The Senses 19
density of mechanoreceptors both in and between the pads and in the soft tissue at
the base of the claws. Specialist mechanoreceptors known as Pacinian or Lamellar
corpuscles in the deeper skin layers also allow cats to detect vibrations through the
pads of the feet (Verrillo, 1966).
Balance
The vestibular system
The vestibular system is the part of the inner ear responsible for the sense of balance
in mammals. It is made up of the semicircular canals, and the saccular and utricular
otolith organs.
The semicircular canals are three fluid-filled tubes that also contain motion-
sensitive hairs (cilia). They are:
● The Horizontal: which detects turns to the left or right.
● The Anterior and Posterior: which detect up and down movement and move-
ments such as putting the head to one side.
As the animal moves its head the vestibular system moves along with it but the
fluid within the semicircular canals stays in place, thereby moving the hairs within the
canals. The information about the movement is then relayed to the brain.
The saccular and utricular otolith organs also have cilia covering their internal
surface, but rather than fluid these organs have tiny calcite crystals covering the sen-
sory surface that brush against the hairs as the animal moves its head. The otolith
organs detect acceleration, deceleration and gravity, allowing the animal to know
when and how fast it is moving, and when it is the ‘right way up’.
Although this is the same system as in other mammals, including humans, the
cat’s semicircular canals are much nearer to being at right angles to each other than
in many other mammals and the horizontal canal is more parallel to the normal head
position. This allows the information detected by the cilia within the canals and
relayed to the brain to be particularly clear and precise. Plus, the utricular otoliths,
and possibly the saccular, are much better attuned to measure gravitational deviations
(Bradshaw et al., 2012).
The ‘Righting’ reflex
The ‘righting reflex,’ sometimes called the ‘air-righting reflex’ is the cat’s ability to
land on its feet from a fall. When a cat starts to fall the movement is instantly detected
by the vestibular system and within one tenth of a second the head starts to turn
towards the ground, allowing the cat to see where it will land. The highly flexible
spine allows the body to then twist, front end first and then the back end so that the
cat is facing the ground. Lastly, as the cat is about the land, the back arches and the
legs extend ready to act as shock absorbers (Fig. 2.3).
If the fall is from a considerable height the legs are initially pushed out sideways
and only extended downwards as the cat is just about to land. This has the effect of
reducing the falling speed and probably accounts for reports of cats surviving falls