Experiments of Needham and Bufon. Hypothesis of Leuwenhoeck, Abandoned. energy of the plants and animals in which they are found ;* whereas, it is now well known that they also originate from parent animals, as well as all others. There is only one set of experiments which seem still to give some countenance to the doctrine of equivocal generation. Needham and Buffon found successively that animal and vegetable infusions, in boiling water, though put into a phial while scalding hot, and kept ever after close corked, generated animalcula in a few days. These experiments were afterwards repeated and confirmed by Mr. Wright. Now it is conceived, that the boiling water would have destroyed any ova of these animalcula, previously deposited upon the animal or vegetable substances employed. And hence it has been inferred, that the animalcula in question have been produced by equivocal generation. But we have no right to suppose that boiling water is capable of destroying the ova of all animals, because it is capable of destroying a great many. The heat of boiling water will not prevent the seeds of many vegetables from germinating. It is likely that the ova of the animalcula in question are capable of resisting its action. The opinion respecting generation, anciently received, was, that there was a female semen as well as a male; that the two were mixed together in the uterus, and that the fœtus was produced by the mixture. But Leuwenhoeck, having observed a vast number of animalcula in the male semen of different animals, was induced, in consequence, to propagate a different opinion; which very speedily acquired celebrity; and was even pretty generally acceded to. According to him, one of these animalcula constitutes the rudiments of the future fœtus. It is merely lodged in a convenient nidus, in the uterus, and all that the female has to do is to supply it with proper nourishment. Dr. Garden, of Aberdeen, made an addition to this theory. According to him, during the period of copulation, one or more ova make their way from the female ovaria to the uterus; such of the animalcula of the male semen, as were fortunate enough to get into these ova, found a proper nidus, were nourished, and gradually increasing in size, became fœtuses; while all the others, amounting by Leuwenhoeck's calculation to several millions, speedily perished for want of nourishment.‡ Subsequent physiologists having examined the semen of various animals, found some in which no traces of animalcula could be found. These were as capable of impregnating the female, as those in which animalcula abounded. This was sufficient to overturn the hypothesis of Leuwenhoeck. Accordingly it was abandoned, and consigned to ridicule as speedily as it had been adopted. *Phil. Trans. 1670. Vol. V. p. 1175. + Phil. Trans. 1755. Vol. XLIX. p. 553. Indeed there is reason to believe, that Leuwenhoeck had sometimes mistaken small crystals of phosphate of lime, which abound in the semen, for animalcula. The opinion of Harvey, that the foetus proceeds from an ovum detached from the female ovarium, and somehow fecundated by the influence of the male semen, came to be universally admitted. This opinion was supported with But De Graff is the anatomist who considerable keenness by Kirkringius.* threw the greatest light on the subject. He ascertained, by numerous experiments on rabbits, that the ovaries are the seat of conception; that one or more of their vesicles become changed; that they are enlarged, lose their transparency, and become opaque and reddish coloured; that the number of vesicles thus altered corresponds with the number of foetuses; that these changed vesicles at a certain period after copulation discharge a substance, which, being laid hold of by the fimbriated extremity of the falopian tube, and conveyed into the uterus, soon assumes a visible vesicular form, and is called an ovum; and that this ovum gradually evolves different organs, and becomes a fœtus. Considerable additional light has been thrown upon this obscure subject Experiments by a curious paper of Dr. Haighton, published in the Philosophical Transac- of Haighton. tions for 1797.† He has demonstrated by very decisive experiments upon rabbits that the male semen does not pass into the falopian tube or ovaries; but merely stimulates the vagina. One or more ova pass in consequence of this stimulus into the uterus, and become the fœtus. These experiments These experiments render the process of generation if possible still more obscure than it was. semen certainly answers some other purposes than merely acting as a stimulus to the parts, otherwise we could not see why it should be so absolutely necessary in all cases as it is. Fowls often lay eggs without impregnation; but unless they have had connection with a male these eggs are incapable of being hatched. Toads, frogs, and fish, exclude the ova out of the body without any previous connection with the male, and the male afterwards sprinkles his semen on the excluded ova, without which they are incapable of being converted into animals. Here the semen is applied not to the female organs, but to the semen, and therefore cannot be intended merely to stimulate these organs. The male Mr. Cruikshanks has demonstrated by a set of well conducted experiments on rabbits, that the ovum is formed in the ovarium, that it comes out of it after conception; that it passes along the falopian tube, and that it takes some days in making its way to the uterus.