Whether cartilage is necessary.
Further one asks whether cartilage is necessary.
1. It seems not. Whatever serves neither motion nor sense is unnecessary to an animal. But cartilage is of this sort, and therefore, etc.
2. Moreover, "nature does nothing in vain and does not lack what is necessary"; but in many animals cartilage is absent and yet nature does not lack what is necessary. Therefore cartilage is not necessary.
As previously, the opposite is evident.
One must reply that cartilage is necessary. For some parts need to flex and to be stabilized. But these two occur together in neither flesh nor bone. For there is no stability in flesh and no flexion in bone, and this is why it is necessary that there exist some part in which the two occur together, and cartilage is like this, and this is why there is cartilage in the nose and in the ear. Therefore, cartilage takes the place of bone for the purpose of stability and takes the place of nerve in them for the purpose of flexion. Similarly, it takes the place of flesh for the purpose of softness.
1. On to the arguments. To the first, one must reply that although cartilage is not an organ of sense or motion, it nevertheless supports organs of sense and motion.
2. To the second argument one must reply that some parts are intended to be so solid that they are inflexible; otherwise, the leg and the hip bone would not support the body, and this is why cartilage does not exist in all the parts.
One next inquires about hair: why it exists on brute beasts from birth, but on a human is present only on certain determinate parts, like the head, eyebrows, and eyelashes.
Second, one asks why the hairs on the head do not always grow, namely, in discrete number, whereas the hairs of the beard grow and increase in number.
Third, one asks why women do not have a beard, as do men.
Fourth, one asks why hair color changes more during old age than it does during youth.
Fifth, one asks why hairs that have been cut off grow back but feathers do not.
To the first, one must reply that of all the animals the human has parts that are moister at the beginning of his life. Hairs are actually caused by hard, dry, and solid matter, and this is why there is more hair on other animals at birth than there is on a human, since they have more dry matter which provides the material for hair, whereas in a human more hair exists on hard and more solid parts, like the skin and the head's skull and the eyelashes and eyebrows.
In addition, the eye is the most fragile of the external members, and this is why nature provides hairs on the eyebrows from the beginningas a protection for the eyes in order to defend them better from external injury such as a bit of straw falling into the eye or the like, etc.
To the second question one must reply that heat is required for the generation of hair, a heat that expels the fumes that form the material for hair and are a material suited to the generation of hair. Yet we know a great number of hairs is caused by the quantitative division of matter because, as the Philosopher says in the third book of the Physics, number is caused by the division of a continuous [quantity]. But the growth in quantity is caused by growth in power. Now, then, heat is powerful in youths and the matter for hair is abundant, namely, the dry fumes, and this is why the hairs on the head increase in number all the way through youth. But in older people the heat is weakened, and a dry and earthy superfluity are more abundant. Thus the weak heat cannot propel the matter for hair as far as the head. Rather, the heat is itself turned back as far as the cheeks and the beard, and the matter, which is heavy, descends that far because the weak heat cannot raise it up; nevertheless it accomplishes what it can and raises the heat to the beard as well as to a nearer location.
And if one objects to this, that therefore the chest and the back will be hairier than the beard because they are nearer, etc., one must reply that this is not correct because although the matter goes there, much of it is not retained but escapes due to the looseness and openness of the pores there, but the beard, which is bony and cold, retains the fumes well until they are converted into hairs, etc. And, moreover, because a warm fume always moves upwards, as it were, this is why more of it goes to the chin than to the chest. Actually it cannot go to the head because it does not have enough power. And this is why the hairs of the beard multiply among old people, whereas in youths they multiply on the head, and the reason is evident.
There is another reason, according to the physicians, which is that semen is drawn from the brain to the seminal vessels, through the cheeks and the beard. And this is why when the generative power begins to dominate and the semen increases in a person, the hairs then begin to grow on the beard, because the heat is rising from the genital members and from the heart, and the spermatic spirit turns back to the brain as far as the beard, and there it is strengthened and generates hair. Thus, when the hairs of the beard are greatly increased in number, then this is an indication that old age is quickly approaching, because the heat is lessening, etc.
To the third question one must reply that moisture, which blocks heat, abounds in women and this is why they do not grow beards or hair on their other parts, because the matter cannot be raised up owing to the coldness of their complexion, except among those in whom heat greatly abounds. Sometimes women like this produce a beard and, similarly, hair on the other parts, like in the armpits, the groin, and around the mouth or the opening of the vulva, owing to the strong heat resulting from the striking of the penis against the vulva during intercourse etc.
To the fourth question one must reply that the cold and moist are the cause of whiteness. Thus congealed water turns white as do snow, hail, and crystal, which is nothing but congealed water. But in old age the heat fades, and as a result the cold is increased, so that the heat does not have the power to expel all the matter of the hair but only that matter that is thinner and more watery. And this is why hairs in old age turn white, owing to the weakness of the heat and the wateriness of the moisture, because this is when excessive cold tinges and whitens them.
To the last question one must reply that feathers are created from a more phlegmatic moisture than that which gives rise to hair. An indication of this is that they resemble organic parts. Now the feathers do not have the same nature in every part, and thus, just as an organic part, like a finger that has been cut off, does not grow back, neither does a feather that has been cut off return, because there is not sufficient power in the remainder of the feather still affixed to the body to produce a part similar to the one cut off. If, however, a feather is completely removed, another can grow in its place because the power capable of producing a whole feather still remains in the entire body, although the power capable of producing another one does not exist in a single part. Therefore, when the feathers are removed at the root they return, as is evident in geese that have been plucked to make pillows, etc. And the reason is evident.
-  Crystal: crystallus, more commonly crystallum.