In addition to its curvature, that part of the urethra which belongs to the perineum pre sents other features of interest to the surgical anatomist. In the dead subject its diameter is naturally far from uniform, whilst in the living its calibre is exceedingly liable to vary, accord ing to the contraction or relaxation of the muscular expansions which in certain situations invest it. Its parietes also exhibit in many places peculiarities of organisation calculated to embarrass the surgeon, and therefore some further notice of this part of the canal becomes here necessary. • The prostatie portion of the urethra is an inch and a quarter or at most an inch and a half in length. In the adult it takes an oblique direction from above and behind downwards and forwards; but in the aged subject it runs more horizontally, a change produced by the developement of the " bas fond" of the bladder ; and in the child its course is nearly vertical in consequence of the position of the bladder at that period of life. The prostatic urethra is slightly contracted at each extremity, whilst in the in termediate space it is somewhat widened, the dilatation being mnst observable near its lower wall. The veruniontanurn or caput gallina ginis, a prominent fold of mucous membrane, extends in the middle line along the floor of this part of the canal : it exhibits anteriorly a depression named the sinus pocularis, the orifice of which is directed forwards, whilst on either side the aperture of the common ejaculatory duct usually opens. The verumontanum is placed between two deep depressions called the prostatic sinuses ; these contain numerous orifices of the prostatic ducts, a few only being observable on the upper wall of the urethra. At the posterior extrernity of each sinus a tmnsverse fold of mucous membrane, of which the free concave rnargin looks fonvards, may be occasionally observed ; this has been called the " pyloric valve" by M. Amussat ; but in the majority of subjects no such structure exists, and when present it is generally occasioned by an incipient enlargement of the third lobe of the prostate gland, which, projecting upwards from below, elevates the mucous membrane at either side, so as to produce the valvular arrangement in question.
From this brief exposition it follows that many impediments to catheterism may be en countered in the prostatic portion of the urethra. The ducts which open upon its walls having their orifices mostly directed forwards, are sometimes morbidly enlarged, when they might easily arrest the point of a fine bougie ; whilst the prostatic sinuses forming depressions below tbe level of the floor of the urethra and the folds of mucous membrane just described, are also calculated at times to entangle a sinall sized instrument. Most of these impediments are situated along the floor of the urethra, and from their very nature they are likely to obstruct none but the smallest instruments. To avoid them, therefore, the surgeon should if possible select an instrument of large size and direct the point along the upper wall of the passage. The difficulties of catheterism are sometimes vastly increased by disease of the prostate gland, but obstiuctions of that description are beyond the scope of this article. The remarkable sympathy so constantly observed in practice between the testicle and the urethm is explained by the manner in which the lining membrane or the vas deferens becomes continuous with that which carpets the urethra at the orifice of the common ejaculatory duct.
The membranous portion of the urethra is intermediate between the prostatic and the spong-y portions of that canal. In situ its length seldom exceeds three quarters of an inch, but when detached and extended it ap pears about an inch long. Its direction is nearly horizontal, but its upper surface presents a very slight curve, concave towards the pubis. Its under surface is overlapped from before by the bulb, a disposition which diminishes somewhat the apparent length of its lower wall. Its anterior extremity is fixed by the triangular ligament of the urethra, a structure of uncom mon strength, through which it passes ; but posteriorly it projects behind the triangular ligament for a short distance, and being there girded by Wilson's muscles, which support it like a sling, it possesses in that situation con siderable mobility. The membranous portion of the urethra is naturally the narrowest part of the canal, presenting in this respect a marked contrast to the prostatic and spongy portions. Its parietes are endowed with considerable powers of resistance, being strengthened in front by the triangular ligament, which sends forwards upon them an expansion continuous with the fibrous covering of the bulb; whilst a still stronger expansion derived from the back of the triangular ligament suriounds the urethra beneath Wilson's muscles, and affords it power ful protection posteriorly. Between this latter investment and the mucous membrane a pecu liar structure exists of which the exact nature is rather doubtful, some considering it a modi fied erectile tissue, whilst others look upon it as muscular.
The membranous portion of the urethra merits from the surgical anatomist an attentive consideration. It is here that the operator lays bare the groove of the staff in lithotorny per formed after the lateral or bilateral methods; this is the situation in which spasm usually arrests the catheter, the obstruction being pro duced by undue action of Wilson's muscles. Foreign bodies, such as calculi, are very likely in consequence of its diminished calibre to be impacted in this part of the canal, and its anterior extremity is frequently the site of per manent stricture.
In usirwr a curved catheter the surgeon should slacken the penis upon the instrument so soon as its point has fairly traversed the triangular ligament; for if, during the further depression of the handle, the penis be forcibly stretched upon the catheter, its point may push the upper wall of the urethra against the back of the pubis, and in that manner produce considerable mischief. It is also of advantage to communi cate a slightly onward movement to the catheter at this part of the passage, as the bladder is situated much more posteriorly, and in the introduction of any instrument, whether curved or straight, it should be borne in mind that spasmodic obstructions yield in general to gentle but continued pressure, and that attempts to force such strictures are usually productive of increased spasm, and, if persisted in, of lacera tion of the urethral canal.