324 THE MAINTENANCE OF THE INDIVIDUAL 



These uriniferous tubules begin in an expansion (Bowman's capsule) 

 about a little arterial knot of capillaries, called a glomerulus, which 

 together make up the functional unit of the excretory system, known 

 as a renal or Malpighian corpuscle. In order to understand the work- 

 ings of these million odd excretory units, it is necessary to understand 

 the anatomy of the kidney. 



The main trunk line of the arterial system gives off a pair of renal 

 arteries that are broken down into many very small afferent vessels 

 each of which enters the glomerulus, leaving as a smaller efferent 

 vessel that breaks down into a typical capillary network over the 

 convoluted surface of the tubule. As the wall of Bowman's capsule 

 surrounding the glomerulus is thin, it is believed that water and 

 inorganic salts are mechanically filtered out into the cavity by means 

 of differences in pressure between the blood vessels and the lumen of 

 the tubule. In the second set of capillaries the urea and other specific 

 urinary constituents are first transferred by the cells and so secreted 

 in the uriniferous tubule. Water and certain salts are reabsorbed 

 into the blood stream at this point. 



In any event, the kidneys remove the waste products from the blood 

 stream, transferring them to the pelvis of the kidney, and thence 

 down the ureters to the bladder. Here the urine is stored until 

 finally released to pass to the outside through the urethra. 



SUGGESTED READINGS 



Clendenning, L., The Human Body, Alfred A. Knopf, Inc., 1930. Chs. III-VII. 



INIore popularized anatom}^ and phj^siology. 

 Haggard, H. H., Devils, Drugs, and Doctors, Harper & Bros., 1929. Ch. VI. 



A popular account of early anatomy and physiology. 

 Howell, W. H., Textbook of Physiology, 17th ed., W. B. Saunders Co., 1933. 



Chs. XXIII, XLI, XLII, XLIII, and XLV. 



A detailed, technical account of physiology. 

 Kimber, D. C, Gray, C. E., and Stackpole, C. E., Textbook of Anatomy 



and Physiology, 9th ed.. The Macmillan Co., 1934. Chs. XVII and 



XXI. 



An anatomy and physiology of the human respiratory system. Technical 



but condensed. 

 Locy, W. A., The Growth of Biology, Henry Holt & Co., 1925. Ch. X. 



An account of Harvey's contribution to our knowledge of the circula- 

 tory system. See also other books by this author, or others on the 



history of biology, 



