a ats 
ae nicl a 
1923. ] Adhesive Pads of Gecko, etc. 143 
upon two chief factors the coefficient of friction and pressure. 
The coefficient of friction, as is apparent, is very high in the 
foot pads of Geckos, and, therefore, a slight pressure on the 
pads results in a very high value of friction. When an animal 
adheres to a vertical surface the weight of the animal itself 
causes pressure on the pads and makes them efficient. On the 
other hand, when a Gecko is made to hang back downwards 
from a smooth surface, the pressure is negative, and hence the 
Gecko falls down. In a case where the limbs are stretched 
outwards, the weight of the body instead of pulling the pads 
directly downwards causes them to slip along the smooth surface 
for a short distance before exerting a vertical pressure on them. 
It is due to this fact that an animal can hang from a smooth 
surface when its limbs are stretched outwards. The pressing 
of the belly against the opposing surfaee is directly correlated 
with the position of the limbs, and is probably an additional 
advantage, since its scaly surface must help in increasing 
friction. Advantage may also be taken of atmospheric 
pressure by adpression. 
bservation 7 clearly shows that the digital pads are 
merely frictional devices. 3 
e digital-pads of the house-lizard are in certain 
respects more highly specialized than either the thoracic 
apparatus of the fish of mountain torrents or the cephalic disc 
of Echeneis. Whereas in the fish the vesulting friction under 
the pad is greatly enhanced by the pressure exerted by the 
flow of water, in the Gecko there is no such external aid. In 
Short, in fishes both the coefficient of friction and the 
pressure play an almost equal part in the production of the 
necessary amount of friction, whereas in the house lizard 
the friction chiefly depends upon a very high coefficient of 
friction. : 
‘is furnished with unstriped, smooth muscular fibres, the 
contraction of which produces ove or more longitudinal 
muscular action the plain finger di conve 
ridge-and-groove type of adhesive apparatus, the = 
of which as a friction device has been e l n severa 
free margins arranged on the exposed surface as thick, closely 
set spines. In my opinion the ridge-and-groove pattern- of 
