136 THE BRAIN OF THE TIGER SALAMANDER 



merous widely spread collaterals. These root fibers are arranged in 

 fascicles, which span almost the entire length of the medulla ob- 

 longata (fig. 7), except for the most dorsal of the three or four 

 lateralis VII roots, which ends in a "dorsal island" of neuropil 

 ("cerebellar crest" of Larsell) at the level of entrance (figs. 7, 33, 45). 

 Lateral-line fibers have not been seen to descend into the spinal cord. 

 Anteriorly, they enter the auricle and end here (fig. 91); none have 

 been traced into the body of the cerebellum, though secondary 

 lateralis fibers after synapse in the auricle enter the com. vestibulo- 

 laterahs cerebelli in company with vestibular fibers (figs. 32, 33, 34, 

 com.cb.l.L). 



The exact functions served by the lateral-line organs are still im- 

 perfectly understood. The organs of the lateral lines and those of the 

 internal ear have many similarities in embryological development, 

 structure of the receptive apparatus, and central connections. They 

 probably have had a common evolutionary origin from a more gen- 

 eralized form of cutaneous sense organ similar to the so-called "sen- 

 sillae" of some invertebrates. This may be the explanation of the inti- 

 mate association in the human ear of sense organs of such diverse 

 functions as the cochlea for hearing and the semicircular canals for 

 equilibration, both being highly refined derivatives of primitive 

 tactile organs. The sense organs of the lateral lines are probably in- 

 termediate in function between tactile sensibihty of the skin and the 

 auditory and equilibrating functions of the internal ear. In fishes 

 they have been shown to be sensitive to mechanical impact, slow 

 vibrations, and currents in the water (Parker and Van Heusen, '17; 

 Parker, '18). Hoagland ('33) and Schriever ('35) have investigated 

 the functions of lateral-line nerves of fishes with the aid of oscillo- 

 graph records of their action currents. Hoagland finds that these 

 organs are in a state of continuous activity and that the nervous dis- 

 charge is increased by application of pressure, by ripples and cur- 

 rents in the water, by movements of the trunk muscles, and by 

 temperature changes. 



In Amblystoma larvae Scharrer ('32) found evidence that the 

 lateral-line organs may participate in the snapping reaction when 

 moving prey is seized; and, subsequently, Detwiler ('45) reports that 

 the lateral-line organs of these larvae constitute an adequate receptor 

 apparatus for the detection of food in motion after extirpation of the 

 eyes and nasal organs. The central connections of these nerves sug- 



