Excretion. 63 



forming a valve which prevents the urine passing back 

 to the kidney; the tubes are known as the ureters. The 

 bladder is the urinary reservoir situated in the pelvic 

 cavity, although when full it may extend into the ab- 

 dominal cavity. It is divided into a fundus (the wide 

 part) and neck which is continuous with the urethra, the 

 latter the tube carrying the urine to the outside of the 

 body. The exit from the bladder is guarded by a circu- 

 lar (sphincter) muscle, which by its contraction prevents 

 the trickling of urine continuously. The coats of the 

 bladder are three in number, serous externally, muscu- 

 lar or middle coat and mucous internally. The ure- 

 thra in cattle and sheep differs from that of the horse, in 

 the former having an S-shaped curve, whereas in the 

 latter it is straight. The urethra in the female of these 

 animals opens on the floor of the vulva, at a short dis- 

 tance (3 to 4 inches) from the outside of the body. In 

 cows the entrance to the urethra is guarded by a valve- 

 like piece of skin, which must be lifted in order to pass 

 the catheter. The difference in length and construction 

 of the urethra in the sexes accounts for the greater fre- 

 quency of stones (calculi) in the bladder and urethra in 

 males. When the bladder becomes filled the mucous 

 membrane comes in contact with the urine, a nervous 

 impulse is sent to the brain and as a consequence the neck 

 (sphincter muscle) of the bladder is relaxed, and the 

 urine is passed, the whole constituting a good illustra- 

 tion of reflex action, the exercise of the will power not 

 being called for. The urine is made use of by chemists 

 and medical men, by the former to estimate the amount 

 of nitrogenous material used or wasted in the body, by 



