Most anatomists have said, that the foetus is constrained to bend its body, because it is too confined in its covering; but this opinion does not appear well founded, for in the first five or six months there is more space than is required for the foetus to extend, and yet during that time it is bent and folded. We also see the chicken is in a curved posture in the liquor of the amnios, although this membrane and its liquor are sufficient to contain a body five or six times as large as the foetus. Thus we may conclude that this curved form of the foetus is natural, and not the effect of force. I am somewhat of Harvey's opinion, who says, it takes this attitude because it is the most favourable to rest and sleep; and as the foetus sleeps almost continually, it naturally takes the most advantageous situation. "Certe (says this famous anatomist) animalia omnia, dum quiescunt & dormiunt, membra sua ut plurimum adducunt & complicant, figuramque ovalem ac conglobatam quærunt: ita pariter embryones qui ætatem suam maxime somno transigunt, membra sua positione ea qua plasmantur (tanquam naturalissima ac maxime indolenti quietique aptissima) componunt[AE]." [AE] Harvey on Generation, page 257. The matrix, as we have already said, takes a very ready growth after conception, and it continues also to increase in proportion with the foetus; but the foetus at length outgrows the matrix, and then, especially when it approaches maturity, it may be too much confined, and agitate the matrix by reiterated motions and violent efforts. The mother sensibly feels the impression of these painful sensations, and which are called periodic pains after the labour commences. The more power the foetus exerts to dilate the matrix the greater it finds the resistance, from the natural compression of the parts. From thence all the effect falls on the orifice, which has been increasing by degrees during the latter months of pregnancy. The head of the foetus, forcibly inclining against the sides of the orifice, dilates it, by a continual pressure, till the moment of delivery, when it opens sufficiently for the child to escape from the womb.