142 PRINCIPLES OF VETERINARY SCIENCE 



is composed of an inner mucous coat, a middle muscular coat of 

 involuntary fibers, and an outer serous covering which does not 

 extend over the entire organ. 



The urethra is a single tube which carries away the urine from 

 the bladder. The orifice of the urethra is guarded by a ring- 

 like muscle which completely closes it when in a state of contrac- 

 tion. In the male this organ is common to the urinary and 

 genital systems, and will be described with the latter. The 

 urethra of the mare and cow is about 2 inches long and connects 

 the bladder with the vulva, into the floor of which it opens by 

 the meatus urinarius, about a hand breadth from the external 

 opening of the vulva. 



THE PHYSIOLOGY OF URINE SECRETION 



The secretion of urine is a continuous process. The blood 

 enters the glomerulus of the malpighian corpuscle under pressure, 

 where it meets with opposition to its outflow. Here most of the 

 water and soluble salts found in urine are secreted. The urini- 

 ferous tubules are lined with cells capable of extracting from the 

 blood the urinary pigments and solids that were not allowed pas- 

 sage at the expanded end of the tubules. These are flushed out 

 by the fluid from above, and are carried to the pelvis of the kid- 

 ney by the collecting tubules. In general all statements made 

 in Chap. IV about the process of secretion hold good for urinary 

 secretion. However, the kidneys are unlike most glands in that 

 their secretion pressure cannot be raised above the arterial blood 

 pressure. This is on account of the arrangement of its minute 

 structures. 



Urine is the most important excretion of the body. The 

 amount and composition varies with the age of the animal, the 

 character of the food and drink, and the state of metabolism. 

 The average amount voided by the horse in twenty-four hours is 

 about 3000 to 4000 cc, but may be as much as 9000 cc. Experi- 

 ments with dairy cows show that only about 13 per cent, of the 

 water taken into the digestive canal is passed through the kidneys. 

 When sweat is profusely excreted the urine is proportionately 

 reduced in amount. Urine is alkaline in reaction in herbivora 

 unless the animal is starved, or on a ration composed entirely 

 of wheat plant products or other unnatural feed combinations, 

 when it is uniformly strongly acid. Equine urine hfts a specific 



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