210 A Manual of Veterma/ry Physiology. 



The waste products, viz., the organic and inorganic matters, 

 are doubtless excreted in the tubules, the water in the 

 Malpighian tufts, and if Ludwig's idea is correct of a partial 

 reabsorption of water after leaving Bowman's capsule of the 

 tuft, then it appears probable that the remarkable con- 

 strictions in the tubules are for this purpose. 



The peculiar elective power so often shown in the 

 tissues of the body is nowhere better demonstrated than in 

 the kidney. After the water has been removed from the 

 blood in the Malpighian tuft, the efferent vessel forms a 

 plexus around the tubules, and from the same blood which 

 has had the water removed, we now have the waste products 

 and inorganic matter separated. 



The urine is always being excreted, and falls drop by 

 drop into the bladder, the ureters penetrating the coats of 

 the latter obliquely, so that as the. organ becomes dis- 

 tended pressure is exercised on these openings, and no re- 

 flux can occur. 



Micturition. — In the lumbar portion of the spinal cord is 

 situated the centre for micturition ; it is by afferent and 

 efferent nerves in direct communication with the bladder. 

 Through it the sphincter of the bladder is controlled re- 

 flexly, tightening its grip consciously and unconsciously 

 on the neck of the bladder as the fluid within increases 

 in quantity, and by means of this centre the mind is 

 made aware of the distension which exists. The desire for 

 micturition having become pressing, the sphincter through 

 the same channel is relaxed as a voluntary act, the dia- 

 phragm is fixed, the abdominal muscles and involuntary 

 muscle uf the bladder wall contract, the urine is forced 

 into the urethra, where it is hurried along by the accele- 

 rator urinse muscle. During the act both the horse and 

 mare stand with the hind-legs extended and apart, resting 

 on the toes of both hind-feet, thereby sinking the posterior 

 part of the body : the penis or vulva is protruded, the tail 

 raised and quivering. The stream which flows from the 

 two sexes is very different in size, depending on the relative 

 diameters of the urethral canal. The urethra, excepting 



