THE NERVOUS SYSTEM. 175 
delicate at the root.” A state of solution and an actual 
contact of the fluid are necessary conditions. 
Smell is the perception of odors, z.¢., certain substances 
in the gaseous state. Many Invertebrates have this sense : 
Snails, ¢. g., seem to be guided to their food by its scent, 
and Flies soon find a piece of meat. 
But it is impossible to say what or 
where the organ is. Most probably it 
is united with the instrument of touch. 
In Vertebrates, it is placed at the en- 
trance to the respiratory tube, in the 
upper region of the nose. There the 0, a oes 
olfactory nerves, which issue from the  «vity. 
front lobe of the cerebrum, and pass through the ethmoid 
bone, or roof of the nasal cavity, are distributed over a 
moist mucous membrane. The odorous substance, in a 
gaseous or finely divided state, is dissolved in the mucus 
covering this membrane. In Fishes and Reptiles gener- 
ally, this organ is feebly developed: Sharks, however, 
gather from a great distance around a carcass. In the 
Porpoises and Whales it is nearly or entirely wanting. 
Among Birds, Waders have the largest olfactory nerves ; 
but Vultures seem to have the keenest scent. It is most 
acute in the carnivorous quadrupeds, and in some wild 
herbivores, as the Deer. In Man it is less delicate, but 
has a wider range than in any brute. 
Hearing is the perception of sound. The simplest 
form of the organ is a sac filled with fluid, in which float 
the soft and delicate ends of the auditory nerve. The 
vibrations of the fluid are usually strengthened by the 
presence of minute hard granules, called ofoliths. The 
Invertebrates have no higher apparatus than this; and it 
is probable that they can distinguish one noise from an- 
other, but neither pitch nor intensity. In all animals the 
organ is double, but not always located in the head. In 
