Herrick, Subdivision of the Brain. 399 



differentiation is however so nearly uniform in the adult through- 

 out the length of the spinal cord that the subdivision of that struct- 

 ure can most conveniently be effected in terms of the transverse 

 segments, these being named after their nerves and the corre- 

 sponding vertebrae, as defined by the classic nomenclature. 



In the rostral end of the body the segmental plan is modified 

 in all lower vertebrates, as w^e have seen, by the presence of gills. 

 These are visceral structures and their innervation belongs wholly 

 to the splanchnic systems.^ In lower vertebrates this branchio- 

 meric system may coexist with the typical somatic systems in the 

 same segments, but in higher forms segments possessing gills, or 

 their derivatives in land vertebrates, have usually lost the somatic 

 components or else these latter have suffered so great modification 

 as to be w^ith difficulty recognized as such. The reduction in 

 number of gills took place early in the phylogeny (they never exceed 

 seven in gnathostomes, and usually are less than five) and the 

 surviving members of the series are so closely associated with 

 cranial structures that the whole gill region in gnathostomes may 

 be considered a part of the head. 



The branchiomeric type of nerve is preserved with least modi- 

 fication in the region of the medulla oblongata, bounded rostrad 

 by the isthmus; and this structural type is clearly evident, though 

 in a highly modified form, in the human medulla oblongata. Ac- 

 cordingly, the rhombencephalon of His is a natural subdivision 

 phyletically as well as embryologically considered. 



The cerebellum and its associated pons are derivatives of the 

 somatic sensory column in the cephalic part of the same region, 

 called forth primarily by the vestibular apparatus (and allied sense 

 organs in fishes). This gives a sound genetic basis for the meten- 

 cephalon of His, as distinguished from the remainder of the rhomb- 

 encephalon (the medulla oblongata). 



The metencephalon should be limited to the cerebellum and its 

 immediate dependencies, a structure which has been added to the 

 much older primary rhombencephalon, or branchiomeric brain. 

 This usage will necessitate some revision of the limits of the meten- 

 cephalon as set by the BNA. It can no longer be regarded as a 



2 These structures have suffered considerable secondary modification. For example, the gustatory 

 system has been derived from the unspecialized visceral sensory, and the visceral motor has in part been 

 specialized parallel with the development of striated branchial muscles from the splanchnic mesoderm. 

 The branchio-motor nerves lack the post-ganglionic neurone and structurally resemble the somatic 

 motor nerves in their mode of connection with their end-organs. 



