If the spongy part of the canal be laid open longitudinally, it is found to be plicated, the folds running from behind forwards, and dovetailing with one another when the urethra is closed. This plicated arrangement of the mucous membrane of the spongy portion was observed by Bichat and others, and is due to the contraction of the submucous tis sue : if the urethra be immersed in alcohol and examined with the aid of a simple lens, an arborescent arrangement of the folds is visible at the anterior part of the canal, in appearance not unlike the plicm in the mu cous membrane of the cervix uteri.
The spongy portion varies in diameter in different parts ; thus, dilated at its commence ment in the bulb, it gradually tapers until it reaches the glans, when it suddenly expands into the fossa navicularis, to be again con tracted at the meatus.
The bulb (pars bulbosa urethrce et pans sub pubica) is the widest part of the spongy por tion. Lisfranc found in twelve subjects it varied in diameter from 5 to 7 lines. Home and most anatomists who have examined the urethra, have come to the general conclu sion that the bulbous portion presents a de cided dilatation, and a simple examination of the part would lead to a similar conclusion. I cannot, therefore, conceive why there should be any doubt on the subject. Injections of wax prove to my mind an undoubted dilata tion of the bulbous part ; Krause and Guthrie, however, deny that any such exists.
Kobelt has shown, that if the bulb be in jected, the corpus spongiosum at this part presents two lateral hemispherical swellings, separated from each other in the middle line by a longitudinal depression. " This is caused by a septum which divides the posterior part of the bulb into two lateral symmetrical halves; but anteriorly it is gradually lost. The two portions of which it indicates that the bulb is composed, are the analogues of the com pletely divided portions of the bulb and corpus spongiosum in the whole of the marsupial tribe."* " In the middle of the bulb there is a slight superficial elevation (colliculus bulbi n2edzus), which is situated above and between them, but does not extend so far backwards as they do, and gives passage to the mem branous part of the urethra, the vessels and nerves of the bulb, and the ducts of Cowper's glands." t The extremity of the caput gallinaginis sometimes reaches the bulbous portion, and the ducts of Cowper's glands terminate in its floor by two minute orifices, extremely dif ficult to find. This part of the canal extends for about the fourth part of an inch or less, the anterior layer of the deep perinmal fascia covers it underneath, and it is completely in vested by the acceleratores urinal.
The remainder of the spongy portion is united to the bulbous at an angle of 45 in the flaccid state of the penis ; but when this organ is erect, this part of the canal is rendered perfectly straight : it terminates at the meatus urinarius.
The extremity of the spongy portion is called the glandular part (pars glandularis), being surrounded by the glans penis. When the urethra has entered the glands, it dilates into a fossa from 4 to 6 lines in length, this is termed the foam navicularis Morgagnii. When cut transversely, it has the appearance of a longitudinal fissure. The urethra is sur rounded unequally by the glans penis ; the floor of the navicular fossa is covered by a very thin layer of this extension of the spongy body, whilst its sides and upper surface have a considerable investment from the glans.
The urethra having traversed the corpus spongiosum and the glands, terminates at the anterior and inferior part of this body, by a small slit-like orifice of two or three lines, the long axis of which is vertical : from this a small fold of membrane passes down to join the prepuce, and is termed the frcenum putii. The effect of this latter is, when the penis is erect, to draw down the opening of the urethra, and thus to narrow the orifice, and direct the contents of the urethra down wards and forwards. The two sides of the meatus urinarius are kept in apposition by the projection of the glans, and they are joined below by a delicate fold of mucous membrane, not unlike the fourchette of the female labia.
The urethral orifice is surrounded, accord ing to Guthrie*, by a peculiar dense struc ture, which he considers analogous to that which forms the edge of the eyelid, and which he believes to be requisite to maintain the patency of the opening ; for if this be de stroyed by ulceration, the part from which it has been removed contracts, and the opening becomes so small, as to give rise to a most troublesome form of stricture. The opening of the urethra is almost invariably the nar rowest part of the canal ; and hence, if an instrument has been introduced through this, it will with facility traverse the remainder of the passage, unless there be some mechanical impediment from spasm or disease ; and hence a trifling division of this part will permit the passage of a large instrument, and the escape of fragments of stone, which are frequently arrested here after the operation of lithotrity. It is but sparingly elastic. After immersion in alcohol, the mucous membrane at the meatus urinarius will be found arranged in circular folds.