320 HUMAN PHYSIOLOGY 



plexity that we shall not undertake to present them. 

 The secreting units are long and tortuous tubules originat- 

 ing near the surface of the organ and conducting the 

 urine toward a cavity in the concave border. A funnel- 

 shaped appendage receives the urine and it passes on into 

 the ureter, a slender and delicate yet definitely contractile 

 vessel. The muscular elements of the ureter are of 

 the smooth variety and they execute a true peristalsis. 

 The travelling contractions propel the urine in small 

 quantities to the bladder. 



This is a contractile sac placed in the pelvis in front of 

 the rectum. The two ureters enter it low down and 

 behind. Their openings are not far from that through 

 which the urine escapes to the exterior. This passage is 

 the urethra. The three openings are at the angles of a 

 small triangle which is not much disturbed by the 

 alternate enlargement and contraction of the bladder. 

 We have used this organ before (Chapter IV) to illustrate 

 what is meant by change of tone. When it has just been 

 emptied it is quite inconspicuous and its upper surface is 

 practically in contact with its base. When it is full it is 

 rounded up and its walls are thin as though stretched. 

 Yet, as we have insisted, we must not think that they are 

 really stretched unless the condition is extreme; they 

 have relaxed or lost tone and are not necessarily reacting 

 with much pressure upon the liquid inside. 



The bladder is often involved in reflexes. It is apt to 

 contract when the hands are dipped in water. The ure- 

 thra is controlled by muscle, both striped and smooth, 

 acting on the principle of a sphincter. When this 

 sphincter is voluntarily or otherwise inhibited, the urine 

 enters the passage and seems to evoke a reflex contrac- 

 tion of the bladder which rapidly completes the dis- 

 charge. Extra pressure may be thrown upon the bladder 

 by contracting the abdominal muscles. 



The Urine. Some characteristics of the urine may now 

 be pointed out. Its color is due to the carrying over to 

 the kidneys of substances having their origin in the liver 



