THE ANATOMICAL RECORD 199~187-195(1981) Histology of the Canine Uretha 11. Morphometry of the Male Pelvic Urethra W. CRAIG CULLEN, THOMAS F. FLETCHER,ANDWILLIAM E. BRADLEY Department of Veterinary Biology, College of Veterimry Medicine, University of Minnesota, St. Paul, Minnesota 55108 (W.C.C., T.F.F.)and Neurology Service, V.A.Hospital, Long Beach, California and Department of Neurology, University of California, Iruine, California (W.E.B.) ABSTRACT Urinary bladders and pelvic urethrae were collected from six adult and two juvenile male dogs. Within two vesical and six urethral sampling regions, volume densities were estimated for smooth and striated muscle, connective tissue and elastic fibers, stratum cavernosum, luminal epithelium, and prostate. The neck had significantly less smooth muscle and more connective tissue than the body of the bladder. In the prostatic urethra, smooth muscle was associated principally with trabeculae surrounding prostate lobules. Smooth muscle was sparse superficially in the prostatic capsule and practically absent in relation to the mid-prostatic urethra. Thus there was no mechanism for active closure of the middle prostatic urethra, and elastic fiber density was correspondingly high in this region. The smooth muscle sphincter needed to maintain urinary continuence and prevent semen reflux was primarily the vesical neck. Caudal to the body of the prostate, striated muscle comprised more than 50% of the urethral wall. Juvenile and adult postprostatic urethrae were similar except for a decreased quantity of stratum cavernosum in the pups. Because of convenient size, availability, and resemblence to the human urethra, the canine male urethra has been the subject of a number of physiologic studies and experimental manipulations. Woodburne ('60, '61) has compared smooth muscle and elastic fibers in the bladder and proximal urethra of canine and human males and has reported similar tissue arrangements in the two species. However, the entire pelvic urethra of male dogs has not been studied thoroughly. The male dog lacks a distinct preprostatic urethra and has a well-developed prostate body, as does man, but the dog lacks vesicular glands (seminal vesicles) (Miller et al., '65).The dog has disseminated prostate and glands in the ampulla of the ductus deferens but does not have bulbo-urethral glands. In addition to prostate, the pelvic urethra of the male dog features epithelium, stratum cavernosum, connective tissue with elastic fibers, and smooth and striated muscle. These tissues were analyzed morphometrically . MATERIALS AND METHODS Urinary bladders and pelvic urethrae were collected from six young adult and twojuvenile 0003-276X/81/1992~0187$02.00 0 1981 ALAN R. LISS,INC. mongrel dogs, all intact males. The adults ranged from 5 to 17 kg in weight. The pups were two months old and averaged 7.3 kg. Following pentobarbital anesthesia, four of the adults received whole-body perfusion of saline followed by 10% buffered formalin or 3% phosphatebuffered glutaraldehyde through a femoral artery. Tissues from the remaining dogs were fixed by immersion in buffered 10%formalin. For all dogs, the region between the level of the ureteral orifices and the cranial extent of the body of the prostate was regarded as vesical neck. The region between the cranial and caudal extents of the prostate body was regarded as prostatic urethra. The region between the prostate body and the bulbus penis enlargement was regarded as postprostatic (membranous)urethra. The prostatic and postprostatic urethrae each were subdivided into three equal-length sampling regions. The vesical neck and the six urethral sampling regions were each potentially divided into ten equallength segments, and a random number between one and ten determined the site where each region was blocked for transverse section. Received May 2 , 1979; accepted Aug 5 , 1980. 