The cord being twisted round the child's neck may not only retard labour, it may destroy the child itself by preventing the free return of blood from the head: this may take place some little time before birth, or during the actual process of labour. That suffocation cannot possibly be the cause of death under these circumstances is sufficiently evident. _Knots upon the cord_ have been mentioned by some authors as a cause of danger to the child shortly before and especially during labour; for the circulation in the umbilical vessels being more or less compressed, the child would either be born dead or in a very weakly state. Experience has, however, shown that these effects have been much over-rated, and that these knots are seldom injurious to the child.[120] Baudelocque has not only met with single, but even triple and very complicated knots tied tightly upon the cord, and yet the child was not only born alive, but remarkably robust and healthy. Circumstances, however, may occur by which the knot is gradually drawn so tight as to destroy the child. Smellie has given a case of this kind; but it is to the late Matthew Saxtorph, of Copenhagen, that we are indebted for an admirable essay on this subject. The result of his observations coincides with those of Baudelocque, viz. that it rarely proves fatal to the child.[121] The manner in which these knots are formed may be easily imagined; when by chance the cord lies in the form of a ring, and the foetus happens to float through it, a noose is made, which, when drawn tight by accident, forms a knot. The most favourable time for the formation of such knots is in the earlier months of pregnancy, when the quantity of liquor amnii, in proportion to the bulk of the foetus, is so much greater than at an after period, and when its movements are consequently less impeded. The circulation in the knot will be obstructed in proportion as the knot is drawn closer: if it be merely somewhat impeded, the vessels on each side of the knot will be distended and varicose, and the cord itself, where it forms the knot, from the constant gradual pressure of one fold against the other, will become more or less flattened.[122] We believe that in every case the cord has been of unusual length. The _placenta_ cannot easily obstruct the birth of the child, although it may render the labour exceedingly dangerous in a great variety of ways: these circumstances will be considered under their respective heads. CHAPTER IV. FOURTH SPECIES OF DYSTOCIA. _Abnormal state of the pelvis.--Equally contracted pelvis.--Unequally contracted pelvis.--Rickets.--Malacosteon, or mollities ossium.-- Symptoms of deformed pelvis.--Funnel-shaped pelvis.--Obliquely distorted pelvis.--Exostosis.--Diagnosis of contracted pelvis.-- Effects of difficult labour from deformed pelvis.--Fracture of the parietal bone.--Treatment.--Prognosis._