The dilatations which in rare cases are observed in the Ductus arteriosus, in every period of life, from the earliest infancy, are simple, and not dependent upon any alteration of texture in the coats of the vessel. They are occasioned by a deficient involution of the duct after birth.

If we except that degree of patency of the Ductus arteriosus, in which, in consequence of a uniformly deficient involution (closure), it remains similar in calibre to a branch of the pulmonary artery in newborn infants, and forms a very secondary cylindrical vessel, we find the following different forms of dilatations, which admit of being referred to an unequally deficient closure of its mouths.

1. In one ease, when the occlusion of both mouths has once commenced, the process goes on more slowly in one - probably the aortic mouth, whose calibre continues permeable after the other mouth has become considerably contracted. Blood now collects here, dilating the vessel, and gradually coagulating within it, forming a spindle-shaped or round, spherical capsule (aneurism), after which this mouth also is finally closed. This anomaly is unquestionably devoid of importance, and does not lead to any secondary consequences, as the coagulum, and the coats of the vessel over it, gradually shrivel together.

2. In other cases the Ductus arteriosus is found to present a funnellike dilatation from the aorta, and the opening into the pulmonary artery is then surrounded by a torn and fringed margin. That this anomaly is not a true patency, - a persistence of the Ductus Botalli in its original form and significance, - is made evident by the above-mentioned relation of the duct, and more especially of its mouths; by its violent reopening from the aorta, towards the pulmonary artery, as indicated by the character of the mouth; by the occlusion of the Foramen ovale, which is observed in such cases; and the existence of a current opposed to the foetal circulation, and inclined from the aorta towards the pulmonary artery.

This condition is owing to the relation of the duct and its openings, that is to say, to the violent reopening of the closed ostium of the pulmonary artery from the dilated aortic portion of the duct; and also to the active dilatation present in these cases in the right side of the heart, which is one of the results of the obstacles produced by the entrance of arterial blood into the current of the pulmonary artery.