520 THE HUMAN BODY 



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 abdominal 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 kidney 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 

 Ua. 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 beginning 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 

 bladder can retain liquid under considerably higher pressure when 

 the spinal cord is intact than after destruction of its lumbar por- 

 tion. The contraction of these urethra constricting muscles can 

 be reinforced voluntarily. When some amount of urine has ac- 

 cumulated in the bladder, it contracts and presses on its content; 

 the ureters being closed in the way above indicated, the elastic 

 fibers closing the urethral exit are overcome, and the urethral 

 muscles simultaneously relaxing, the liquid is forced out. 



Naked Eye Structure of the Kidneys. These organs have ex- 

 ternally a red-brown color, which can be seen through the trans- 

 parent capsule of peritoneum which envelops them. When a 

 section is carried through a kidney from its outer to its inner 

 border (Fig. 139) it is seen that a deep fissure, the hilus, leads into 

 the latter. In the hilus the ureter widens out to form the pelvis, 

 D, which breaks up again into a number of smaller divisions, the 

 cups or calices. The cut surface of the kidney proper is seen to 

 consist of two distinct parts: an outer or cortical portion, and an 



