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 larve 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 Mee eens 
division of physiological labour between SS ee 
the different parts of the complex laby- presenting the outer part of the original 
q . ° . ingrowth; a.s., Z.s., and 4., anterior ver- 
rinth that a discussion of details would ticat, posterior vertical, and horizontal 
be out of place here. But experiments misty cls: » wide: se, se 
have shown that the semicircular canals *:* ‘nd olher dotted patches), groups 
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 
Sxin-SeEInc.—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 
