From the deion of inanimate bodies and plants,I passed to animals,and particularly to man.But since I had not as yet sufficient knowledge to enable me to treat of these in the same manner as of the rest,that is to say,by deducing effects from their causes,and by showing from what elements and in what manner nature must produce them,I remained satisfied with the supposition that God formed the body of man wholly like to one of ours,as well in the external shape of the members as in the internal conformation of the organs,of the same matter with that I had described,and at first placed in it no rational soul,nor any other principle,in room of the vegetative or sensitive soul,beyond kindling in the heart one of those fires without light,such as I had already described,and which Ithought was not different from the heat in hay that has been heaped together before it is dry,or that which causes fermentation in new wines before they are run clear of the fruit.For,when I examined the kind of functions which might,as consequences of this supposition,exist in this body,I found precisely all those which may exist in us independently of all power of thinking,and consequently without being in any measure owing to the soul;in other words,to that part of us which is distinct from the body,and of which it has been said above that the nature distinctively consists in thinking,functions in which the animals void of reason may be said wholly to resemble us;but among which I could not discover any of those that,as dependent on thought alone,belong to us as men,while,on the other hand,I did afterwards discover these as soon as I supposed God to have created a rational soul,and to have annexed it to this body in a particular manner which I described.
But,in order to show how I there handled this matter,I mean here to give the explication of the motion of the heart and arteries,which,as the first and most general motion observed in animals,will afford the means of readily determining what should be thought of all the rest.And that there may be less difficulty in understanding what I am about to say on this subject,I advise those who are not versed in anatomy,before they commence the perusal of these observations,to take the trouble of getting dissected in their presence the heart of some large animal possessed of lungs (for this is throughout sufficiently like the human),and to have shown to them its two ventricles or cavities:in the first place,that in the right side,with which correspond two very ample tubes,viz.,the hollow vein (vena cava),which is the principal receptacle of the blood,and the trunk of the tree,as it were,of which all the other veins in the body are branches;and the arterial vein (vena arteriosa),inappropriately so denominated,since it is in truth only an artery,which,taking its rise in the heart,is divided,after passing out from it,into many branches which presently disperse themselves all over the lungs;in the second place,the cavity in the left side,with which correspond in the same manner two canals in size equal to or larger than the preceding,viz.,the venous artery (arteria venosa),likewise inappropriately thus designated,because it is simply a vein which comes from the lungs,where it is divided into many branches,interlaced with those of the arterial vein,and those of the tube called the windpipe,through which the air we breathe enters;and the great artery which,issuing from the heart,sends its branches all over the body.I should wish also that such persons were carefully shown the eleven pellicles which,like so many small valves,open and shut the four orifices that are in these two cavities,viz.,three at the entrance of the hollow veins where they are disposed in such a manner as by no means to prevent the blood which it contains from flowing into the right ventricle of the heart,and yet exactly to prevent its flowing out;three at the entrance to the arterial vein,which,arranged in a manner exactly the opposite of the former,readily permit the blood contained in this cavity to pass into the lungs,but hinder that contained in the lungs from returning to this cavity;and,in like manner,two others at the mouth of the venous artery,which allow the blood from the lungs to flow into the left cavity of the heart,but preclude its return;and three at the mouth of the great artery,which suffer the blood to flow from the heart,but prevent its reflux.Nor do we need to seek any other reason for the number of these pellicles beyond this that the orifice of the venous artery being of an oval shape from the nature of its situation,can be adequately closed with two,whereas the others being round are more conveniently closed with three.Besides,I wish such persons to observe that the grand artery and the arterial vein are of much harder and firmer texture than the venous artery and the hollow vein;and that the two last expand before entering the heart,and there form,as it were,two pouches denominated the auricles of the heart,which are composed of a substance similar to that of the heart itself;and that there is always more warmth in the heart than in any other part of the body-and finally,that this heat is capable of causing any drop of blood that passes into the cavities rapidly to expand and dilate,just as all liquors do when allowed to fall drop by drop into a highly heated vessel.
