclature is technical and frequently unintelligible 

 except to specialists, the attempt is made in Part I to present the 

 salient features with a minimum of confusing detail and, so far as 

 practicable, in terms of familiar mammalian structure. This is not an 

 easy thing to do, and no clear and simple picture can be drawn, for 

 the texture of even so lowly organized a brain is bafflingly compli- 

 cated and many of these structures have no counterparts in the hu- 

 man body. Homologies implied by similar names are rarely exact, 

 and in many of these cases the amphibian structure is regarded as the 

 undifferentiated primordium from which the mammalian has been 

 derived. This is emphasized here because homologies are usually 

 defined in structural terms and because organs which are phylo- 

 genetically related are regarded as more or less exactly homologous, 

 regardless of radical changes in their functions. Thus the "dorsal 

 island" in the acousticolateral area of the medulla oblongata of 

 Necturus is regarded as the primordium of the dorsal cochlear nu- 

 cleus of mammals, despite the fact that Necturus has no recognizable 

 rudiment of a cochlea or cochlear nerve. This is because, when the 