188 W. CRAIG CULLEN, THOMAS F. FLETCHER, AND WILLIAM E. BRADLEY For comparison, a transverse section was collected also from either the right or left midbody of each bladder. Tissues were prepared and measured as described in the preceding paper presenting morphometric measurements of the female canine urethra. OBSERVATIONS The body and neck of the bladder blended without sharp demarcation. The neck appeared to penetrate the body of the prostate gland, so that a morphologically distinct preprostatic urethra was not observed. The postprostatic (membranous) pelvic urethra was distinct, extending from the caudal aspect of the body of the prostate to the abrupt enlargement produced by the bulbus penis covered by the M. bulbospongiosus.Except for differences in size, the urethrae of adult males appeared grossly and histologically similar. Among the dogs of different size, however, prostatic glandular mass was more similar than vesical and urethral mass. Thus it appears that prostatic volume has varied more conservatively than body size in canine evolution. Whether fixed by perfusion or immersion, comparable tissue regions appeared similar, except for greater dilation of the stratum caverosum with perfusion. The wall of the vesical body in the male dogs was similar in histologic appearance to that described for bitches (preceding paper). A discontinuous muscularis mucosae was evident in bladders of three of the adult males and both pups, whereas this structural feature was seen in only one of the females. In the vesical neck, small longitudinal muscle fascicles were observed adjacent to the submucosa along with two thicker longitudinal fascicles associated with the trigone and urethral crest. Superficial to this, the main muscle coat consisted of circularly or obliquely oriented muscle fascicles. The layering of muscle fascicles was irregular and, typically within a plane of section, individual fascicles did not completely encircle the neck in males as they did in females. Thus the principal musculature of the bladder neck in males consisted of obliquely oriented muscle bundles that continued on to blend with trabecular muscle in the prostate. Superficial to the principal musculature, a few longitudinal muscle fascicles were scattered in the vesical neck and prostatic capsule. In the body of the prostate, smooth muscle was associated principally with trabeculae separating the glandular lobules (Fig. 1).Only delicate muscle fascicles were present in the outer prostatic capsule. In relation to the urethra, circular smooth muscle formed a thin sphincter restricted to the cranial sixth of the prostatic urethra. The middle prostatic urethra was devoid of smooth muscle, except for occasional, delicate, longitudinal fascicles. In the caudal prostatic urethra, an increase in submucosal smooth muscle was evident, but the muscle was either related to prostatic lobules or oriented longitudinally to the urethral lumen and did not form a complete sphincter. In the postprostatic urethra, smooth muscle persisted mainly as isolated longitudinal fascicles located adjacent to the submucosa. Striated muscle (M. urethralis) first appeared in the caudal prostatic region as isolated fascicles in the prostatic capsule and as transversely oriented fascicles coating the urethra ventrally, where the urethra first emerged from the prostate body. Interdigitating a t first with smooth muscle, the striated muscle coat then developed superficial to smooth muscle. Striated muscle was quite thick throughout the postprostatic urethra. The muscle fascicles were oriented mainly circularly but also obliquely. The two orientations were mixed, and consistent circular and oblique muscle layers were not found within the striated muscle coat. (Fig. 2). Morphometry For quantitation, a vesical neck and six urethral sampling regions were defined. Because sections were taken randomly within each region in each dog, the entire length of the pelvic urethra and vesical neck was sampled