CRANIAL NERVES 135 



LATERAL- LINE, LABYRINTHINE, AND COCHLEAR SYSTEMS 



These special somatic sensory systems are closely related geneti- 

 cally, structurally, and physiologically, but much remains obscure 

 about their relationships. The labyrinthine apparatus seems to be 

 at the focus of these systems. It is very conservative, except for the 

 cochlear part, showing relatively little change in structure and func- 

 tion from lowest to highest vertebrates; moreover, its physiological 

 properties have been thoroughly explored. The lateralis system at- 

 tains its maximum in fishes, persists in larval amphibians and adults 

 of some urodeles, and disappears entirely in all higher groups, both 

 embryonic and adult. Organs of hearing are poorly developed in 

 fishes. Auditory functions seem to be performed by the vestibular . 

 apparatus and also (for slow vibration frequencies) by the organs of 

 the lateral line, which undoubtedly have other functions also. 



The peripheral end-organs of all these systems are specialized 

 epithelial structures, in contrast with the free nerve endings of the 

 general somatic system. The vestibular end-organs of the internal 

 ear resemble the end-organs of the lateral lines, in that in both cases 

 there are specialized epithelial cells which are the receptive elements. 

 The epithelium is thickened, and among the slender elongated sup- 

 porting elements there are shorter ovoid cells with ciliated outer 

 ends. These specific nerves have thick myelinated fibers, the 

 branched unmyelinated terminals of which closely embrace the cell 

 bodies of the specific receptive elements (Larsell, '29; Chezar, '30; 

 Speidel, '46). 



The lateral-line organs of Amblystonia are papillae, some of which 

 are depressed in pits but are not inclosed in canals as in most fishes. 

 Their arrangement conforms with the general pattern in fishes, with 

 rows above and below the eye, on the lower jaw, and extending into 

 the trunk as far back as the tail. The related nerves comprise one of 

 the largest systems of the larva, which is reduced but not lost at 

 metamorphosis. These thick and heavily myelinated fibers enter the 

 brain in two large roots spiimlward of the VIII roots and three or 

 four which enter dorsally and slightly rostrally of the VII roots. 

 They are conventionally assigned to the VII and X pairs of nerves, 

 though they are more properly aligned with the VIII roots. 



The arrangement of these roots is shown in figures 7, 9, 89, 90. 

 Most of their individual fibers bifurcate immediately upon entrance 

 into the brain into ascending and descending branches with nu- 



