1308 THE UEO-GENITAL SYSTEM. 



urethras projects backwards and overlaps the posterior wall of the membranous 

 part of the urethra to a considerable extent (Fig. 1024). 



The membranous portion of the urethra is the most firmly fixed and least 

 dilatable part of the passage. 



A slight medial elevation, which is continuous above with the crista urethras, 

 projects into the canal from its posterior wall, and, becoming less marked as it is 

 traced downwards, is often seen to divide into two faint ridges. When the canal 

 is empty other longitudinal folds or ridges are usually to be seen on the mucous 

 membrane, but these become obliterated when the passage is distended. The 

 lumen of the empty tube, in transverse section, presents a stellate outline. 



It is important to note that the terminal portion of the pars membranacea urethrse, 

 where it is overlapped posteriorly by the urethral bulb, lies in front of the urogenital 

 diaphragm. It is considerably wider than the upper part of this subdivision of the canal, 

 and is very thin-walled. This is the part of the canal which is most liable to rupture 

 (Figs. 1024 and 1026). 



Pars Cavernosa Urethrse. The third, or cavernous portion, of the urethra 

 is much the longest of the three subdivisions. It begins at a point about half 

 an inch in front of the posterior end of the bulbus urethras, and ends at the 

 external urethral orifice on the glans penis. Its proximal, or perineal, portion 

 has a fixed position and direction, while its distal part varies with the position 

 of the penis. The canal is about six inches in length, and is related throughout 

 its whole extent to the erectile tissue of the corpus cavernosum urethras and 

 of the glans penis. Directed at first forwards through the bulbus urethras, the 

 canal turns downwards and forwards at the point where it comes to lie in front 

 of the lower part of the symphysis pubis (Fig. 1025). This bend in the direction 

 of the canal, roughly speaking, corresponds to the place of attachment of the 

 suspensory ligament to the dorsum of the penis. When the penis is drawn 

 upwards towards the front of the abdomen, the direction of the terminal half of 

 the canal is, of course, changed, and at the same time the whole length of this 

 subdivision of the urethra becomes more uniformly curved. 



The urethra passing obliquely downwards and forwards enters the bulb at a point 

 nearly half an inch from its posterior extremity. Immediately after the canal has 

 pierced the fascia inferior of the urogenital diaphragm its posterior aspect becomes 

 surrounded by the erectile tissue of the bulb, but the anterior wall remains un- 

 covered for a distance of about a quarter of an inch (Fig. 1025). The wall of the 

 urethra is here very thin, and the passage is more readily dilatable than in other 

 parts. In this region the urethral wall may readily be torn through, if undue 

 force is used, or if the handle is depressed too soon when attempting to pass an 

 instrument into the narrower more fixed part of the canal. The urethra lies at 

 first in the upper part of the erectile tissue, but as it passes forwards it sinks 

 deeper, and comes to occupy the middle part of the corpus cavernosum urethras 

 (Fig. 1025). In the glans, on the other hand, the erectile tissue lies on the dorsal 

 and lateral aspects of the urethra. Like the other parts of the urethral passage, 

 the pars cavernosa is closed except during the passage of fluid, the closure being 

 effected by the apposition of its dorsal and ventral walls except in the portion 

 of the canal which lies in the glans penis, where the side walls of the canal 

 come into contact. Thus the lumen of the first part of the canal, when empty, 

 is represented in cross section by a transverse slit, and that of the terminal part 

 by a vertical slit (Fig. 1027). The cavernous part of the urethra does not 

 present a uniform calibre throughout, but is narrower in its intermediate part, 

 where it traverses the corpus cavernosum urethras, than it is in those portions 

 of its course which are surrounded by the bulb and the glans. The terminal 

 dilated part of the passage is termed the fossa navicularis urethrae, and opens on 

 the surface by the vertically placed slit-like orificium urethrse externum, or external 

 urethral orifice, which is the narrowest and least dilatable part of the whole 

 urethral canal. 



The ducts of the bulbo-urethral glands open by very minute apertures in the 

 inferior wall of the proximal part of the cavernous portion of the urethra. Before 