over all dogs (Fig. 3). Also the wall of the midbody region of the bladder was sampled and quantified in terms of volume fraction (Vv). The body of the bladder averaged 52% smooth muscle and 46%connective tissue. The neck of the bladder had significantly less smooth muscle and more connective tissue than the body (Table 1).Both regions were similar in Vv of elastic fibers and epithelium, but the index of elastic fiber density in submucosa for the neck was more than twice that of the body of the bladder. A small volume of disseminated prostate was found in the vesical neck. Prostatic glandular epithelium comprised the most abundant tissue of the prostatic urethral region, followed by total connective tissue and smooth muscle. Nearly all of the smooth muscle was associated with trabeculae surrounding prostatic lobules rather than arranged for urethral encirclement. The three sampling regions of prostatic urethra were significantly different in Vv of glandular epithe- MORPHOMETRY OF THE! CANINE MALE URETHRA 189 Fig. 1. Transverse section through the midprostatic urethra of an adult male dog. Most of the smooth muscle is associated with trabeculae separating lobules of prostate gland. Very little smooth muscle is in the outer prostate capsule or in the urethral submucosa. A distinct uterus masculinus is evident in the dorsal submucosa deep to the colliculus seminalis. x 5.4. Fig. 2. Transverse section through the middle of the postprostatic urethra of an adult male dog. The submucosa features a voluminous stratum cavernosum. The thick striated muscle coat has predominantly obliquely oriented muscle bundles; layering of muscle bundles is not distinct. x 11. 56.6 (3.0) 1.3 (0.1) 0.8 (0.5) 1.0 (0.4) 69.2 (14.1) 0.59 (0.15) 14.3 (6.6) 8.4 (1.3) 1.50 (0.58) 21.1 (4.8) 45.7 (2.3) 1.2 (0.01) - - 3.4 (0.6) - - - 0.6 (0.2) 0 1.9 (0.4) 39.8 (3.4) 0 Bladder neck 1.7 (0.2) 52.0 (2.1) 0 Bladder M Y ~ ~ ~ 23.5 (2.4) 0.9 (0.1) 67.8 (2.5) 1.3 (0.7) 223.7 (12.1) 0.13 (0.02) 8.8 (5.7) 21.2 (2.5) 0.53 (0.12) 13.9 (1.0) 0.2 (0.1) 7.3 (2.4) 0 Proximal ~ ~ 9.5 (3.4) 19.7 (3.4) 0.43 (0.15) 18.3 (1.4) (0.04) 20.0 (1.1) 1.0 (0.1) 75.4 (1.3) 1.4 (0.9) 290.0 (26.8) 0.18 0.2 (0.1) 3.1 (0.8) 0 Prostatic Urethra Middle 0.29 (0.12) 9.1 (1.0) (2.6) 0.2 (0.1) 12.1 (2.8) 11.8 (2.7) 21.5 (2.6) 1.1 (0.1) 53.0 (6.1) 1.6 (1.0) 156.5 (36.0) 0.22 (0.06) 16.7 (6.4) 22.0 Distal (1.0) 0.9 (0.1) 4.9 (1.0) 53.7 (3.2) 39.9 (2.3) 1.4 (0.1) 0.2 (0.2) 0.7 (0.2) 22.0 (3.0) 1.86 (0.39) 10.4 (3.6) 13.1 (1.4) 0.18 (0.02) 8.8 0.8 (0.1) 0.5 (0.2) 58.4 (2.0) 39.9 (1.7) 1.7 (0.1) 0.1 (0.1) 0.7 (0.4) 19.3 (2.5) 2.21 (0.57) 14.5 (5.9) 14.2 (1.7) 0.16 (0.02) 8.0 (0.9) 19.5 (4.9) 11.3 (1.2) 0.15 (0.01) 9.0 (0.8) (0.46) 0.8 (0.2) 27.5 (7.0) 1.70 0.5 (0.1) 0.3 (0.2) 54.5 (3.4) 44.3 (3.4) 1.6 (0.1) 0 Postl'rostatic Urethra Proximal Middle Distal >Meanvolume-fraction expressed as percent (standard error shown parenthetically). 2Elastic fiber volume-fraction percent i s included also in total connective tissue. 'Also includes some lymphatic tissue, but excludes stratum cavernosum. 'Mean section area (mmz)~susefulforcomparingregions,butthemeans arecompositesofdogssignificantlydifferentin size.Meanmmlengths (standarderrors)were: 4.8 (0.7)forneck; 8.9(0.6)foreach prostatic region; and 13.4 (1.8)for each postprostatic repon. 'Sv = endothelial surface area per unit volume of tissue (mmYmm9. @Index of density of elastic fibers in the submucosa = (mean number of proximal ends of elastic fiberslmm')ilOJ. Smooth muscle' Striated muscle' Total connective tissue' Elastic fibers';2 Prostate gland' Nerves and vessels'." Mean section area4 Sv stratum cavernosumj Vv stratum cavernosum' Index of elastic fiber density6 Mean epithelial area (mm2) Mean lumen perimeter (mm) Epithelium1 Tissue TABLE 1 . Tissue constituents rn regions of the bladder and pelvic urethra for six adult male dogs c 0 (D 191 MORPHOMETRY OF THE CANINE MALE URETHRA lium and smooth muscle, and the caudal