CHAPTER IV. 

 SECRETION BY THE KIDNEYS. 



THE epithelium of the kidney, like that of the alimentary canal, 

 is a secreting tissue. The protoplasmic cells which line at least a 

 large portion of the tubuli uriniferi elaborate from the blood, in a 

 manner which we shall presently discuss, certain substances, and 

 discharge them into the channels of the tubules. Besides these 

 distinctly active secreting structures, however, the kidney exhibits 

 in its Malpighian bodies an arrangement very analogous to that 

 which obtains in the lungs. Just as in the latter the functions of 

 the alveolar epithelium are reduced to a minimum, and the en- 

 trance and egress of the gases of respiration are mainly carried on 

 by diffusion, so in the former the epithelium covering the glomerulus 

 has probably but little secreting activity, and the passage of 

 material from the interior of the convoluted blood-vessels into the 

 cavity of the tubule is chiefly carried on by processes which 

 more closely resemble ordinary filtration. What substances 

 pass in this way, and what substances are secreted by the 

 direct action of the epithelium of the secreting tubules, we 

 shall shortly consider. The various substances passing, in company 

 with a large amount of water, in either the one or the other way, 

 into the ducts of the gland, constitute the secretion called urine. 

 And since none of the substances so thrown out are of any further 

 use in the economy, but are at once carried away, urine is generally 

 spoken of as an excretion. 



