232 



THE CAT. 



[CHAP, vin, 



different secretions as are those, for example, of the sweat glands, 

 salivary glands, synovial membrane, and liver. The undivided tube 

 of a gland by which its secretion is poured out is its duct. The se- 

 cretions, and therefore their glands may, as we have seen, simply 

 serve to aid the process of assimilation. They may also aid the 

 function of generation, or, finally, they may merely serve for ex- 

 cretion, i. c. } to get rid of waste products or excreta. Certain large 

 and small glands have already been described in the sixth chapter, 

 namely, the liver, the pancreas and the various salivary glands. 

 The anal glands were also therein noticed. It remains to describe 

 those very important glands, the kidneys. 



9. As the foods and the tissues of the body may both be 

 divided into nitrogenous and non-nitrogenons substances, so also 

 the excreta of the body may be similarly divided. The non- 



Fig. 109. THE CAT'S KIDNEY, ENTIRE AND IN SECTION. 



A. The outer surface of the kidney, showing the 



network of blood-vessels. 

 a. Renal artery. 

 V: Ureter. 

 r. Renal vein. 



B. Vertical section through the kidney. 



c. Cortical substance. 



i. Expanded end of the calix surrounding 



the mammilla. 



o. Dark spots in cortical substance. 

 p. Papilla, or mammilla. 

 pi. Pelvis. 

 t. Tubules of the kidney. 



nitrogenous products of waste are eliminated by the lungs, and to a 

 very much less degree by the skin in the form of water and carbonic 

 acid. But a very large portion of the waste products are nitrogenous. 

 These are eliminated in a trifling degree also by the skin, but the 

 special organs for their elimination are the renal organs or kidneys. 

 A process of oxidation in the innermost substance of the body 

 converts the nitrogenous waste matter into urea, uric acid, ammonia, 

 and certain other acids and salts which are crystalloidal derivations 

 from colloidal tissues. The kidneys extract all these, with much 

 water, from the blood, and so form urine. 



10. The KIDNEYS differ from the lungs in that they are organs 

 of excretion only. The lungs excrete, but as we have just seen, they 

 also take in. The secretion of the kidney, the urine, passes down 

 from those organs by two tubes into a receptacle the bladder where 

 it accumulates, and whence it is expelled at intervals. 



The kidneys are two organs placed one 011 either side of the 