prostatic urethra had equal volumes of smooth and striated muscle. The three sampling regions were not significantly different in Vv of total connective tissue, Vv of elastic fibers, or index of elastic fiber density in the submucosa. Striated muscle was the dominant tissue constituent of the postprostatic urethra, occupying over 50% of the urethral wall in all three sampling regions. Total connectivetissue occurred next in abundance, comprising approximately 40%0of the urethral wall. The cranial third of the postprostatic urethra was significantly different from the distal two-thirds in having a greater amount of smooth muscle. A trace amount of disseminated prostate was present in the proximal half of the postprostatic urethra. Stratum cavernosum, quantified by surface area of endothelium per unit volume of tissue (Sv) or by absolute endothelial surface area, was best developed in the postprostatic urethra. The regions of prostatic urethra and vesi- cal neck, which were not significantly different from one another in absolute endothelial surface area for stratum cavernosum, averaged only half the endothelial surface area of the postprostatic urethra. Although Vv of stratum cavernosum also was measured, Vv was highly variable generally and particularly as a consequence of perfusion. Furthermore, the Vv statistic probably incurred bias, since a fixative-induced contracture of striated muscle seemed t o impede vascular distension of stratum cavernosum in the postprostatic urethra. Vv of epithelium was not significantly different between the body and the neck of the bladder. For the urethra, epithelial Vv was not a meaningful parameter because of the massive prostate, and epithelial area per section, estimated by number of points falling on epithelium, was regarded as a better parameter for comparison. In terms of area per section, epithelium declined through the prostatic urethra and remained low in the postprostatic 0 0 7 600 / 0 0 - 50- 8 0 0 0 - 0 .-C0 4 0 - 8 0 0 0 0 0 0 (D t 0 0 0 0 0 30- 0 0 0 0 OO 0 0 0 0 8 0 0 0 0 w 0 0 i.. . *. -*. 5 0. Body I Neck Bladder . I 0 I c .* 2 1 3 Prostatic Urethra ma .* .. . . . . 0 8 I 0. 2 4 3 Postprostatic Urethra Fig. 3. The volume-fraction (Vv)of total connection tissue (0) and of elastic fibers ( 0 )is plotted for each of the six adult males with respect to location within each samplingregion. Notice that by random sampling within each region, the entire length of the urethra was examined over all dogs. For total connective tissue, the bladder neck was significantly different from the body of the bladder and from the postatic and postprostatic urethral regions. Despite the mass of prostate gland, elastic fiber Vv was not greatly reduced in the prostatic urethra because of an increased density of elastic fibers in the submucosa of this region. 192 W. CRAIG CULLEN, THOMAS F. FLETCHER, AND WILLIAM E. BRADLEY urethra. In general, epithelial area was proportional to lumen perimeter except in the midprostatic urethra, where epithelium decreased despite an increase in lumen perimeter (Fig. 4).Thus the epithelial layer was appreciably thinner in this region. But this could be artifactual, owing to the lack of urethral sphincter muscle allowing the lumen to balloon preferentially in the midprostatic region. Pelvic urethrae of the two pups were smaller than those of the adult males, but the pup prostates were disproportionately small. Vv of prostatic glandular epithelium averaged only 25% of the prostatic urethral region for the pups compared to 65% for adult males. Consequently, Vv's of smooth muscle, connective tissue, and urethral epithelium were proportionately greater in pup compared to adult prostatic urethrae (Table 2). Proportions of the various tissues in the postprostatic urethra and in the neck and body of the bladder were quite similar in pups and adults, with one exception. The stratum cavernosum was much better developed in adults, presumably a reflection of sexual maturity. However, the pups displayed the adult pattern of postprostatic cavernosum exceeding prostatic stratum cavernosum. 