CHAPTER XXVIII. 

 THE KIDNEYS AND SKIN. 



General Arrangement of the Urinary Organs. These 

 consist of (1) the kidneys, the glands which secrete the 

 urine; (2) the ureters or ducts of the "kidneys, which carry 

 their secretion to (3) the urinary bladder, a reservoir in 

 which it accumulates and from which it is expelled from 

 time to time through (4) an exit tube, the urethra. The 

 general arrangement of these parts, as seen from behind, is 

 represented in Fig. 132. The two kidneys, R, lie in the 

 dorsal part of the lumbar region of the abdominal cavity, 

 one on each side of the middle line. Each is a solid mass, 

 with a convex outer and a concave inner border, and its 

 upper end a little larger than the lower. From the ab- 

 dominal aorta, A, a renal artery, Ar, enters the inner border 

 of each kidney, to break up within it into finer branches, 

 ultimately ending in capillaries. The blood is collected from 

 these into the renal veins, Vr, one of which leaves each kid- 

 ney and opens into the inferior vena cava, Vc. From the 

 concave border of each kidney proceeds also the ureter, U, a 

 slender tube from 28 to 34 cm. (11 to 13.5 inches) long, 

 opening below into the bladder, Vu, on its dorsal aspect, and 

 near its lower end. From the bladder proceeds the urethra, 

 at ffrt. The channel of each ureter passes very obliquely 

 through the wall of the bladder to open into it; accordingly 

 if the pressure inside the latter organ rises above that of the 

 liquid in the ureter, the walls of the oblique passage are 

 pressed together and it is closed. Usually the bladder, 

 which has a thick coat of unstriped muscular tissue lined by 

 a mucous membrane, is relaxed, and the urine flows readily 

 into it from the ureters. While urine is collecting, the be- 

 ginning of the urethra is kept closed, in part at least, by 

 bands of elastic tissue around it: some of the muscles which 

 surround the commencement of the urethra assist, being kept 

 in reflex contraction ; it is found that in a dog the urinary 



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