Whether birds and fish ought not to chew.
One inquires further whether birds and fish ought not to chew.
1. It seems not. Because mastication is a certain preparation for digestion. But fish, since they are cold, have a weak digestion, and similarly the nutriment of birdslegumes, for exampleis difficult to digest. Therefore, they need to chew.
2. Moreover, the reason why fish do not masticate is, according to the Philosopher, that then too much of the water that surrounds them would enter their bellies and suffocate them. But this conclusion does not follow, because then it would work to prove the same for animals living in air.
3. In addition, the reason why birds do not masticate is that they lack teeth. But this does not suffice, because birds have hard beaks in place of a mouth and teeth. Therefore, just as others chew with teeth, so birds can chew with their beaks.
The Philosopher says the opposite.
One must reply that birds and fish do not chew. There are many reasons for this with respect to fish. One relates to the water that surrounds them, because if they were to chew, water would enter excessively and would work to suffocate and drown them. Another reason is related to their nutriment, which is soft and easily convertible, and this is why they do not need to chew. A third reason is related to gluttony, since these animals are gluttonous and snatch their food so eagerly that they swallow their nutriment almost whole.
The reason related to birds is that they do not have teeth, and even if they had them they would get in their way because birds have curved beaks, or beaks that hardly follow a straight line. Therefore, if birds had teeth, their mouths would remain always open, and as a result curved beaks would be useless to them for retaining food.
1. On to the arguments. To the first argument one must reply that although chewing is preparation for digestion, nevertheless fish do not need to chew because their nutriment is soft and digestible. Nor do birds need to do this, because either their nutriment is easily digestible or they have some instrument in which their nutriment is prepared, and this is why, etc.
2. To the second argument one must reply that water is, accidentally, softer than air and there is a great deal of windiness in fish because it contributes to their swimming. Now those learning to swim and experienced swimmers alike place on themselves bladders and other light things that are not easily submerged in water. They do this so that they will be supported by them. So too, nature provides fish with a great windiness inside their body so that they may be supported better while swimming. This windiness would be easily corrupted by water entering excessively. And for this reason, while walking animals chew, air exits the same way that it enters, because the lungs are in continuous motion. But this is not the case for fish, because they cannot expel water through their gills as easily as it enters through their mouth, on account of the gills' narrowness.
3. To the third argument one must reply that although birds have beaks in place of teeth, teeth can exist for many reasons for which beaks cannot, because beaks are uniform, as it were, whereas teeth are diverse, because some are sharp for breaking up and some are broad for grinding, and this is why, etc.
Whether birds ought to have a double stomach, like a liver and crop.
One inquires further whether birds ought to have a double stomach, like a liver and crop.
1. And it seems not. Because if a weaker thing has power over something, a stronger thing has power over the same thing, and even more so. But a crop is weak and tender, and a liver is strong and thick. Therefore, whatever a crop can do, a liver can do, too. Therefore, it does not need a crop.
2. In addition, what is natural is the same for all. But not all birds have a crop, as is evident among the heron and cranes. Therefore, it is not natural for birds to have a crop.
3. Moreover, possession of a crop is only necessary owing to a weak digestion. But digestion is weaker in fish than it is in birds. Therefore, fish need a crop more than birds do.
The Philosopher says the opposite.
One must reply that many birds do need a crop, for many reasons. One reason is that the warmer they are during the third digestion, the more heat they lack in the first and second digestions. Therefore, generally the liver in birds is particularly temperate and this is why they need [a crop], so that the nutriment may be softened and prepared before it reaches the iecur [liver], that is, the hepar [liver]. Another reason is that they lack teeth. Now, because they lack teeth and their nutriment is hardfor example, whole grain and similar sand, stones, and things of this sortand these are not broken up by teeth, therefore it is necessary that they be softened in something before they arrive at the liver, which is a solid member. A third reason is that many birds do not live as predators or acquire nutriment by violence, and this is why nature provides them with an instrument in which the nutriment can be preserved until the time when it is needed.
Nevertheless, not all birds have a crop. The nutriment for some birds is soft and easily digestible and convertible. Examples are those birds that live on worms, or others that live off the fruit of trees, and still others that live off flesh. And in the same way some birds have short and straight intestines, and the food they receive quickly passes out of ones like this, and animals like this, for this reason, do not need a crop. But those animals do need one who are nourished on solid grains or stones or things of this sort. And the reason for this has been given.
1. On to the arguments. To the first, one must reply that the liver is ordered for digestion, but the crop is ordered for retaining and softening, and this is why the crop is tender whereas the liver is hard and solid. Therefore, although the liver is more powerful than the crop, nevertheless this is not entirely so, just as the intellect can do more than a sense, yet it still requires a sense, because a sense can act on something on which the intellect cannot act in the same way, just as sight can see color but the intellect cannot. This is so in many other instances as well. Therefore, although the liver is stronger, nevertheless it cannot do what the crop does because it is not ordered for that, just as the intellect cannot see.
2. To another argument one must reply that it is already apparent that the nutriment of some birds is more convertible than the nutriment of others, and those who live more as predators live on cruder foods.
3. To the third argument one must reply that the nutriment for fish is liquid and easily convertible; they have short and straight intestines, and this is why they do not need a crop. Nevertheless, some fish that are not gluttonous live more on harder foods, and they have something like a crop, as the Philosopher says, and this is why, etc.
-  "Worms": vermes. The word can also indicate insects or any of several ignoble creatures. Cf. DA 26 ( SZ 2: 1739-64), where these are listed.