THE MALE URETHRA 585 



ischial arch, between the bulbo-urethral glands, it contracts again, forming the 

 isthmus urethras. It is related dorsally to the rectum and the prostate, ventrally 

 to the internal obturator muscles, and laterally to the bulbo-urethral glands. It 

 is enclosed, except at its origin, by the urethral muscle. ^ 



The extrapelvic part (Pars externa) passes between the two crura of the penis 

 and runs along the groove on the ventral surface of the corpus cavernosum penis, 

 enclosed by the corpus cavernosum urethra and the bulbo-cavernosus muscle. It 

 passes through the glans penis and projects forward about an inch in the fossa 

 glandis as a free tube, the processus lu-ethrse; this part is covered by a delicate 

 integument, under which there is a thin layer of erectile tissue. 



The lumen of the tube is largely obliterated in the inactive condition of the parts. When 

 naoderately distended, its dimensions in a horse of medium size are as follows: At its origin the 

 diameter is about haK an inch (1 to 1.5 cm.). The pelvic dilatation at its widest part measures one 

 and a haK to two inches (ca. 3.5 to 5 cm.) transversely, and about an inch (2 to 3 cm.) vertically; 

 it is elliptical in cross-section when fully distended. The isthmus at the ischial arch is a little 

 smaller than the initial part. Beyond this the lumen is about one-half to three-fourths of an 

 inch (ca. 1.5 cm.) in diameter, and is fairly uniform to the glans penis. Here there is a slight 

 fusiform dilatation (Fossa navicularis), beyond which the tube contracts. 



The opening from the bladder into the urethra is termed the internal urethral 

 orifice (Orificium urethrse internum); it is closed except during urination. The 

 terminal opening is the external urethral orifice (Orificium urethree externum) or 

 meatus urinarius. The colliculus seminalis is a rounded prominence, situated 

 medially on the dorsal wall, about two inches (ca. 5 cm.) behind the internal ure- 

 thral orifice. On either side of the colliculus there is a small diverticulum, in which 

 the ductus deferens and the duct of the vesicula seminalis open. The small orifice 

 of the uterus masculinus is placed centrally on the colliculus; it is inconstant. The 

 orifices of the prostatic ducts are on two groups of small papillae, placed lateral to 

 the ejaculatory openings. The ducts of the bulbo-urethral glands open on two 

 lateral series of small papillae, about an inch (ca. 2.5 cm.) further back and close to 

 the median line. The small orifices of the lateral urethral glands are situated 

 laterally in the wide pelvic portion. 



Structure. — The mucous membrane contains a large amount of fine elastic 

 fibers, and in its pelvic part there are tubulo-alveolar urethral glands (Glandula; 

 urethrales).^ The epithelium is at first like that of the bladder, then becomes 

 cylindrical, and in the terminal part is stratified squamous. In the dorsal wall the 

 membrane forms a median ridge, the urethral crest (Crista urethralis) ; this termi- 

 nates about two inches from the internal urethral orifice in the colliculus seminalis. 



At the origin of the urethra there is a layer of circular unstriped muscle-fibers 

 outside of the mucous coat. Beyond this the latter is enclosed by a layer of erectile 

 tissue (Stratum cavernosum), which contains plexuses of veins supported by 

 trabeculae of elastic and unstriped muscular tissue ; in its peripheral part there are 

 numerous small arteries. There is a slight thickening of the erectile tissue at the 

 ischial arch, producing an enlargement known as the bulb of the urethra (Bulbus 

 urethrae). This is continued by the corpus cavernosum urethrae. Outside of the 

 erectile tissue there is a continuation of the intrinsic unstriped muscular coat, 

 consisting of external and internal longitudinal strata, with a layer of circular 

 fibers between them. 



lit has been customary to divide the pelvic part of the urethra into prostatic and mem- 

 branous parts. These terms apply well in human anatomy, but have no special value m com- 

 parative anatomy. In the horse a prostatic part hardly exists, unless we assume that it and the 

 neck of the bladder together are only about an inch in length. There is no membranous part m 

 the sense in which that term is used in regard to man, since the tube has a continuous envelop of 

 erectile tissue. 



2 Two sets of glands can be distinguished in the pelvic part of the urethra. Two rows of dor- 

 sal glands occur close to the median plane; their ducts open into those of the bulbo-urethral glands. 

 A series of lateral glands extends on either side from a point near the prostatic ducts to the end of 

 the pelvic urethra; their ducts open laterally, as mentioned above. 



