Whether the liver is generated from semen or from menstrual blood.

One inquires further into the complexion and generation of the liver, whether the liver is generated from semen or from menstrual blood.

1. And it seems to be from the blood. For the liver is coagulated blood. But coagulation does not remove the nature or species of a thing, as is clear in the case of frozen water and coagulated milk and, similarly, glass. Therefore, the liver is generated from blood.

2. Moreover, the complexion and accidents of a thing attest to its origin. But semen is white, as is said in the third book, and those that are generated from semen have a white color, like bones, veins, and the like. But the liver is reddish and has the color of blood. Therefore, it is generated from blood and not semen.

To the contrary. A principal member is generated from semen and not from blood. But the liver is a principal member; therefore, etc.

One must reply that the liver is not generated from blood but first from semen. Because there are four principal powers in the body, which the four principal members serve immediately: for example, the brain serves the animal power, the liver the natural power, the heart the vital power, and the testicles the generative power. If the other members are from semen, then these members arise from semen. And an indication of this is that a principal member that has been amputated does not regenerate. And so it is for the liver, so that if it is wounded it cannot be healed, and this is clear whether medicaments are applied or whether physicians treat it. Because in the first generation of the embryo two bubbles are found in the semen: the heart arises from one of these, and the liver from the other, and this is why one must say that the liver arises from the semen.

1. On to the arguments. To the first argument one must reply that the liver is not said to be coagulated blood, because it is blood, but because it resembles blood in color.

2. To the second argument one must reply that a similarity in color or even complexion does not argue for a unity of substance, and this is why, etc.

Whether the motion of the lungs is natural.

One inquires further into the lungs. And first, whether the motion of the lungs is natural.

It seems not. Natural motion occurs simultaneously in both the artery and the heart, because the artery's motion comes from a natural motion in the heart. But the lungs' motion does not occur at the same time with the motion of the heart. Therefore, it does not proceed from the natural power, which is in the heart.

In addition, natural motion is not subject to speeding up or slowing down. But the lungs' motion can be speeded up or slowed down, since a person can hold in his breath for a long time and, as a result, slow down the lungs.

To the contrary. The lungs' motion is on account of the heart's motion. But the heart's motion is natural, and therefore so too is the lungs'.

One must reply that one motion in an animal is solely natural, and another is only animal, and still another is composed of both natural and animal. The motion of the heart and spirit in the arteries is only a natural motion. Animal motion is only a progressive motion. A composite motion is like the motion of urine and excrement. And the motion of the lungs is like this sort of motion. Now, if one should consider the first principle of this motion, this comes from the heart and this is why it is a natural motion; if, however, we should consider the mode, this is an animal motion because it can be speeded up or slowed down.

In this way a solution is apparent to the arguments.

Whether a pulse exists in every animal.

One inquires further whether a pulse exists in every animal.

It seems so. For the pulse exists on account of the heart's motion, just as breathing does. But breathing is present in every animal having lungs. Therefore, so too is the pulse.

In addition, the branches are moved once the root is moved. But the arteries arise from the heart. Therefore, once the heart is moved, the arteries are moved. But the motion of the arteries is the pulse. Since, then, motion is present in the heart of every animal that has a heart and arteries, the pulse's motion will exist in every animal.

Moreover, the pulse is nothing other than the motion of the spirit inside the arteries. But this sort of motion is present in every animal that has arteries. Therefore, etc.

The Philosopher says the opposite.

One must reply that a pulse can be understood in two ways: either on behalf of an extrinsic motion, and thus it is said in contrast to a pulling and twirling movement in the seventh book of the Physics, and this kind of pulse can be found in every animal. Pulse can be understood in another way on behalf of an intrinsic motion, and this type is double: either accidental or natural. An accidental pulse arises from "windiness," and a pulse like this can exist in any animal, and it is said to be a twitch in the members or a hiccough. If it is a natural pulse, it will be either in the lungs or in the heart. If in the heart, then again it can exist in every animal that has a heart. If in the lungs, then this can exist in two ways: either according to the customary course of natureand so again a pulse exists in every animal that has lungs just as does movement of the lungsor according to a non-customary course of nature, and this is nothing other than a sigh or a deep breath. And a pulse like this exists in an animal only as a result of fear or hope. Now when a human is afraid, he can see by his senses a quick and unaccustomed motion of his lungs. And the Philosopher talks about this sort of pulse. And because a human alone properly has hope for future goods and fear of evils, this is why the Philosopher says that a pulse like this exists only in a human.