The chick in the shell begins to move its feet and legs on the sixth day of incubation (Mattreican, p. 138); or on the seventh day, (Langley); afterwards they are seen to move themselves gently in the liquid that surrounds them, and to open and shut their mouths, (Harvei, de Generat. p. 62, and 197. Form de Poulet. ii. p. 129). Puppies before the membranes are broken, that involve them, are seen to move themselves, to put out their tongues, and to open and shut their mouths, (Harvey, Gipson, Riolan, Haller). And calves lick themselves and swallow many of their hairs before their nativity: which however puppies do not, (Swammerden, p. 319. Flemyng Phil. Trans. Ann. 1755. 42). And towards the end of gestation, the fœtus of all animals are proved to drink part of the liquid in which they swim, (Haller. Physiol. T. 8. 204). The white of egg is found in the mouth and gizzard of the chick, and is nearly or quite consumed before it is hatched, (Harvie de Generat. 58). And the liquor amnii is found in the mouth and stomach of the human fœtus, and of calves; and how else should that excrement be produced in the intestines of all animals, which is voided in great quantity soon after their birth; (Gipson, Med. Essays, Edinb. V. i. 13. Halleri Physiolog. T. 3. p. 318. and T. 8). In the stomach of a calf the quantity of this liquid amounted to about three pints, and the hairs amongst it were of the same colour with those on its skin, (Blasii Anat. Animal, p.m. 122). These facts are attested by many other writers of credit, besides those above mentioned.
[III]. It has been deemed a surprising instance of instinct, that calves and chickens should be able to walk by a few efforts almost immediately after their nativity: whilst the human infant in those countries where he is not incumbered with clothes, as in India, is five or six months, and in our climate almost a twelvemonth, before he can safely stand upon his feet.
The struggles of all animals in the womb must resemble their mode of swimming, as by this kind of motion they can best change their attitude in water. But the swimming of the calf and chicken resembles their manner of walking, which they have thus in part acquired before their nativity, and hence accomplish it afterwards with very few efforts, whilst the swimming of the human creature resembles that of the frog, and totally differs from his mode of walking.
There is another circumstance to be attended to in this affair, that not only the growth of those peculiar parts of animals, which are first wanted to secure their subsistence, are in general furthest advanced before their nativity: but some animals come into the world more completely formed throughout their whole system than others: and are thence much forwarder in all their habits of motion. Thus the colt, and the lamb, are much more perfect animals than the blind puppy, and the naked rabbit; and the chick of the pheasant, and the partridge, has more perfect plumage, and more perfect eyes, as well as greater aptitude to locomotion, than the callow nestlings of the dove, and of the wren. The parents of the former only find it necessary to shew them their food, and to teach them to take it up; whilst those of the latter are obliged for many days to obtrude it into their gaping mouths.
[IV]. From the facts mentioned in No. [2]. of this Section, it is evinced that the fœtus learns to swallow before its nativity; for it is seen to open its mouth, and its stomach is found filled with the liquid that surrounds it. It opens its mouth, either instigated by hunger, or by the irksomeness of a continued attitude of the muscles of its face; the liquor amnii, in which it swims, is agreeable to its palate, as it consists of a nourishing material, (Haller Phys. T. 8. p. 204). It is tempted to experience its taste further in the mouth, and by a few efforts learns to swallow, in the same manner as we learn all other animal actions, which are attended with consciousness, by the repeated efforts of our muscles under the conduct of our sensations or volitions.
The inspiration of air into the lungs is so totally different from that of swallowing a fluid in which we are immersed, that it cannot be acquired before our nativity. But at this time, when the circulation of the blood is no longer continued through the placenta, that suffocating sensation, which we feel about the precordia, when we are in want of fresh air, disagreeably affects the infant: and all the muscles of the body are excited into action to relieve this oppression; those of the breast, ribs, and diaphragm are found to answer this purpose, and thus respiration is discovered, and is continued throughout our lives, as often as the oppression begins to recur. Many infants, both of the human creature, and of quadrupeds, struggle for a minute after they are born before they begin to breathe, (Haller Phys. T. 8. p. 400. ib pt. 2. p. 1). Mr. Buffon thinks the action of the dry air upon the nerves of smell of new-born animals, by producing an endeavour to sneeze, may contribute to induce this first inspiration, and that the rarefaction of the air by the warmth of the lungs contributes to induce expiration, (Hist. Nat. Tom. 4. p. 174). Which latter it may effect by producing a disagreeable sensation by its delay, and a consequent effort to relieve it. Many children sneeze before they respire, but not all, as far as I have observed, or can learn from others.
At length, by the direction of its sense of smell, or by the officious care of its mother, the young animal approaches the odoriferous rill of its future nourishment, already experienced to swallow. But in the act of swallowing, it is necessary nearly to close the mouth, whether the creature be immersed in the fluid it is about to drink, or not: hence, when the child first attempts to suck, it does not slightly compress the nipple between its lips, and suck as an adult person would do, by absorbing the milk; but it takes the whole nipple into its mouth for this purpose, compresses it between its gums, and thus repeatedly chewing (as it were) the nipple, presses out the milk, exactly in the same manner as it is drawn from the teats of cows by the hands of the milkmaid. The celebrated Harvey observes, that the fœtus in the womb must have sucked in a part of its nourishment, because it knows how to suck the minute it is born, as any one may experience by putting a finger between its lips, and because in a few days it forgets this art of sucking, and cannot without some difficulty again acquire it, (Exercit. de Gener. Anim. 48). The same observation is made by Hippocrates.
A little further experience teaches the young animal to suck by absorption, as well as by compression; that is, to open the chest as in the beginning of respiration, and thus to rarefy the air in the mouth, that the pressure of the denser external atmosphere may contribute to force out the milk.
The chick yet in the shell has learnt to drink by swallowing a part of the white of the egg for its food; but not having experienced how to take up and swallow solid seeds, or grains, is either taught by the felicitous industry of its mother; or by many repeated attempts is enabled at length to distinguish and to swallow this kind of nutriment.
And puppies, though they know how to suck like other animals from their previous experience in swallowing, and in respiration; yet are they long in acquiring the art of lapping with their tongues, which from the flaccidity of their cheeks, and length of their mouths, is afterwards a more convenient way for them to take in water.