30 DISCUSSION The morphometric method used in this study of normal urethrae could be adapted to examine dysfunctional canine urethrae, or could be used to compare urethrae among species. In sampling to compare urethrae, one should avoid urethral regions of abrupt transition where tissue variability is high. For the male canine urethra, the middle third of the prostatic urethra and the caudal two-thirds of the postprostatic urethra are optimal sampling regions for comparing urethrae because of minimal transition of tissue proportions. Some of the morphologic arrangements we observed had implied functional correlates. The principal distribution and orientation of prostatic smooth muscle around lobules of the gland implied that the muscle functions to increase intralobular pressure. Muscle in the prostatic capsule was skimpy, indicating no significant mechanism for active compression of the overall gland and the included urethra. The observations that smooth muscle encircled urethral submucosa only at the cranial extent of the prostatic urethra and that the midprostatic urethra was particularly deficient in 70 T m 25 -E 0. rc 1.5 2o E v L 2 15 .-i? L e, Q 10 5 0 1 B f : c l k T Prostatic 1 2 Urethra 1 3 2 Postprostatic Urethra 3 0 53.3 (0.8) 1.3 (0.2) 0 1.0 (0.1) 35.3 (3.2) 0.14 (0.003) 0.2 (0.1) 48.0 (2.2) 1.3 (0.03) 0 1.o - - - (0) 1.8 (0.1) 44.0 (0.9) 0 Bladder neck 1.8 (0.2) 49.3 (2.1) 0 Bladder M Y 58.3 (0.2) 1.0 (0.2) 23.0 (1.3) 1.7 (1.1) 33.8 (3.6) 0.018 (0.001) 0.3 (0) 0 1.4 (0.6) 16.4 (0.8) Proximal 0.014 (0.002) 0.4 (0.3) (1.6) 53.8 (1.8) 1.2 (0.1) 33.7 (3.1) 0.6 (0.2) 54.2 0.9 (0.3) 11.0 (1.3) 0 Prostatic urethra Middle (0) 0 0.8 (0.2) (0.004) 0.2 (0.1) 20.1 (0.3) 0.031 (0.1) (0.1) 0.8 (0.5) 4.0 (1.6) 53.1 (1.5) 41.8 (3.4) 1.3 0.8 (0.2) 16.1 (2.2) 0.030 (0.008) 0.8 (0.2) 0.9 (0.2) 1.3 (0.3) 56.5 (4.9) 40.6 (4.3) 1.5 (0.2) 0 0.8 (0.3) 16.6 (0.7) 0.041 (0.006) 1.4 (0.2) 0 (0.1) 52.1 (1.9) 46.5 (2.3) 1.7 (0) 0.1 (0) 0.7 PostProstatic urethra Proximal Middle Distal 17.8 (1.4) 0.5 (0.1) 39.0 (2.2) 0.014 (0.001) 0.2 15.2 (1.1) 12.8 (2.2) 53.2 (2.0) 1.3 (0) 0.3 Distal ‘Mean volume-fraction expressed as percent (standard error). *Elastic fiber volume-fraction is included also in total connective tissue. 3Excludes stratum cavernosum. ‘Sv = endotheIialsurfaceareaperunit volumeoftissue(mms/mm’l.Meanmm lengths(standarderrors1 were:5.0(1.0)forneck;3.7(O)foreach prostat1cregion;and lO.O(O1foreachpostprostaticregjon. Smooth muscle‘ Striated muscle’ Total connective tissue’ Elastic fibers‘J Prostate gland’ Nerves and vessels’.3 Mean section area (mm? Sv stratum cavernosum% Vv stratum cavernosum’ Epithelium’ Tissue TABLE 2. Tissue constituents of bladder and pelvic urethra for twojuvenile male dogs z 8 E A -. a 6 c 0 * i 8 < % m 8z - 8 z 194 W. CRAIG CULLEN. THOMAS F. FLETCHER, AND WILLIAM E. BRADLEY urethral related smooth muscle, indicated that closure of the lumen of the prostatic urethra is passive. The abundant elastic fibers found in the submucosa of the prostatic urethra implied that elastic fibers hold the lumen closed, with closure facilitated by an apparently firm colliculus seminalis that indents the urethral lumen dorsally. With regard to urinary continence, canine males and females had somewhat different arrangements. In females, the vesical neck was augmented by a long smooth muscle sphincter extending throughout the cranial half of the urethra. This gradually was replaced by a striated sphincter caudally. The entire urethra contained a uniform amount of elastic fibers. In males, the bladder neck appeared to be the primary smooth muscle sphincter; it was augmented only by a brief extension of the sphincter into the cranial sixth of the prostatic urethra. Caudal to this there was no smooth muscle sphincter. The postprostatic pelvic