152 



PRINCIPLES OF ANIMAL BIOLOGY 



Relation of the Structure of the Kidney to Excretion. — From the 

 preceding account of excretion it may be inferred that the kidney is the 

 chief organ for the eUmination of soluble wastes from the body. The 

 kidney contains a large number of Malpighian corpuscles (Fig. 113) each 

 located at the end of a much coiled uriniferous tubule, which discharges 

 through a collecting tubule into the ureter. The Malpighian corpuscle 

 consists of a glomerulus (a knot of minute blood vessels) surrounded by 

 two layers of a very dehcate membrane (Bowman's capsule) which is the 

 expanded and invaginated end of the uriniferous tubule. The vessels of 

 the glomerulus take their origin in an afferent arterial bloodvessel and dis- 

 charge into an efferent arterial vessel. The uriniferous tubules (Fig. 



113) are composed of more or less cubical or 

 pyramidal cells which have a granular proto- 

 plasm. Near the Malpighian corpuscle the 

 cells of the tubules, in lower vertebrates at 

 least, are ciliated. The tubule may vary con- 

 siderably in size in different parts of its length. 

 In addition to the blood vessels which form 

 the glomeruli there are very numerous minute 

 blood vessels in intimate contact wdth other 

 parts of the uriniferous tubule. It is probable 

 that there is no other organ in the body which 

 is more richly suppUed with blood than this 

 organ. It has been estimated that under the 

 „ ,,^ „^ ^ . influence of diuretics (substances which cause 



Fig. 113. — Structures from , ^ 



vertebrate kidney. A, Mai- increased flow of Urine) the amount of blood 

 pighian corpuscle; B and c, g^^ ^^ ^^^ kidney may in one minute equal 



This is said to be 



cross-sections of uriniferous 



tubules at different levels; av, the Weight of thoSC OrganS. 



afferent vessel; 6c, Bowman's 4 ^^ ^g ^^^^^ ^^le average relative blood SUpply 



capsule; cap, capillary; ctl, " itl ^ 



cilia; ev, efferent vessel; gl, of other Organs of the body. 



glomerulus; ui, uriniferous Theories of Urine Elunination.— There are 



tubule. 



two principal theories as to the action of the 

 parts of the kidney in elimination of urine. The earliest and 

 simplest one of these theories, that proposed by Ludwig, is known as 

 the mechanical theory. According to it urine elimination is explained 

 by the filtration of water and dissolved substances, as salts, urea, etc., 

 from the blood through the thin membranes of the glomerulus and the 

 capsule. The urine according to this theory is dilute as it enters the 

 tubules but is modified during its passage through the tubule by diffu- 

 sion. The other theory, developed by Bowman and Hcidcnhain, as- 

 serts that water and certain salts are eliminated from the l)lood through 

 the membranes of the glomerulus by an act of secretion, and that the 

 urea and other organic compounds found in the urine are secreted by 

 the cells of the tubules. Many experiments have been performed to test 



