This is evidently prov'd from the Torricellian Experiments, and those made upon Animals in Mr. Boyle's Engine; where, as soon as the Air is withdrawn, and the pressure thereby taken off, the Intercostal Muscles and Diaphragm are contracted, and the Ribs elevated in an instant, and can't by any Power of the Will be made to subside, till the Air is again let in to bear them forcibly down.
It were scarce worth while to take notice here of a Mistake of the Learned Dr. Willis, were it not for the great Authority of the Man, which is almost sufficient to keep Error in countenance. |De Respirationis
Organis & Usu. | The Doctor having observ'd, that the Fibres of the External and Internal Intercostal Muscles ran in a contrary order, as it were, decussating each other, takes occasion from thence to fansie, that there was an opposition in their Office; and that as the External serv'd to raise up the Ribs, the Internal drew them down again, forgetting at that time, That, when a contractile Body is fasten'd at the several ends to Points unequally moveable, let the Contraction happen in what part or manner soever, the more moveable Point must be drawn towards the less moveable: By which Rule, whether External or Internal Intercostals be contracted, the lower Ribs will be forc'd to approach the upper, that is, be rais'd up.
As in the Elevation of the Costæ, the Blood, by the passage that is open'd for it, is in a manner solicited into the Lungs; so in the Depression of them, by the subsidence of the Lungs, and the Contraction of the Blood-Vessels, both which are consequent thereof, the Blood is forcibly driven, as it were with an Embolum, through the Pulmonary Vein into the Left Ventricle of the Heart. And this, together with the general Compression of the Body by the weight of the Atmosphere, which surrounds and presses upon the whole Surface of it, is that Power which causes the Blood to mount in the Veins, after the force impress'd upon it by the Heart is broken and spent, and which is sufficient to force the Heart from its natural State to Dilatation.
He that is able to compute the weight of a Column of Air, equal to the Surface of the whole Body, will readily grant it a power sufficient for the Effects, which are here ascrib'd to it. And when he considers, that the Bodies of Animals are compressible Machines, he will find that it must of necessity affect them in the manner here laid down. But though our Bodies be entirely compos'd of Tubuli, or Vessels fill'd with Fluids; yet this pressure, how great soever, being equal, cou'd have no effect upon them, if the superficial Dimensions were not easily variable; because being compress'd on all parts with the same degree of Force, the contain'd Fluids cou'd not any where begin to recede, and make way for the rest to follow, but wou'd remain as fix'd and immoveable as if they were actually solid. But by the Dilatation of the Thorax, room is made for the Fluids to move, and by the Coarctation of it, fresh motion is imprest, which is the main Spring whereby the Circulation is set and kept going.
This reciprocal Dilatation and Contraction of the superficial Dimensions of the Body, seems so necessary to Animal Life, that there is not any Animal so imperfect as to want it, at least none to the inward Structure, of which our Anatomical Discoveries have yet reach'd. For, tho' most kinds of Fish and Insects, want both moveable Ribs and Lungs, and consequently have no dilatable Thorax, yet that want is made up to 'em by an Analogous Mechanism, answering sufficiently the Necessities of their Life.
Those Fishes which have no Lungs, have Gills, which do the Office of Lungs, receiving and expelling alternately the Water, whereby the Blood-Vessels suffer the same alteration of Dimensions, that they do in the Lungs of more perfect Animals.
The Lungs or Air-Vessels of Insects, are yet exceedingly more different in Structure, Distribution, and Situation from those of perfect Animals, than those of Fishes are, and yet in their Use and Action agree perfectly with both; that is, receiving and expelling the Air, and varying the Dimensions and Capacities of the Blood-Vessels. These having no Thorax, or separate Cavity for the Heart and Air-Vessels, have the latter distributed through the whole Trunk of their Bodies, by which they communicate with the External Air through several Spiracula or Vent-holes, to which are fasten'd so many little Tracheæ, or Wind-pipes, which thence send their Branches to all the Muscles and Viscera, and seem to accompany the Blood-Vessels all over the Body, as they do in the Lungs only of perfect Animals. By this disposition in every Inspiration, the whole Body of these little Animals is inflated, and in every Expiration compress'd; and consequently the Blood-Vessels must suffer a Vicissitude of Extension and Contraction, and a greater motion must thereby be impress'd upon the Fluids contain'd in them, than the Heart, which does not in those Creatures appear to be Muscular, seems capable of giving.
