Of this somewhat frequent condition two forms may be distinguished.

(a) General dilatation of the tube is a result of obstruction at any part of its course. The obstruction is mostly low down in the oesophagus and may be even at the cardiac end of the stomach. With the dilatation in these cases there is usually considerable thickening of the muscular coat, although this is not always present. There is also sometimes a general dilatation without obstruction, probably due to diminution of contractility in the muscular coat.

(b) Partial dilatation or Diverticulum occurs in two principal forms, according as the force causing the dilatation is pressure from within the tube or traction from without.

The Pulsion-diverticulum arising by pressure from within may sometimes owe its origin to a slight congenital pouching of the tube. This may be due to an imperfect closure of the communication between the oesophagus and the trachea during foetal life. If this closure be delayed, then the wall of the oesophagus may remain bulged out in front and from this beginning a larger diverticulum may arise.

More commonly, however, diverticula form on the posterior wall and usually at the junction of pharynx and oesophagus. It is probable that for the most part this form originates by a piece of hard food lodging in a fold of the mucous membrane, and being gradually pushed outwards, carrying the wall of the tube with it. As the sac enlarges, it hangs downwards with its mouth presenting upwards, and so it is always ready to receive the food in its passage downwards. It is probable that in most of these cases there is, to begin with, a separation of the fibres of the muscular coat of the oesophagus or pharynx, and the mucous membrane is pushed, as it were, through the muscular coat, which is thinnest at the junction of pharynx and oesophagus. If the sac is of any considerable size and filled with food, it will press on the part of the oesophagus situated below its mouth, and of itself produce an obstruction. In this way the food is prevented passing down the oesophagus and goes readily into the sac. We have therefore, for the most part, a continually increasing enlargement of the diverticulum which may reach the dimensions of a child's head or larger. It contains the remains of the decomposing food with mucus, sometimes in considerable masses. If the diverticulum is moderate in size its wall may still contain some muscular fibre, although the muscular coat is mostly absent except just at the neck.

The Traction-diverticulum arises by cicatricial contraction of structures adherent to the wall of the oesophagus. The contracting structure is mostly a lymphatic gland at the root of the lung, which has been affected with tuberculosis, usually in connection with phthisis pul-monalis. After healing of the tuberculosis, the gland has undergone cicatricial contraction, and by dragging on the wall of the oesophagus has produced a funnel-shaped pouch, whose apex is usually composed of cicatricial tissue.

This form of diverticulum is peculiarly prone to Perforation at the apex if hard food impinges on it. This leads to acute inflammation in the neighbourhood, resulting in an abscess-like cavity, which may perforate the trachea or a bronchus, or the neighbouring pleura or pericardium. In the former case there will probably be gangrene of the lungs, and in the latter acute inflammation of the serous membrane.