THE EXCRETORY SYSTEM l8/ 



covered partially by a serous coat derived from the 

 peritoneum. The muscular coat has three layers, the 

 principal fibers of which run longitudinally and circu- 

 larly, the circular fibers being collected into a layer of 

 some thickness around the constricted portion or neck, 

 where the bladder becomes continuous with the urethra. 

 These circular fibers around the neck form a sphincter 

 muscle that normally is in a state of contraction, relaxing 

 only at intervals, when the accumulation of urine within 

 the bladder renders its expulsion necessary. 



The Urethra. The urethra is a narrow, membran- 

 ous canal, about \\ inches in length in the female, and 

 extending from the neck of the bladder to the external 

 orifice, or meatus urinarius. It is situated beneath the 

 symphysis pubis, and is embedded in the anterior wall of 

 the vagina. It runs obliquely downward and forward, its 

 course being slightly curved, the concavity directed for- 

 ward and upward. While it admits of considerable di- 

 latation, its normal diameter is about J inch. It is 

 lined by a mucous coat that is continuous, externally, 

 with that of the vulva, and, internally, with that of the 

 bladder. The external muscular coat is also continuous 

 with that of the bladder, but between the mucous and 

 muscular coats is a layer of thin, spongy tissue, containing 

 a network of large veins. In the male, the urethra ex- 

 tends from the neck of the bladder to the meatus urinarius 

 at the end of the penis, measuring 8 or 9 inches. It is 

 divided into a prostatic portion, a membranous portion, and 

 a spongy portion. The latter portion is the longest, being 

 contained in the corpus spongiosum of the penis (see Fig. 

 156). (By referring to the illustrations, Figs. 145, 146, 

 the structures that make up the urinary organs may be 

 clearly seen.) 



Micturition. This act is normally caused by the 

 accumulation of urine within the bladder. The accumu- 