The only Animal that is exempted from this necessary condition of Breathing, or receiving and expelling alternately some Fluid into and out of the Body, is a Fœtus. But this, while included in the Womb, has little more than a vegetative Life, and ought scarce to be reckon'd among the number of Animals. For, were it not for that small share of Muscular Motion, which it exercises in the Womb, it might without absurdity be accounted for as a Graft upon, or Branch of the Mother.
Concerning the immediate Matter, and Means of Life, and Nutrition, Authors are not agreed, nor is it the business of this place to reconcile, or decide their Differences, but to account for the Motion of the Blood through the Vessels only. In order to this, it will be necessary to observe, that the Pulsation of the Heart in a Fœtus is so very weak and obscure, and the Motion of the Blood so extream slow and languid, as to be scarce, if at all perceivable, as has been experienced in the Dissection of Puppies before Respiration had. To |Boyle of the Elasticity of Air. Pechlinus de Aeris & Alimenti defectu.| produce such a feeble Palpitation, and creeping Motion, no greater force seems to be required, than may be deriv'd from the Communication between the Vessels of the Mother and Fœtus in the Placenta. I am not ignorant, that divers very Learned Anatomists (whom the Crowd have implicitly follow'd) have absolutely rejected all Communication between these Vessels. But, with submission to Great Authorities, I think they have acted arbitrarily, and without sufficient Warrant from Reason or Experiment: For neither are the Arguments which they bring against it conclusive, nor the Office which they assign to the Umbilical Vessels in lieu of it, proper, or natural to those Vessels, or the reality of the Fact made out by any substantial Reasons. Those that reject this Communication usually do it in favour of one or both of these Opinions, that the Arteries of the Uterus do deposite a Nutritive Juice, or a Juice impregnate with Air in the Placenta, which is suck'd in by the Umbilical Vein, and convey'd to the Fœtus, for the necessary Uses of Nutrition and Life. Now those that patronize either of these Opinions, lead Nature an unnecessary Dance. For if the Maternal Blood does really contain any such Nutritious, or any such necessary Aerial Particles, why shou'd they be separated and extravasated, to be with difficulty receiv'd into the Umbilical Vein, and again mixt with the Blood, when they might more easily have been imparted by the plain simple way of Transfusion from the Arteries of the Mother to the Veins of the Fœtus. And, that this is the course which Nature takes in this Case, I am perswaded from the easiness and simplicity of the Method, which readily performs what might be perhaps in vain expected from the other, and wou'd over and above find them, what they seem to grope so blindly about for, a first Mover of the Blood in a Fœtus.
Those that contend for the conveyance of the Nutricious Juice, through the Umbilical Vein from the Placenta, are forc'd upon two Difficulties next to Absurdities. For first they are oblig'd to make this Vein, which, as all other Veins, seems dedicated to the Re-conveyance of Blood only, the proper and immediate Chanel, thro' which a very different Liquour is to be carried; and next, to give a Power of Attraction or Suction to it; because the Nutricious Juice, which it is thus destin'd to carry, is both viscous and stagnant, and has neither force to drive, nor subtilty to penetrate, or insinuate it self into the Capillary Veins; and therefore must be drawn or suck'd as Milk is from the Breast, to which the Placenta and its Nutricious Juice are by the Favourers of them expresly compar'd. But if this were the sole use of the Placenta, and Umbilical Vessels, why were the Umbilical Arteries sent along with the Vein? Their business is not to bring any thing back to the Fœtus, nor can they contribute any thing to the benefit of the Mother; for the Uterine Arteries bring all to the Placenta, the Umbilical Vein carries it to the Fœtus, and the Uterine Veins convey back again the Surcharge of the Mother's Blood; the Umbilical Arteries only, have nothing to do, and are superfluous and impertinent, which is contrary to the constant Practice of Nature. Yet if Autopsie did in the least countenance this Hypothesis, some Defence might still be made; but we find in the Umbilical Vein of a Fœtus nothing but Florid Blood, such as in all probability it received immediately from the Arteries of the Mother without any mixture. And therefore I can't help concluding, that this Opinion engages its Favourers in some Absurdity, without Necessity and without Proof.