SIGHT 



39 



ear we shall find that it begins as a pit in the skin, and by 

 closure of the mouth of this a vesicle is produced, which if it 

 underwent no further modification would be called an otocyst. 

 As it is, however, a very complex shape is assumed, the final 

 result being known as the membranous labyrinth, or internal ear 

 (fig. 1050). This sometimes, as in a Skate or Dog- Fish, remains 

 in communication with the exterior throughout life. It is sig- 

 nificant that in Fishes the auditory pit arises in close connection 

 with the " lateral line", which is a groove 

 or tube containing groups of sense-cells 

 belonging to the skin. And this suggests 

 that the ear is no more than a bit of this 

 line which has sunk beneath the surface 

 and become specialized as regards struc- 

 ture and function. It is extremely pro- 

 bable that the lateral line of Fishes and 

 Amphibian larvae has to do with main- 

 tenance of balance and direction of move- 

 ments, and if so, the fact that the ear has 

 to do with these functions is quite intelli- 

 gible. We know so little about the 



\ 



Fig. 1050. Diagram of the Left Mem- 

 diviSlOn Of physiological labour between branous Labyrinth of a Lower Vertebrate, 



seen from the outer side, t., A tube re- 

 presenting the outer part of the original 

 ingrowth; a.s., f.s., and k., anterior ver- 

 tical, posterior vertical, and horizontal 



be out of place here. But experiments 3^Kti-tV^ 



s.,s. (and other dotted patches), groups 

 of sense-cells. 



rl 1 



different partS OI the COmpleX 



rinth that a discussion of details would 



have shown that the semicircular canals 

 have some connection with movement and 

 equilibrium, and it is interesting to note that they lie in three 

 planes which are mutually at right angles. It is also certain that 

 the labyrinth is the sense-organ of hearing proper. In land- verte- 

 brates there are more or less perfect arrangements for conducting 

 air- waves from the exterior to the deeply-seated and well-protected 

 internal ear. This has already been sufficiently illustrated by the 

 brief account of the auditory organs of Man given elsewhere (see 

 vol. i, p. 56). 



SIGHT 



SKIN-SEEING. The simplest kind of sight is literally that of 

 " seeing without eyes", and it amounts to no more than the 

 power of distinguishing between light and darkness, or detecting 