urethra featured a long, thick, striated sphincter, occupying in males more than twice the relative volume of the striated sphincter in females. The striated muscle was relatively just as well developed in the pups, which were too young to have possibly exercised the muscle in squirting urine for territorial marking as adult males do. Except for the submucosa of the prostatic urethra where elastic fibers were concentrated, elastic fibers were less numerous in the male than in the female urethra and bladder. The numerical deficiency of submucosal elastic fibers in the male was moderated by a larger mean fiber diameter in the male (0.8pm) than in the female (0.6 pm). However, the volume density of elastic fibers was still significantly less in males than in females. The perimeter of the urethral lumen was not significantly different along the length of the female urethra. In males, luminal perimeter in the midprostatic region and, to a lesser extent, the cranial prostatic region was significantly larger than that observed more caudally in the pelvic urethra. In both sexes, epithelial area per section was proportional to lumen perimeter, except in the midprostatic region where epithelium appeared thin in our specimens. Whether this thin epithelium is a consistent structural feature or merely a consequence of lumen distension in our specimens was not clear. Male-female urethral comparisons were complicated by presence of the prostate and greater development of striated muscle in the male. In the vesical neck, the higher muscle connective tissue ratio for males vs. femalesdid not prove to be significant statistically. In the vesical body, muscle and connective tissue were not significantly different between the sexes, but epithelium was relatively decreased in the males. Investigating the mechanics of the canine pelvic urethra during erection and ejaculation would pose an extraordinary experimental challenge. Based on histology of the organ, we hypothesize the following sequence of events: With erection the urethral stratum cavernosum expands, reducing the urethral lumen and firming the urethral wall for conveyanceof a relatively small volume of ejaculate under relatively high pressure. Contraction of smooth muscle in the vesical neck and the most cranial prostatic urethra maintains urinary continence and prevents ejaculate reflux. Contraction of smooth muscle surrounding prostatic lobules forces prostatic fluid into the elastically closed midprostatic urethral lumen. Simultaneous contraction of the postprostatic striated muscle traps fluid in the midprostatic urethra, which is ballooned by entering fluid. Peristalsis of each ductus deferens adds spermatozoa to the ejaculate pool. At this point, relaxation of striated muscle allows expulsion of the ejaculate pool into the postprostatic urethra, powered by elastic recoil of the ballooned prostatic urethral wall. A striated peristaltic wave conveys ejaculate into the penile urethra. Then, striated muscle tetanus initiates another cycle of ejaculate pooling during continued contraction of smooth muscle in the prostate and vesical neck. Woodburne ('60, '61) reported that deep longitudinal muscle of the vesical neck-urethra was thick and, along with a rich supply of elastic fibers, it constituted a myoelastic tissue. We were not impressed by the amount of longitudinal muscle, which formed only delicate fascicles adjacent to the submucosa in our specimens. Woodburne also reported that there was no sphincter at the canine (and human) vesical neck, because muscle bundles extending from the vesical body to the neck arched past rather than around the vesico-urethral junction. Because of this arrangement and because stimulation of muscle bundles at the vesical body opened the urethral orifice, Woodburne concluded that neck musculature does not serve as a sphincter and that elastic tissue must maintain continence. Woodburne had discountedthe concept