182 



THE HUMAN MECHANISM 



- Vena Cava 



- Ureter 



the ureter enters the kidney at the center of the depression 

 in that organ it expands to form a basin, known as the pelvis 

 of the ureter. Into this basin open the hundreds of glandular 

 tubules of which the bulk of the kidney is composed. Each 

 tubule, like the alveolus and ducts of the gland described in 

 Chapter III, consists of a single layer of cells, which separate 

 the blood and lymph from the lumen of the tubule ; and the 



formation of urine by the kidney 

 is essentially an act of secretion. 

 4. The secretion of urine. 

 The urine is secreted continu- 

 ously from the blood, at one 

 time more rapidly than at an- 

 other, but under normal condi- 

 tions never ceasing altogether. 

 Passing down the tubules, it 

 collects in the upper portion of 

 the ureter, and successive peri- 

 staltic waves carry it from this 

 point to the urinary bladder, an 

 organ with muscular walls in 

 which the urine accumulates 

 and from which it is from time 

 to time discharged. 



In one very important re- 

 spect, however, secretion by the 

 kidney presents a sharp contrast to secretion by the stomach 

 and the submaxillary gland. While an adequate blood supply 

 to the two latter glands accompanies secretion and, indeed, 

 is necessary to maintain the secretion for any length of time, 

 yet these glands secrete only as they are stimulated to ac- 

 tivity by their nerves ; merely increasing their blood supply 

 does not produce increased secretion. In the case of the 

 kidney there seem to be no secretory nerves, and the activity 

 of the gland seems to be determined to a large extent ly the 



FIG. 81. Dorsal aspect of the kid- 

 neys, ureter, urinary bladder, and 

 abdominal aorta and vena cava 



