A simple dilatation may occur from a sudden obstruction to the urethra, or from paralysis of the muscles concerned in emptying the bladder. In this way a very extreme general dilatation may result.

Hypertrophy of the muscular coat is of very frequent occurrence as a result of some obstruction either at the neck of the bladder or in the urethra. The commonest cause is enlargement of the prostate leading to the prominence of the so-called middle lobe at the internal orifice of the urethra, but stricture of the urethra is also a frequent cause. The muscular coat of the bladder is in the form of bundles of muscular fibre-cells which run in special directions. The muscular coat is therefore not a homogeneous layer, but more like a network of interlacing bands. It is so at least when the bladder is distended, the bands coming closer together as the bladder contracts. When hypertrophy occurs these bundles increase greatly in size, and the internal ones raise the mucous membrane into elongated prominences. As the bundles interlace, the result is that the internal surface of the bladder presents a network or prominent trabecule which suggest the appearance of the internal surface of a ventricle of the heart, as shown in Figure 443.

Enlargement of prostate and hypertrophy of bladder.

Fig. 443. - Enlargement of prostate and hypertrophy of bladder. The prostate is traversed by a false passage, through which a probe has been passed.

As these trabecule interlace, little spaces are left between them in the form of small pouches. Sometimes these pouches undergo considerable enlargement, and we may have Diverticula formed in this way. The diverticulum is originally formed of the mucous membrane pushed out between the thickened muscular trabecular When small it will be contained in the thickened wall of the bladder and emptied during micturition. But as it deepens and projects outside the wall of the bladder it becomes free of the muscular coat, and as it possesses no muscular coat of its own the effect of the contraction of the bladder during micturition will be to force the urine into it, just as it is forced into the urethra. The diverticulum is liable in this case to periodical dilatation. The urine also will stagnate in it, and if decomposition occurs, then there will be inflammatory disturbances in the wall of the diverticulum. New-formation of connective tissue occurs, and, as this tissue is at first soft, the recurring dilatation during micturition causes it to yield, so that a continuous enlargement goes on. The diverticula so formed are usually of small or moderate dimensions, but cases occur in which there is a large sac, usually behind the bladder and communicating with it by a narrow neck (Fig. 444). The sac may be larger than the bladder itself, and it presents a somewhat thick fibrous wall with signs of recent inflammation internally.

Largediverticulum of urinary bladder: a, bladder with greatiy thickened wall.

Fig. 444. - Largediverticulum of urinary bladder: a, bladder with greatiy thickened wall; e, prostate and prostatic urethra; b, diverticulum with fibrous wall; c, aperture between bladder and diverticulum; d, rectum. (From a preparation in Museum of Western Infirmary).

In diverticula of moderate or large size calculi are liable to form from stagnation of the urine. Or a calculus may slip into such a pouch and escape detection with the sound.