Whether every animal that has urine has excrement.
Second, one asks about the converse, whether every animal that has urine has excrement.
1. It seems not. For Avicenna says that a fetus [fetus] in the uterus has urine and sweat, but not excrement. But a child [ puer] in the uterus is an animal, and therefore, etc.
2. Again, excrement is left over from the digestion that occurs in the stomach. But according to both Avicenna and the
Philosopher a child in the womb is not nourished from its stomach but through the umbilical cord, and it therefore will not have excrement.
The Philosopher says the opposite. For he says that "every animal that has a bladder has a stomach," but not contrariwise.
To the first question one must reply that not every animal that has a vessel reserved for the first superfluity has a vessel reserved for the second superfluity. And the reason for this is twofold. One reason is that in every animal there is a second superfluity just as there is a first. But in some animals the second superfluity descends into something necessary for them, just as in birds the second superfluity descends to the areas for wings and feathers. And this is why they do not have a designated vessel capable of receiving it; instead, in such animals, as quickly as the urine is isolated in the liver, by nature's wisdom it is resolved into fumes and vapors and travels to the body's surface to generate feathers.
Another cause is that some animals are warm and moist, like the human and the sheep, and some are warm and dry, like birds. In warm and moist animals moisture abounds in the liver, and this is why the superfluity that is left over after the digestion that occurs in the liver is sent to a designated vessel such as a bladder. But in warm and dry animals the entire moisture barely suffices to temper their excessive heat. So the superfluity of the second digestion is consumed by their own complexional heat, and this is why birds do not urinate but only defecate. Thus, briefly: food is taken in to restore what has been lost, But drink is taken in for two reasons: so that together with it the food may better penetrate to the members, and in like manner, so that the members will be moistened or the heat tempered. And because it is more necessary for that which has been lost to be restored than for the members to be moistened or for the heat to be tempered, not every animal therefore has the same necessity for drinking that it has for eating.
And besides this, the birds of prey live on raw and moist flesh, and this is why they do not need to drink in the way walkers do, and because they live and they fly in the clean, pure, and subtle air and this air, according to Galen in the Tegni, satisfies their thirst more than water. This is why the air moistens them in their chests, an air that is cold in act but which internally is some measure of moisture mixed with food. For this reason, etc. Thus, universally across their genus, flyers are warmer than walkers; nevertheless, in a species a given walker is warmer than a given flyer.
And further, since the entire moisture which some animals take in barely suffices to moisten their members or temper their heat, they do not, for this reason, emit urine. But because a second digestion presupposes a first, and that which is digested first is not converted entirely into the substance of a food source, the Philosopher therefore says that "every animal that has a bladder has a stomach." From this a solution to each question is thus evident.
1. On to the arguments. To the first, one must say that nourishment is purified in the second digestion. But what is superfluous there is either immediately consumed by the strength of the heat in such animals, or it is sent to moisten certain designated parts, like the feathers and wings on birds. Thus every animal has a second superfluity, but not every one has a vessel reserved for it. And the reason for this has already been seen.
2. One must reply to the second in the same way, that even if there occurs a passage from the first digestion to the second, nevertheless it is not necessary that there be a designated vessel for the superfluity of the second digestion.
To the arguments for the second question one must say that the Philosopher is speaking about an animal that is nourished by means of a stomach existing outside the womb, and living on things produced from the earth. But the fetus in the uterus is nourished by the umbilical cord and on menstrual blood, which needs no purification and does not have the first superfluity,
but only the second and the third, and the second and third digestions suffice for the purification of its menstrual blood. And this is why the fetus in the uterus does not have the first superfluity, etc., and this is why he does not assert that it does.
-  Galen, Tegni 2.