UROGENITAL DIAPHRAGM 449 



Variations. — The muscle shows great individual variation in structure which causes it to 

 be variously described by different authors. 



The sphincter ani externus (figs. 392, 394, 396, 397, 399) is made up of bundles of muscle 

 fibres which surround the anus for nearly two centimetres. It is elliptical in form. Behind 

 the anus the fibre-bundles of each side in part interdigitate, forming a ring. They are attached 

 to the skin, and in part are attached through a tendon, the ligamentum ano-coccygeum, to the 

 back of the cocc3^x. In front of the anus the fibre-bundles also in part interdigitate with one 

 another, in part are inserted into the skin and in part interdigitate with the fibre-bundles of the 

 transverse perineal and bulbo-cavernosus muscles. At the point where these muscles meet, 

 about two and a half centimetres in front of the anus, there may be a visible mass of fibrous 

 tissue, the central tendon of the perineum, but this is not always distinct. It is usually better de- 

 veloped in the female than in the male perineum. The external sphincter is divisible into three 

 portions, a subcutaneus, a superficial and a deep (fig. 397). The three parts are connected by 

 fibre-bundles, and are not always distinct. The subcutaneous division is small and immediately 

 encircles the anal orifice. The superficial division lies external to the subcutaneous ring and de- 

 scends further toward the rectum. It is shown in figs. 392, 394. It is the only part attached 

 to the coccyx. In front it is attached to the central tendon of the perineum, but some fibres 

 are continued into the bulbo-cavernosus. The deep portion forms a heavy ring above the rec- 

 tum beneath the superficial part. It is distinctly, though not completely, separated from the 

 pubo-rectal portion of the levator ani by fascial tissue containing the inferior hsemorrhoidal 

 vessels. Some of the fibre-bundles of the deep portion may be traced in front of the anus 

 across the mid-line to the ascending ramus of the opposite side and form part of the superficial 

 transverse perineal muscle. 



Nerve-supply. — From the inferior haemorrhoidal branches of the pudendal (internal pudic) 

 and frequently also by a perineal branch from the fourth sacral. 



Action. — To keep the anus closed. 



Relations. — Externally it is surrounded by the fat of the ischio-rectal fossa, internally near 

 the skin it surrounds the sphincter ani internus, composed of smooth muscle, deeper it Ues 

 next the mucous membrane, for a distance of two centimetres from the skin. 



Variations. — The muscle shows considerable individual variation in structure. 



The recto-coccygeus or muscle of Treitz, is a triangular bundle of smooth muscle fibres. 

 The origin of the muscle is from the second and third coccygeal vertebrae. It is inserted by 

 its apex into the posterior wall of the rectum and the perirectal fascia. It retracts and elevates 

 the rectum. 



B. Muscles of the Urogenital Diaphragm 



The urogenital diaphragm is composed of two closely united muscles, the deep transverse 

 perineal muscle and the urogenital sphincter. 



The transversus perinei profundus (fig. 398) is a flat muscle which arises from the inner side 

 of the inferior ischial ramus and is inserted into the median raphe. Many of the fibre-bundles 

 interdigitate with those of the opposite side and some may be followed into the external sphinc- 

 ter of the anus and into the urogenital spincter and other perineal muscles. 



Nerve-supply. — By the perineal branch of the pudendal (internal pudic). 



Action. — The pair of muscles draw back and fix the central tendon of the perineum and thus 

 give firm support for the action of the urogenital sphincter. 



Relations. — The inferior surface is separated (often incompletely) by the inferior layer of 

 the urogenital trigone from the superficial transverse perineal muscle. The superior surface 

 is covered by the deep layer of the urogenital trigone, into which the superficial layer is reflected 

 about the anal margin of the muscle. 



Variations. — The muscle is variable in structure and may be absent. (It is more frequently 

 absent in the female than in the male.) 



The sphincter urogenitalis differs in the male and female owing to the passage of the vagina 

 through the perineum in the latter. In each sex it is convenient to consider the muscle as 

 divided into two parts, a peri-urethral and an infra-urethral (vaginal). 



In the male (fig. 398) the peri-uretlu-al part, the m. sphincter urethroe membranacece is com- 

 posed of fibre-bundles which are circularly placed about the membraneous m-ethra. The more 

 external fibre-bundles are attached to the crura of the penis near their junction, to the trans- 

 verse ligament of the pubis and to the fasciae of the trigone. Some of them partially ensheath 

 the lower part of the prostate, and others envelop the bulbo-urethral (Cowper's) glands. Some 

 of the fibre-bundles take a longitudinal course along the urethra. Bundles of smooth muscle 

 fibres are intermingled with the striated, and the fibrous framework of the musculature is marked 

 by the large amount of elastic tissue which it contains. The infra-urethral part, the m. trans- 

 versus urethrce, is closely associated with the urethral part. The fibre-bundles arise on each side 

 from the inferior ramus of the pubis. They pass for the greater part beneath the urethra and 

 interdigitate with that of the opposite side or are insei-ted into a median raphe. A few fibre- 

 bundles may pass above instead of below the m-ethra. The transverse urethral muscle, first 

 described by Guthrie (On the anatomy and diseases of the neck of the bladder, London, 1834) 

 is inconstant. Its existence as a normal constituent of the male perineal musculaturehas been 

 disputed by Delbet (Poirier and Charpy) and others. ■ 



In the female the peri-urethral part, sphincter urethrce, differs in no essential respects from 

 the corresponding muscle in the male. Some of the fibre-bundles form a true sphincter about 

 the urethra. The infra-urethral part, on the other hand, seems to vary greatly in different indi- 

 viduals so that the descriptions given by different authors are somewhat contradictory. It is 

 better developed in women who have not borne children than in those who have. It may be 

 looked upon as composed of two portions, a m. transversus vagince and an m. constrictor vagince. 



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