Aristotle

by Various Works [lit]

  gazelle. But in all our speculations concerning nature, what we have

  to consider is the general rule; for that is natural which applies

  either universally or generally. And thus when we say that the largest

  animals have most earthy matter, we say so because such is the general

  rule. Now this earthy matter is used in the animal body to form

  bone. But in the larger animals there is an excess of it, and this

  excess is turned by nature to useful account, being converted into

  weapons of defence. Part of it necessarily flows to the upper

  portion of the body, and this is allotted by her in some cases to

  the formation of tusks and teeth, in others to the formation of horns.

  Thus it is that no animal that has horns has also front teeth in

  both jaws, those in the upper jaw being deficient. For nature by

  subtracting from the teeth adds to the horns; the nutriment which in

  most animals goes to the former being here spent on the augmentation

  of the latter. Does, it is true, have no horns and yet are equally

  deficient with the males as regards the teeth. The reason, however,

  for this is that they, as much as the males, are naturally

  horn-bearing animals; but they have been stripped of their horns,

  because these would not only be useless to them but actually

  baneful; whereas the greater strength of the males causes these

  organs, though equally useless, to be less of an impediment. In

  other animals, where this material is not secreted from the body in

  the shape of horns, it is used to increase the size of the teeth; in

  some cases of all the teeth, in others merely of the tusks, which thus

  become so long as to resemble horns projecting from the jaws.

  So much, then, of the parts which appertain to the head.

  3

  Below the head lies the neck, in such animals as have one. This is

  the case with those only that have the parts to which a neck is

  subservient. These parts are the larynx and what is called the

  oesophagus. Of these the former, or larynx, exists for the sake of

  respiration, being the instrument by which such animals as breathe

  inhale and discharge the air. Therefore it is that, when there is no

  lung, there is also no neck. Of this condition the Fishes are an

  example. The other part, or oesophagus, is the channel through which

  food is conveyed to the stomach; so that all animals that are

  without a neck are also without a distinct oesophagus; Such a part

  is in fact not required of necessity for nutritive purposes; for it

  has no action whatsoever on the food. Indeed there is nothing to

  prevent the stomach from being placed directly after the mouth.

  This, however, is quite impossible in the case of the lung. For

  there must be some sort of tube common to the two divisions of the

  lung, by which--it being bipartite--the breath may be apportioned to

  their respective bronchi, and thence pass into the air-pipes; and such

  an arrangement will be the best for giving perfection to inspiration

  and expiration. The organ then concerned in respiration must of

  necessity be of some length; and this, again, necessitates there being

  an oesophagus to unite mouth and stomach. This oesophagus is of a

  flesh-like character, and yet admits of extension like a sinew. This

  latter property is given to it, that it may stretch when food is

  introduced; while the flesh-like character is intended to make it soft

  and yielding, and to prevent it from being rasped by particles as they

  pass downwards, and so suffering damage. On the other hand, the

  windpipe and the so-called larynx are constructed out of a

  cartilaginous substance. For they have to serve not only for

  respiration, but also for vocal purposes; and an instrument that is to

  produce sounds must necessarily be not only smooth but firm. The

  windpipe lies in front of the oesophagus, although this position

  causes it to be some hindrance to the latter in the act of

  deglutition. For if a morsel of food, fluid or solid, slips into it by

  accident, choking and much distress and violent fits of coughing

  ensue. This must be a matter of astonishment to any of those who

  assert that it is by the windpipe that an animal imbibes fluid. For

  the consequences just mentioned occur invariably, whenever a

  particle of food slips in, and are quite obvious. Indeed on many

  grounds it is ridiculous to say that this is the channel through which

  animals imbibe fluid. For there is no passage leading from the lung to

  the stomach, such as the oesophagus which we see leading thither

  from the mouth. Moreover, when any cause produces sickness and

  vomiting, it is plain enough when the fluid is discharged. It is

  manifest also that fluid, when swallowed, does not pass directly

  into the bladder and collect there, but goes first into the stomach.

  For, when red wine is taken, the dejections of the stomach are seen to

  be coloured by its dregs; and such discoloration has been even seen on

  many occasions inside the stomach itself, in cases where there have

  been wounds opening into that organ. However, it is perhaps silly to

  be minutely particular in dealing with silly statements such as this.

