In briefly reviewing the condition of the urethra as connected with the structures in vesting it, we observe that the prostatic por tion, being surrounded by a body possessing considerably dilatability, remains under almost every condition passive and yielding; and hence it happens that a catheter, having reached this part of the canal, must, if its point be properly directed, enter the bladder, unless there be some enlargement of the prostate itself; and hence the inference that no force is to be employed in passing the catheter under these circumstances.
On the contrary, the membranous portion of the urethra is a part of high irritability. This depends partly on the nature of the tissue itself, but mainly on the muscular apparatus connected with it. This part of the canal is most liable to irritation and spasm from general and local causes ; and is well known to be the most frequent seat of stricture. The remainder of the urethra is entirely sur rounded by the corpus spongiosum, and must necessarily be influenced by the condition of that body. When the latter is distended, as in erection, the diameter of the urethra must be proportionally diminished ; and, in the undis tended state of the former, it assumes its greatest degree of dilatability. Consequently, in the introduction of the catheter, the chief difficulty, even independent of organic disease, is experienced at the meatus, or at the angle formed at the junction of the bulb and anterior part of the spongy portion, or at the mem branous part, the remainder of the spongy por tion in the healthy state seldom offering any resistance to the progress of the instrument.
It will also be further remarked, that the dorsal surface of the urethra is smooth and even throughout, except in the situation of the lacuna magna, whilst its depressions and eleva tions are found upon the floor of the canal, and hence the importance in catheterisation of keeping the beak of the instrument against the superior part of the urethra.
Mucous Membrane.— The urethra is es sentially a mucous canal, forming an important part of the genitourinary mucous system. It is continuous with the mucous mem brane of the bladder, and blends at the meatus with the cutaneous covering of the glans penis. There is always a distinct line of de marcation at this part, the urethral surface being moistened by a mucous secretion. When laid open, it can be frequently seen to present two distinct white lines, one at the upper, and the other at the under surface, running longi tudinally; these are supposed to indicate the original lateral division of the canal. Inde
pendent of this, the mucous membrane will be generally found arranged in longitudinal folds, with furrows between them. The folds vary in depth and breadth, the larger are found on the under surface, and they are more numerous in this situation than above. They commence at the bulb and run forwards, sometimes continuing of the same size through out, frequently tapering and sending off, as they approach the extremity of the penis, delicate processes, which present a somewhat arborescent arrangement, not unlike that pre sented by the lining membrane of the uterus ; many on the dorsal aspect will be seen to terminate on either side of the lacuna magna. The folds of the urethra, which are not al ways visible after death, result from the con traction of the submucous layer, and are so arranged, that, when the two surfaces are in ap position, especially if the vessels of the urethra be injected, a sort of dovetailing of the mucous membrane occurs, bearing a slight resemblance to the plicated arrangement of the oesophagus of the whale.
The urethra after death is whitish in colour, and in some parts so transparent as to permit the veins of the corpus spongiosum to be seen through it. Its bloodvessels are readily injected, when the whole canal presents a beau tiful vermilion tint. At the membranous por tion, Shaw has described a considerable plexus of veins beneath the mucous membrane of the urethra: these veins are spread all over the canal, but are accumulated at the membranous part, lying one over the other in its long axis, so as to form two distinct columns with a groove between them : they unite and sur round the sinus pocularis. Wilson had ob served this arrangement of vessels, before Shaw published his description of them. These veins communicate with the plexus surrounding the prostate and neck of the bladder. They are best seen by inserting a tube beneath the mucous membrane, and in jecting them with mercury. Shaw supposes them capable of considerable distension, and thus of producing a species of erection of this part of the tube.* In rude attempts to pass a catheter, the vessels of this part are often lacerated, and pour out a large quantity of blood. They have no communication with the veins of the corpus spongiosum itself, properly so called.