[ 345 ] THE MUSCULATURE OF THE HUMAN PROSTATIC URETHRA BY E. J. CLEGG Department of Anatomy, the University of Liverpool INTRODUCTION Henle (1866) described two distinct sphincters in the wall of the prostatic urethra. The proximal, composed entirely of smooth muscle, was continuous above with the circular muscle layer of the bladder and formed a complete ring around the internal urinary meatus. He named it the 'sphincter vesicae internus'. Distally, and over- lapping the lower fibres of the internal sphincter on their outer surface, he described a ' sphincter vesicae externus', which lay mainly anterior to the urethra, and which near the apex of the gland blended with the deep transverse muscle of the perineum, completely surrounding the urethra. The fibres of the external sphincter were mainly striped, and originated in the stroma of the anterior parts of the prostate gland. Subsequent workers have generally agreed with the above findings, although there is disagreement on the more detailed anatomy of the muscles (Walker, 1906; Young & Wesson, 1921; Graning, 1936; Macleod, 1942; and others). During a previous study of the blood vessels of the prostate gland (Clegg, 1956) it was noted incidentally that, contrary to Henle's view, comparatively few trans- verse fibres, either of muscle or connective tissue, were seen anterior to the urethra, although posteriorly a thick band of transversely running fibres was observed. In addition, the number of longitudinally directed fibres, especially in the anterior and lateral urethral walls, was much greater than Henle's description warranted. It seemed desirable therefore to re-investigate this problem. MATERIALS AND METHODS Prostate glands were removed from fresh post-mortem material. The ages of the subjects, seven in number, ranged from 53 to 82 years. None of the glands showed significant pathological changes, other than small adenomata; these did not affect the findings. The glands, together with adjoining portions of the bladder, were fixed for periods from 2 to 6 months in 10 % formol saline. In six, transverse sections 50 , thick were cut on the freezing microtome, and one section in ten was stained by Masson's trichrome method. The remaining gland was sectioned longitudinally and treated in the same way. In a few instances, sections were stained by Mallory's method, but this proved unsatisfactory. RESULTS Striped muscle In no case is striped muscle seen to enter into close relationship with the urethra except near the apex of the gland, where bands of fibres infiltrate the smooth muscle anterior, lateral and sometimes posterior to the urethra. Striped muscle fibres make 346 E. J. Clegg their appearance in the anterior region of the gland at the level of the termination of the ejaculatory ducts and progressively increase in number towards the apex. Smooth muscle With regard to the arrangement of its smooth muscle, it is convenient to consider the prostatic urethra in three parts, a proximal immediately below the internal urinary meatus, an intermediate, comprising that part above the opening of the ejaculatory ducts, and a distal part extending to the apex of the gland. Proximal part In this section of the urethra the main feature is the thick band of muscle running obliquely downwards and medially behind the lumen (P1. 1, fig. 1). The considerable amount of connective tissue which separates its individual fibres gives it an 'onion- skin' appearance, and it will subsequently be referred to as the 'concentric muscle'. Its fibres are derived from two sources. Peripherally they have their origin in the muscular stroma of the antero-lateral part of the prostate gland and its capsule. Centrally they are continuous with the transverse fibres of the trigone and with the anterior longitudinal muscular layer of the bladder. Some of these latter fibres pass directly backwards lateral to the internal urinary meatus. Others pass downwards in the anterior urethral wall (P1. 1, figs. 2, 3) for varying distances before passing obliquely downwards and backwards to the posterior aspect of the urethra. From the trigone a number of longitudinally directed fibres pass downwards immediately deep to the mucous membrane of the posterior urethral wall (P1. 1, fig. 3). They are of smaller diameter than the fibres of the concentric muscle. In the lower part of this segment of the urethra a few transverse fibres, again of small diameter, may be seen in the submucosa of the anterior urethral wall. Some of these are continuous with the innermost concentric fibres, and thus a complete although very thin ring of smooth muscle passes around the urethra a short distance above the crest. There is no anatomical sphincter immediately below the bladder neck, except in the sense that the peripheral part of the concentric muscle is in continuity with the capsular fibres of the prostate, some of which cross the midline anterior to the urethra. These anterior transverse fibres, however, are not in such intimate relationship with the urethra as are