‡ Female animals have two ovaries, one on each side. Mr. John Hunter extirpated one of these organs from a sow, and compared her breeding powers with another sow of the same age, and treated in every respect the same as the first sow; but retaining both her ovaries. The Phil. Trans. 1672. Vol. VII. p. 4018. + Vol. LXXXVII. p. 159. Nourishment of the fœtus. Children cry in the womb. spayed sow continued to breed till she was six years of age; the perfect sow till she was eight the spayed sow had 76 pigs; the perfect sow 162. * From this experiment it would appear that a determinate number of ova exist at first in each ovarium, and that when these are exhausted the animal ceases to breed. The loss of an ovarium must of course have a considerable influence both on the time of breeding, and upon the quantity of young produced. Needham published a work giving an anatomical account of the vessels, liquors, coverings, nourishment, &c. of the fœtus, from which he acquired a high reputation. Of this work there is a good analysis in the Philosophical Transactions. There can be no doubt that the foetus is nourished, at least chiefly, by means of the umbilical canal. Hence the reason why respiration is unnecessary, and why the foetus can live and grow without some of its most essential organs, as is well known sometimes to happen. The fœtus in utero is surrounded by a liquor called the liquor amnii, and it has been supposed by some that this liquor makes its way into the mouth of the fœtus, and serves it for nourishment. Dr. Fleming mentions a fact which at first sight seems to prove the truth of this opinion, at least with regard to some animals. Examining the meconium of a still born calf, he found it full of white hairs similar to the hairs with which the skin of the animal was covered. It is well known that upon calves there is always a great deal of very loose hair. Hence he supposed that this loose hair had mixed with the liquor amnii, been swallowed by the calf, and thus made its way to the meconium. But that this mode of nourishment is only partial, if it exists at all, is obvious from a multitude of facts which might be produced; fœtuses have come into the world entirely destitute of a head, and of course incapable of swallowing. Mr. Brady gives an example of a living puppy whelped without any mouth whatever,§ of course incapable of swallowing or of being nourished by the liquor amnii. The fœtus in utero is surrounded with a liquid, and consequently excluded from air. Voice, it is well known, depends upon the action of air on the throat. Accordingly fish and all animals that live constantly under water are absolutely dumb. It is on that account very difficult to explain a fact which sometimes happens, namely, a child crying in the womb. Yet there are examples of this upon record, supported by evidence which cannot be set aside. Dr. Derham relates an instance which came under his own examination. The child cried almost every day for six weeks before delivery, and so loud that it could be heard in the next room. Chickens sometimes cry before they burst their shell; but this is not so surprising, as these animals are at that time obviously surrounded * Phil. Trans. 1787. Vol. LXXVII. p. 233. | Phil. Trans. 1709. Vol. XXVI. p. 485. Phil. Trans. 1667. Vol. II. p. 503. § Phil. Trans. 1705. Vol. XXIV. p. 2176. with air; probably even breathing has begun in them some little time before they make their way out of their shell. In certain animals, as man, and most quadrupeds, the foetus is attached to the uterus, and nourished by the mother by means of the navel string. In others, as in birds and fishes, the ovum is excluded at once, and the fœtus is nourished by means of the food deposited in the egg without any connection with the parents whatever. The animals of the opossum tribe seem to be intermediate in this respect between quadrupeds and birds. Mr. Home has given us Organs of the kanguroo. a curious account of the generative organs of the kanguroo, an animal belonging to this tribe. The embryo in these animals does not appear to be attached to the uterus. When not more than 21 grains in weight, it is excluded and received into the false belly of the mother; the mouth of the fœtus attaches itself to the nipple of the mother, and the young animal continues in that situation for nine months, when it is expelled.