CHAPTER XIII. 

 EXCRETION. 



WE have now considered the methods by which the food is digested 

 and prepared for absorption, as well as the methods by which the changed 

 materials reach the general blood-stream, either by means of the lymph- 

 atics of the intestinal wall or by the capillaries of the portal circulation. 

 We have also discussed the most difficult problems of physiology, viz., 

 those concerned with the exact changes which take place in the tissues 

 and organs of the body, when they are supplied with the food necessary 

 for life. We have mentioned the chief forms in which the waste mate- 

 rials resulting from the metabolism of the tissues leave the body. We 

 have seen how carbon dioxide and other matters are eliminated by the 

 lungs, and, further, we have devoted some time to the consideration of 

 the amount and composition of the fasces. The highly important func- 

 tion of the kidneys, in excreting the urine, and thus removing certain 

 waste materials, and the functions of the skin remain, and it is to these 

 that we must now direct our attention. 



The Structure and Functions of the Kidneys. 



The kidneys are two in number, and are situated deeply in the lum- 

 bar region of the abdomen on either side of tlje spinal column behind 

 the peritoneum. They correspond in position to the last two dorsal and 

 two upper lumbar vertebrae; the right being slightly below the left in 

 consequence of the position of the liver on the right side of the abdo- 

 men. They are about 4 inches long, 2-J- inches broad, and 1|- inches 

 thick. The weight of each kidney is about 4 oz. 



Structure. The kidney is covered by a tough fibrous capsule, which 

 is slightly attached by its inner surface to the proper substance of the 

 organ by means of very fine fibres of areolar tissue and minute blood- 

 vessels. From the healthy kidney, therefore, it may be easily torn off 

 without injury to the subjacent cortical portion of the organ. At the 

 hilus or notch of the kidney, it becomes continuous with the external 

 coat of the upper and dilated part of the ureter (fig. 287). 



On dividing the kidney into two equal parts by a section carrieti 



460 



