OF THE SPERM WHALE. 
109 
one that is rarer, and is to water a species of vacuum, 
the pressure being much greater on the external surface 
than the counter pressure from within. But expiration 
on the other hand must be much more easily per- 
formed, 5 ? 
OF THE BLOW-HOLE OR PASSAGE FOR THE AIR, 
66 As the nose in every animal that breathes air is a com- 
mon passage for the air, and is also the organ of smelling, 
I shall describe it in this tribe as instrumental to these 
purposes. The membranous portion of the posterior 
nostril is one canal, but when in the bony part, in most 
of them it is divided into two. The spermaceti whale, 
however, is an exception. At its beginning in the 
fauces it is a roundish hole, surrounded by a strong 
sphincter muscle, for grasping the epiglottis. In the 
spermaceti whale, which has a single canal, it is thrown 
a little to the left side. This opening forms a passage 
for the air to and from the lungs, for it would be impos- 
sible for these animals to breathe through the mouth ; 
indeed, I believe the human species alone breathe by 
the mouth, and in them it appears mostly from habit, 
for in quadrupeds the epiglottis conducts the air through 
the nose. 
In the whole of this tribe the situation of the opening 
on the upper surface of the head is well adapted for this 
purpose, being the first part that comes to the surface of 
the water ; in the natural progressive motion of the 
animal, therefore, it is to be considered principally as 