* young pro The number of young produced at once differs according to the animal. Number of Some fish produce several thousands at a time. Dogs, cats, swine, &c. produce duced at onee. from four to 20 at a time. But man and the large quadrupeds are usually limited to one. In the human race twins are by no means uncommon. They occur at an average about once in every 80 births. Three at a time is much more uncommon, though it sometimes happens. An instance lately occurred in Berwickshire, in which all the children survived. Four at a birth is a very uncommon occurrence; but there is a well authenticated case of that kind in the Philosophical Transactions. It happened in Lancashire, and the children were delivered by Mr. Hull, a surgeon, in the neighbourhood. Two of them were still born, and two were alive but died soon after. Dr. Garthshore has attached to the case a number of curious examples of numerous births recorded by medical authors. One of them an example of no fewer than eight children at a birth, one of whom grew up to manhood, and was alive when the account was drawn up.t Physiologists have laid it down as a maxim that when animals are capable of Whether dif breeding together, and of producing young perfect in all their organs, and ca- can breed. ferent species pable of propagating, they belong to the same species; but when the young animal is incapable of propagating, then the parents belong to different species. Thus the horse and the ass belong to different species, because the mule which they produce is incapable of propagating. Relying upon these data, Mr. John Hunter has published a variety of documents showing that wolves, jackals, and dogs, breed together, and produce an offspring capable of propagating. Hence he concludes that these animals belong to the same species. The wolf he conceives to be the original, the dog to be the wolf tamed, and the jackal the dog * Phil. Trans. 1795. Vol. LXXXV. p. 221. + Phil. Trans. 1787, Vol. LXXVII. p. 344. Why females are mole nunerous than males. run wild. But this mode of reasoning, though strenuously insisted upon by Mr. Buffon, and though it became popular in consequence, scems entirely hypothetical and incapable of proof. This capacity of propagating perfect. animals certainly demonstrates a similarity in the organs of generation of the animals which breed; but this is not sufficient to demonstrate identity of species. It is conceivable that a perfect animal might be bred between man and the ourang outang; but this surely would be insufficient to demonstrate identity of species. There is only one other paper in the Transactions connected with this subject which we shall touch upon. It is well known that in most European countries the number of male births exceed that of females; yet if a census be taken of the inhabitants of a country, the number of females is always found to exceed that of males. This is commonly accounted for by the more exposed life which males lead. Hence it is conceived that they are liable to many more casualties than the females. Wars, emigrations, the sea, are mentioned, and these are conceived fully to account for the difference. But Dr. Clarke has shown that other causes intervene besides these. For from a table of births and causualties in the Dublin Lying-in-Hospital, for a series of years, he has shown that there is a greater proportion of still-born males than females, and that more male children die during their infancy than females. He accounts for this by showing that male children are larger than female, and in particular that the head of a male fœtus has a greater circumference than that of a female. Hence he conceives that male children are more likely to be stunted of nourishment before delivery than female, and from the greater size of the head they are much more likely to be injured during delivery than females. The last we conceive to be the true reason of the greater mortality of male than female children. SECTION V.-Of Medicine and Surgery. Medicine must have been, in some measure, coeval with mankind. For such is the frailty of our nature, and so numerous are the accidents to which we are exposed, that diseases and wounds must have speedily made their appearance, and the sufferers must have cast about on every side for assistance and relief. Surgery was probably the first department of medical knowledge which mankind attempted to acquire. Diseases were naturally enough ascribed to the direct agency of the offended deities, whose resentment they attempted to remove by sacrifices and prayers. But a wound received in war, or the bite of an enraged or poisonous animal, did not require any such explanation. They were injuries obviously inflicted by their fellow creatures. Accordingly we find that the medical men celebrated for their skill by Homer were in fact only sur * Phil. Trans. 1787. Vol. LXXVII. p. 253. + Phil. Trans. 1786. Vol. LXXVI. p. 349. |