the concentric fibres, as they are separated from it by the fibres derived from the anterior longitudinal layer of the bladder. Intermediate part The urethral crest is a fibromuscular structure containing the ducts of prostatic glands which open on its sides. The musculature consists of bundles of fibres sepa- rated by connective tissue, which run downwards in the line of the urethra. They are in the main continuous with the posterior longitudinal fibres of the proximal part of the urethra, but in addition longitudinal fibres anterior to the urethra, which run obliquely round to its posterior aspect, and a few fibres from the concentric muscle contribute to the muscle of the crest. As the height of the crest increases, the ejaculatory ducts and prostatic utricle pass forwards and downwards through the substance of the prostate towards the base of The musculature of the human prostatic urethra 347 the crest. The ducts and utricle are contained in a sheath, the composition of which changes proximo-distally. Above, it consists mainly of connective tissue with a small number of smooth muscle fibres (PI. 1, fig. 4). The walls of the ejaculatory ducts contain smooth muscle, but the utricle in five of the six glands sectioned trans- versely contains no muscle. In the remaining gland, the utricle wall contains well- defined bands of circular muscle (P1. 2, fig. 5). Below the point where the ducts and utricle turn almost directly forwards the sheath contains considerable amounts of muscle, in which longitudinal, circular, and oblique fibres may be seen, although separate layers cannot be distinguished (P1. 2, figs. 6-8). This muscle is quite distinct from that of the walls of the ejaculatory ducts. As the ducts pass forwards into the crest, there lies behind them in the median plane a thick bar of muscular tissue which blends anteriorly with the posterior part of the sheath and which posteriorly appears to be continuous with the concentric muscle. In the crest, this bar, together with the posterior half of the sheath, assumes a 'Y' shape (P1. 3, fig. 9). The limbs of the 'Y' extend to the urethral submucosa, and the crest is thus divided into an anterior part, containing the terminations of the ejaculatory ducts and utricle, and a posterior, divided by the stem of the ' Y' into two halves, each containing the ducts of the prostatic glands. At this level the amount of smooth muscle in the submucosa of the anterior urethral wall increases. Both longitudinal and transverse fibres are present, the latter being especially prominent opposite the openings of the ejaculatory ducts and prostatic utricle (PI. 3, fig. 9). They are continuous with the transverse fibres noted in the proximal segment. The concentric fibres in the upper part of this segment form a thick mass behind the urethra. As the ejaculatory ducts and prostatic utricle pass forward to their terminations thev traverse this muscle. That part anterior to the ducts terminates at or above the level of their entrance into the urethra (P1. 3, fig. 10). The much thinner posterior part generally persists, although in an attenuated form, into the next segment of the urethra. Distal part Immediately below the level of entrance of the ejaculatory ducts and prostatic utricle into the urethra the muscle of the crest still retains its 'Y' shape. With the gradual disappearance of the utricle and ducts, the limbs of the 'Y' approach one another and fuse in the midline, the anterior end of the bar so formed being immedi- ately deep to the urethral submucosa (P1. 3, fig. 11). This bar persists, although of diminishing height, to the lowest part of the prostatic urethra (PI. 3, fig. 12). In this part, the amount of circular muscle surrounding the urethra increases considerably (P1. 3, figs. 11, 12). In five cases out of seven it is directly continuous with the concentric muscle referred to above, although the connexion immediately below the openings of the ejaculatory ducts is but a tenuous one. The fibres of the muscle arise entirely in the stroma of the prostate in the antero-lateral region, generally a short distance from the mid-line. As noted previously, striped fibres are present in this segment of the urethra. They are most numerous anteriorly, but a few pass backwards to the lateral and occasionally the posterior aspects of the urethra. 22 Anat. 91 348 E. J. Clegg DISCUSSION The smooth muscle of the prostatic urethra Apart from Griffiths (1891) and Scher (1950) who considered that there was no sphincter at the bladder neck (although the latter author considered that the fibro- muscular tissue of the prostate as a whole could act as one), all previous authors are agreed that some type of sphincter muscle exists in this position. Most workers agree with Henle (1866) that the bladder neck sphincter is continu- ous above with the circular muscle of the bladder and that its fibres pass obliquely downwards and forwards on either side of the internal urinary meatus to become continuous with one another anterior to the urethra.