PHYSIOLOGY OF TO-DAY 



known physical or chemical laws. In many cases, this 

 cannot be done, and we can then only express the process 

 in terms of what may be called "physiological laws." 

 A good example of these two types of explanation will 

 be found in recent work on the physiology of kidney 

 excretion. 



To understand the problem, a little must be explained 

 about the intimate structure of the kidney. This organ 

 is made up of many identical small units — in the human 

 about one million in each kidney — called "renal tubules." 

 Each tubule commences in a peculiar structure formed by 

 an invagination of a tuft of very small blood vessels into 

 its expanded end. This structure is called the renal cor- 

 puscle or glomerulus. The remainder of the renal unit is 

 composed of a long tube formed of epithelial cells, which, 

 in the mammal, are found by microscopic examination to 

 be of at least three distinct types. This tube is designated 

 the tubule proper, and has a markedly different structure 

 from the glomerulus. Ever since Bowman, nearly a century 

 ago, discovered the true structure of the kidney unit, phy- 

 siologists have interested themselves in two related 

 problems concerning the production of urine; namely, 

 the assigning of the proper functions to each distinct part 

 of the renal unit and the "explaining" how each part 

 carried out its function. 



All theories which have been proposed to explain the 

 secretion of urine by the kidneys have assigned different 

 functions to the glomerulus and to the tubule. It has been 

 generally agreed that the glomerulus eliminates most of 

 the water or fluid part of the urine, but there has been, 

 until recently, quite a controversy as to the nature of the 

 process by which fluid is elaborated by this structure. 

 One school has held that the process occurring at the glo- 

 merulus is a physical filtration or ultra-filtration, such as 

 can readily be carried out in a dead collodion or parchment 



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