  The windpipe then, owing to its position in front of the oesophagus,

  is exposed, as we have said, to annoyance from the food. To obviate

  this, however, nature has contrived the epiglottis. This part is not

  found in all sanguineous animals, but only in such of them as have a

  lung; nor in all of these, but only in such as at the same time have

  their skin covered with hairs, and not either with scaly plates or

  with feathers. In such scaly and feathered animals there is no

  epiglottis, but its office is supplied by the larynx, which closes and

  opens, just as in the other case the epiglottis falls down and rises

  up; rising up during the ingress or egress of breath, and falling down

  during the ingestion of food, so as to prevent any particle from

  slipping into the windpipe. Should there be the slightest want of

  accuracy in this movement, or should an inspiration be made during the

  ingestion of food, choking and coughing ensue, as already has been

  noticed. So admirably contrived, however, is the movement both of

  the epiglottis and of the tongue, that, while the food is being ground

  to a pulp in the mouth, the tongue very rarely gets caught between the

  teeth; and, while the food is passing over the epiglottis seldom

  does a particle of it slip into the windpipe.

  The animals which have been mentioned as having no epiglottis owe

  this deficiency to the dryness of their flesh and to the hardness of

  their skin. For an epiglottis made of such materials would not admit

  of easy motion. It would, indeed, take a longer time to shut down an

  epiglottis made of the peculiar flesh of these animals, and shaped

  like that of those with hairy skins, than to bring the edges of the

  windpipe itself into contact with each other.

  Thus much then as to the reason why some animals have an

  epiglottis while others have none, and thus much also as to its use.

  It is a contrivance of nature to remedy the vicious position of the


  windpipe in front of the oesophagus. That position is the result of

  necessity. For it is in the front and centre of the body that the

  heart is situated, in which we say is the principle of life and the

  source of all motion and sensation. (For sensation and motion are

  exercised in the direction which we term forwards, and it is on this

  very relation that the distinction of before and behind is founded.)

  But where the heart is, there and surrounding it is the lung. Now

  inspiration, which occurs for the sake of the lung and for the sake of

  the principle which has its seat in the heart, is effected through the

  windpipe. Since then the heart must of necessity lie in the very front

  place of all, it follows that the larynx also and the windpipe must of

  necessity lie in front of the oesophagus. For they lead to the lung

  and heart, whereas the oesophagus leads to the stomach. And it is a

  universal law that, as regards above and below, front and back,

  right and left, the nobler and more honourable part invariably is

  placed uppermost, in front, and on the right, rather than in the

  opposite positions, unless some more important object stands in the

  way.

  4

  We have now dealt with the neck, the oesophagus, and the windpipe,

  and have next to treat of the viscera. These are peculiar to

  sanguineous animals, some of which have all of them, others only a

  part, while no bloodless animals have any at all. Democritus then

  seems to have been mistaken in the notion he formed of the viscera,

  if, that is to say, he fancied that the reason why none were

  discoverable in bloodless animals was that these animals were too

  small to allow them to be seen. For, in sanguineous animals, both

  heart and liver are visible enough when the body is only just

  formed, and while it is still extremely small. For these parts are

  to be seen in the egg sometimes as early as the third day, being

  then no bigger than a point; and are visible also in aborted

  embryos, while still excessively minute. Moreover, as the external

  organs are not precisely alike in all animals, but each creature is

  provided with such as are suited to its special mode of life and

  motion, so is it with the internal parts, these also differing in

  different animals. Viscera, then, are peculiar to sanguineous animals;

  and therefore are each and all formed from sanguineous material, as is

  plainly to be seen in the new-born young of these animals. For in such

  the viscera are more sanguineous, and of greater bulk in proportion to

  the body, than at any later period of life, it being in the earliest

  stage of formation that the nature of the material and its abundance

  are most conspicuous. There is a heart, then, in all sanguineous

  animals, and the reason for this has already been given. For that

  sanguineous animals must necessarily have blood is self-evident.

  And, as the blood is fluid, it is also a matter of necessity that

  there shall be a receptacle for it; and it is apparently to meet

  this requirement that nature has devised the blood-vessels. These,

  again, must necessarily have one primary source. For it is

  preferable that there shall be one such, when possible, rather than

  several. This primary source of the vessels is the heart. For the

  vessels manifestly issue from it and do not go through it. Moreover,

  being as it is homogeneous, it has the character of a blood-vessel.