that sympathetic innervation operates to constrict an internal urethral sphincter. Since that time, several investigators have MORPHOMETRY OF THE CANINE MALE URETHRA demonstrated sympathetic induced localized contraction of the vesical neck-proximal urethra i n dogs (Awad and Downie, ’76; Kleeman, ’70; Koraitim et al., ’77; Raz et al., ’72; Rohner et al., ’71). We would agree with Woodburne’s observation that muscle bundles tend to arch rather than encircle the vesical neck; however, when viewed in transverse section, most of the fascicles a t least partially surround and some even completely encircle the neck. Of particular significance are the muscle bundles that arch around the neck and continue into the body of the prostate. It seems likely that localized contraction of these muscle bundles, anchored cranially a t the distended bladder and caudally at the prostate, is sufficient to squeeze the vesical neck closed for continence. In addition to physiologic evidence for a n active sphincter at the vesical neck, our finding that elastic fibers are reduced in the neck compared with the prostatic urethra argues for an active proximal sphincter to prevent ejaculate reflux. The primary sphincter must reside in the vesical neck since the prostatic urethral sphincter is so abbreviated. Woodburne’sobservation that stimulation of muscle bundles at the vesical body produces opening of the vesical neck is not pertinent to continence since the latter is related physiologically to localized contraction of musculature a t the neck. However, the observation does reinforce appreciation for the dual mechanical role of vesical neck muscle cells: During active continence they must shorten; during voiding they must elongate mechanically to allow distention of the lumen of the vesical neck. This dual role of muscle cells pertains throughout the bladder, but muscle cells of the vesical neck must function out of phase to those in the rest of the 195 bladder, and this relates to the great dependency of the bladder-urethra on properly coordinated innvervation. Thus, in the male dog, the internal “urethral” sphincter resides in the vesical neck, except for a minor component encircling submucosa of the proximal prostatic urethra. The sphincter is a functional entity generated by reciprocal innervation. Awad and Downie (’76)studied the effects of adrenergic drugs and hypogastric nerve stimulation on fluoroscopically visualized urethrae of male dogs. Consistent with our findings, they demonstrated that the lumen of the midprostatic urethra was more dilated than the rest of the urethra and that sympathetic stimulation obliterated the lumen only of the vesical neck and most cranial prostatic urethra. They also reported that norepinephrine in sufficient dosage could produce complete occlusion of the lumen of the prostatic urethra, and we presume this resulted from elastic recoil following complete blockage of perfusion fluid a t the vesical neck. LITERATURE CITED Awad, S.A., and J.W. Downie (1976) The effect of adrenegic drugs and hypogastric nerve stimulation on the canine urethra. Invest. Urol., 13:298-301. Kleeman, F.J. (1970)The physiology of the internal urinary sphincter. J. Urol., 104t549-554. Koraitim, M., W. Schaffer, H. Melchoir, and W. Lutzeyer (1977) Urogenitodynamics of the male bladder neck. 1. During continence. Invest. Urol., 14r496-497. Miller, M.E., G.C. Christensen, and H.E. Evans (1965)Anatomy of the Dog. W.B. Saunders Co., Philadelphia. Raz, S.,M. Zeigler, and M. Caine (1972) Isometric studieson canine urethral musculature. Invest. Urol., 9:443-446. Rohner, T.J., D.M. Raezer, A.J. Wein, and H.W. Schoenberg (1971) Contractile responses of dog bladder neck muscle to adrenergic drugs. J. Urol., 105r657-661. Woodburne, R.T. (1960) Structure and function of the urinary bladder. J. Urol., 84:79-95. Woodburne, R.T. (1961) The sphincter mechanism of the urinary bladder and urethra. Anat. Rec., 141:ll-20.