In regard to the heart

there is an exceptional phenomenon
observable in oxen. In other words, there is one species of ox
where, though not in all cases, a bone is found inside the heart. And,
by the way, the horse's heart also has a bone inside it.

The genera referred to above are not in all cases furnished
with a lung: for instance, the fish is devoid of the organ, as is also
every animal furnished with gills. All blooded animals are furnished
with a liver. As a general rule blooded animals are furnished with a
spleen; but with the great majority of non-viviparous but oviparous
animals the spleen is so small as all but to escape observation; and
this is the case with almost all birds, as with the pigeon, the
kite, the falcon, the owl: in point of fact, the aegocephalus is
devoid of the organ altogether. With oviparous quadrupeds the case
is much the same as with the viviparous; that is to say, they also
have the spleen exceedingly minute, as the tortoise, the freshwater
tortoise, the toad, the lizard, the crocodile, and the frog.