  Again its position is that of a primary or dominating part. For

  nature, when no other more important purpose stands in her way, places

  the more honourable part in the more honourable position; and the

  heart lies about the centre of the body, but rather in its upper

  than its lower half, and also more in front than behind. This is

  most evident in the case of man, but even in other animals there is

  a tendency in the heart to assume a similar position, in the centre of

  the necessary part of the body, that is to say of the part which

  terminates in the vent for excrement. For the limbs vary in position

  in different animals, and are not to be counted with the parts which

  are necessary for life. For life can be maintained even when they

  are removed; while it is self-evident that the addition of them to

  an animal is not destructive of it.

  There are some who say that the vessels commence in the head. In

  this they are clearly mistaken. For in the first place, according to

  their representation, there would be many sources for the vessels, and

  these scattered; and secondly, these sources would be in a region that

  is manifestly cold, as is shown by its intolerance of chill, whereas

  the region of the heart is as manifestly hot. Again, as already

  said, the vessels continue their course through the other viscera, but

  no vessel spreads through the heart. From this it is quite evident

  that the heart is a part of the vessels and their origin; and for this

  it is well suited by its structure. For its central part consists of a

  dense and hollow substance, and is moreover full of blood, as though

  the vessels took thence their origin. It is hollow to serve for the

  reception of the blood, while its wall is dense, that it may serve

  to protect the source of heat. For here, and here alone in all the

  viscera and indeed in all the body, there is blood without

  blood-vessels, the blood elsewhere being always contained within

  vessels. Nor is this but consistent with reason. For the blood is

  conveyed into the vessels from the heart, but none passes into the

  heart from without. For in itself it constitutes the origin and

  fountain, or primary receptacle, of the blood. It is however, from

  dissections and from observations on the process of development that

  the truth of these statements receives its clearest demonstration. For

  the heart is the first of all the parts to be formed; and no sooner is

  it formed than it contains blood. Moreover, the motions of pain and

  pleasure, and generally of all sensation, plainly have their source in

  the heart, and find in it their ultimate termination. This, indeed,

  reason would lead us to expect. For the source must, when. ever

  possible, be one; and, of all places, the best suited for a source

  is the centre. For the centre is one, and is equally or almost equally

  within reach of every part. Again, as neither the blood itself, nor

  yet any part which is bloodless, is endowed with sensation, it is

  plain that that part which first has blood, and which holds it as it

  were in a receptacle, must be the primary source of sensation. And

  that this part is the heart is not only a rational inference, but also

  evident to the senses. For no sooner is the embryo formed, than its

  heart is seen in motion as though it were a living creature, and

  this before any of the other parts, it being, as thus shown, the

  starting-point of their nature in all animals that have blood. A

  further evidence of the truth of what has been stated is the fact that

  no sanguineous animal is without a heart. For the primary source of

  blood must of necessity be present in them all. It is true that

  sanguineous
animals not only have a heart but also invariably have a

  liver. But no one could ever deem the liver to be the primary organ

  either of the whole body or of the blood. For the position in which it

  is placed is far from being that of a primary or dominating part; and,

  moreover, in the most perfectly finished animals there is another

  part, the spleen, which as it were counterbalances it. Still

  further, the liver contains no spacious receptacle in its substance,

  as does the heart; but its blood is in a vessel as in all the other

  viscera. The vessel, moreover, extends through it, and no vessel

  whatsoever originates in it; for it is from the heart that all the

  vessels take their rise. Since then one or other of these two parts

  must be the central source, and since it is not the liver which is

  such, it follows of necessity that it is the heart which is the source

  of the blood, as also the primary organ in other respects. For the

  definitive characteristic of an animal is the possession of sensation;

  and the first sensory part is that which first has blood; that is to

  say is the heart, which is the source of blood and the first of the

  parts to contain it.

  The apex of the heart is pointed and more solid than the rest of the

  organ. It lies against the breast, and entirely in the anterior part

  of the body, in order to prevent that region from getting chilled. For

  in all animals there is comparatively little flesh over the breast,

  whereas there is a more abundant covering of that substance on the

  posterior surface, so that the heat has in the back a sufficient

  amount of protection. In all animals but man the heart is placed in

  the centre of the pectoral region; but in man it inclines a little

  towards the left, so that it may counterbalance the chilliness of that

  side. For the left side is colder in man, as compared with the

  right, than in any other animal. It has been stated in an earlier

  treatise that even in fishes the heart holds the same position as in

  other animals; and the reason has been given why it appears not to

  do so. The apex of the heart, it is true, is in them turned towards

  the head, but this in fishes is the front aspect, for it is the