All viviparous quadrupeds,

then, are furnished with an oesophagus
and a windpipe, situated as in man; the same statement is applicable
to oviparous quadrupeds and to birds, only that the latter present
diversities in the shapes of these organs. As a general rule, all
animals that take up air and breathe it in and out are furnished
with a lung, a windpipe, and an oesophagus, with the windpipe and
oesophagus not admitting of diversity in situation but admitting of
diversity in properties, and with the lung admitting of diversity in
both these respects. Further, all blooded animals have a heart and a
diaphragm or midriff; but in small animals the existence of the latter
organ is not so obvious owing to its delicacy and minute size.